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Life history evolution and comparative developmental biology
Abstract
Evolutionary biologists studying life history variation have used echinoderms in experimental, laboratory, and field studies of life history evolution. This focus on echinoderms grew originally from the tradition of comparative embryology, in which echinoderms were central. The tools for obtaining and manipulating echinoderm gametes and larvae were taken directly from comparative embryological research. In addition, the comparative embryologists employed a diverse array of echinoderms, not a few model species, and this diversity has led to a broad understanding of the development, function, and evolution of echinoderm larvae. As a result, this branch of life history evolution has deep roots in comparative developmental biology of echinoderms. Here two main aspects of this relationship are reviewed. The first is a broad range of studies of fertilization biology, dispersal, population genetics, functional morphology, and asexual reproduction in which developmental biologists might take a keen interest because of the historical origins of this research in echinoderm comparative embryology. The second is a similarly broad variety of topics in life history research in which evolutionary biologists require techniques or data from developmental biology in order to make progress on understanding patterns of life history variation among echinoderm species and higher taxa. Both sets of topics provide opportunities for interaction and collaboration.
... Each class of echinoderms has its characteristic planktotrophic larva; however, larval morphology poorly correlates with the phylogeny accepted in this phylum that is based on adult morphology and molec ular genetic analysis [24,28]. Thus, only sea urchins and brittle stars, which are not sister groups within the phylum Echinodermata, form planktotrophic larvae named plutei, which have a larval skeleton and similar morphology [24,25]. The study of the larval anatomy and neuroanatomy of sea urchins and brittle stars is important for understanding the evolutionary patterns of the larval forms in these groups and the degree of independence of origin of larvae with a skeleton [25]. ...
... Thus, only sea urchins and brittle stars, which are not sister groups within the phylum Echinodermata, form planktotrophic larvae named plutei, which have a larval skeleton and similar morphology [24,25]. The study of the larval anatomy and neuroanatomy of sea urchins and brittle stars is important for understanding the evolutionary patterns of the larval forms in these groups and the degree of independence of origin of larvae with a skeleton [25]. In recent years, many works have appeared on the neuroanatomy and neurogenesis of larval echino derms, among them ophiuroids [11,14,17,19,23,24,26,44], suggesting an interest in the study of the larval development of echinoderms. ...
... For electron microscopic observations, larvae at different developmental stages were fixed for 1.5 h in 2.5% glutaraldehyde in 0.05 M cacodylate buffer with the addition of 21.4 mg/ml NaCl and then postfixed for 1.5 h in 1% OsO 4 in the same buffer with the addi tion of 27. 25 Larvae were further washed in PB (3 × 20 min), embedded in glycerol, and mounted onto glass slides. The slides were examined in a Zeiss LSM 510 META confocal laser scanning microscope. ...
The differentiation of the ectodermal, entodermal, and mesodermal cell lines in developing plutei of the ophiuroid Amphipholis kochii was examined using electron microscopy and the immunochemical staining technique. The ectodermal cells form the pseudostratified epithelium of the ciliary band, the flattened epithelium of the body wall, and the esophageal epithelium. The epithelium of the ciliary band consists of ciliated and mucous cells; at its base is an axonal tract formed of the processes of neurons. The serotoninergic neurons form two lateral ganglia located along the paraoral ciliary band and the posterolateral arms’ ciliary band. The prominent features of the neurons are large size, the presence of a cilium, an electron-light cytoplasm filled with microvesicles with neurotransmitters, and a large nucleus with a predominant euchromatin. The ectoderm cells (except mucous cells) are characterized by the presence of a cilium surrounded by a collar of microvilli and a thin layer of apical extracellular matrix. The entodermal cells form the digestive tract epithelium and differentiate into four cell types: type I and II cells probably function in the nutrient uptake and assimilation; type III cells perhaps secrete digestive enzymes; and myoepithelial cells that constitute the cardiac and pyloric sphincters and the anus. Sclerenchymatous cells, which are the descendants of the primary mesenchyme, form a syncytium around the developing spicules. The biomineralization process is intrasyncytial, the ophioplutei spicules retain the cytoplasmic covering throughout the period of larval development. The secondary mesenchyme gives rise to smooth muscle cells and amebocytes. Muscle cells compose the circumesophageal musculature, the cell processes of each “muscle band” seem to fuse together. At the base of the preoral band are two symmetrically located groups of muscles, viz., the anterior dilators. Amebocytes function in excretion either near the epidermis or are able to penetrate through the epidermis and excrete wastes into the external environment. The mesoderm formed by the enterocoely gives rise to three pairs of coeloms; their cells remain unspecialized during the entire period of larval development. Results of this study are compared with the micro- and neuroanatomy of the larvae of other echinoderms.
... Similarities in morphology and feeding biology among the disparate larvae of echinoderms (and even their sister group the hemichordates) suggest that the ancestral mode (plesiomorphy) of development in the echinoderms is development via a feeding larva, and that non-feeding development has evolved independently on many occasions (Strathmann 1985, see Figure 1-3 for echinoids; Wray 1995). Much has been written regarding the ecological and evolutionary consequences of feeding versus non-feeding larval development (Hart 2002;Strathmann 1985;Wray 1995), including trade offs between egg size and egg number, differences in survival in the plankton, limitations to dispersal, as well as differences in juvenile growth and mortality in feeding and non-feeding taxa. Still, the developmental mechanisms involved in the multiple evolutionary transitions from feeding to non-feeding have remained obscure. ...
... Many scenarios have been discussed concerning how non-feeding development evolved from feeding development (review in Hart 2002). Egg size can be a strong predictor of developmental mode in echinoderms and other taxa, in some cases (such as the echinoids), however, this correlation is less strong (Emlet et al. 1987;Levitan 2000a), ...
... suggesting that there are other factors involved as well (further discussed in chapter 2 and 3). Ecological considerations such as trade offs between egg size and egg number, differences in survival in the plankton, limitations to dispersal, as well as differences in juvenile growth and mortality in feeding and non-feeding taxa have thus far dominated the discussion about the evolution of alternative life history modes in marine invertebrates (reviewed in previous chapters and (Hart 2002), while mechanistic (ontogenetic) changes have received relatively little attention. Our data presented here once again highlight thyroid hormone signaling as a possible mechanism involved in this evolutionary transition. ...
... In contrast, we know much less about how life histories evolve in marine invertebrates (Hart 2002;Wray 2002). The developmental and genetic bases for life history transitions have been studied in the greatest detail within 2 groups, the echinoid genus Heliocidaris (for example, Raff 1992;Kauffman and Raff 2003) and the ascidian genus Molgula (for example, Swalla and others 1993;Swalla and Jeffery 1996), where a few of the relevant genes and developmental processes have been identified. ...
... Of these 3 approaches, the first has so far contributed the most to understanding the evolution of life histories of marine invertebrates. A clearer understanding of phylogenetic relationships has helped to reconstruct the history of life history transformations and to clarify the evolutionary mechanisms that have operated on them (Strathmann and Eernisse 1994;McEdward and Miner 2001;Hart 2002). For some clades, it has been possible to infer ancestral life history states and to estimate how many times parallel changes in life histories have evolved (Wray 1996;Rouse 1999;Huber and others 2000). ...
... This knowledge base partially alleviates the problem of missing information, by delineating which sequences within a cis-regulatory region are functional. Echinoid development has also been studied extensively by larval ecologists and evolutionary biologists (Smith 1997;Strathmann 2000;Hart 2002), which provides an important context for understanding how selection operates on anatomy and life history. Finally, the phylogenetic relationships and divergence times of echinoids are relatively well understood, due to their unusually good fossil record and recent efforts by molecular phylogeneticists (Smith and others 1995;Jeffery and others 2003;Biermann and others 2003). ...
Many evolutionary modifications in development and life history derive from changes in embryonic gene expression. However, the genetic variation affecting gene expression in natural populations is not well understood, nor are the evolutionary mechanisms that operate on that variation. The early embryonic gene network of the purple sea urchin (Strongylocentrotus purpuratus) has been studied in considerable detail, providing an informative basis for analyzing the developmental and evolutionary mechanisms that alter gene expression. Comparative functional analyses have been carried out for several genes. These case studies indicate a complex relationship between sequence divergence and gene expression: in some cases, gene expression is conserved despite extensive divergence in cis-regulatory sequences, while in others the basis for a change in gene expression does not reside locally but rather in the expression or activity of transcription factors that regulate its expression. Diverse evolutionary mechanisms apparently operate on cis-regulatory regions, including negative, balancing, and stabilizing selection.
... For many phyla (e.g. echinoderms, some molluscs and polychaetes), planktotrophic development in species with small eggs is considered to be the ancestral life history from which independent evolutionary transitions to a large egg and lecithotrophic development have occurred (Strathmann 1985, Em let et al. 1987, Haszprunar et al. 1995, Hart 1996, Duda & Palum bi 1999, McEdward & Miner 2001, Hart 2002, Byrne 2006. Evolutionary change in egg and larval traits (evolution of development, 'evo-devo') profoundly influences the biogeography and population structure of marine invertebrates, with the plankto trophic− lecithotrophic dichotomy being of major im portance for marine ecology and life-history theory (O'Connor et al. 2007, Marshall et al. 2012, Barbosa et al. 2013, Puritz et al. 2017. ...
... In particular, the DAGE content of these eggs would be insightful to determine if change in this lipid class was a key innovation associated with increased egg size to allow facultative feeding. The current paradigm is that a gradual in crease in egg size reduced larval dependence on exogenous food, with the eventual loss of feeding structures, morphological simplification of larvae and finally full dependence on maternal provisions (Wray 1996, Hart 2002. This change is also likely to have been associated with oogenic and developmental plasticity. ...
... Echinoderms are a diverse group of organisms that includes five classes (echinoids, asteroids, crinoids, ophiuroids and holothuroids) and together with Hemichordates belong to Ambulacraria, sister group to Chordates. Studies on echinoderm embryos and larvae have made fundamental contributions to our knowledge of embryonic patterning, organ development and cell-type evolution (reviewed in Hart, 2002;Arnone et al., 2015, Arnone et al., 2016. Although belonging to the same phylum, each echinoderm larva evolved distinct cell types, such as the skeletogenic cells that support long arms in sea urchin and brittle star pluteus larvae, or the large, neuron-rich oral hoods present in the auricularia and brachiolaria larvae of sea cucumbers and sea stars. ...
To explore animal diversity, new experimentally tractable organisms must be established. Echinoderms include five groups of marine animals that have been used as developmental models for over a century thanks to their low costs, high fecundity, optically clear larvae and genetic tractability. An additional advantage of echinoderms is that their larval forms display diverse morphologies. This rich diversity enables comparative studies to investigate the evolutionary relationships among cell types, tissues, and organs. However, reproducible protocols to obtain gametes, detailed information on embryogenesis, and genomic tools have been optimized only for selected species of sea urchins and sea stars. To address this gap, we established the abundant Mediterranean sea cucumber Holothuria tubulosa as a new experimental system. Here we describe a method to reliably obtain gametes and make embryonic cultures multiple times from the same animal and characterize unique larval tissues combining immunohistochemistry and high-resolution microscopy. This work represents a step forward in our understanding of holothurian development and establishes H. tubulosa as an emerging experimental system for evo-devo and other biological disciplines.
... Experimental biology with echinoderms has driven major discoveries in the past 100 years, significantly contributing to our understanding of cell, developmental and regulatory biology [24,41,46,72] and reviewed in [5,14,15,32,65,67]. This group of animals includes sea urchins, sea stars, sea lilies, brittle stars and sea cucumbers and together with hemichordates belong to ambulacraria, the sister group to chordates (Fig. 1). ...
A challenge for evolutionary developmental (evo-devo) biology is to expand the breadth of research organisms used to investigate how animal diversity has evolved through changes in embryonic development. New experimental systems should couple a relevant phylogenetic position with available molecular tools and genomic resources. As a phylum of the sister group to chordates, echinoderms extensively contributed to our knowledge of embryonic patterning, organ development and cell-type evolution. Echinoderms display a variety of larval forms with diverse shapes, making them a suitable group to compare the evolution of embryonic developmental strategies. However, because of the laboratory accessibility and the already available techniques, most studies focus on sea urchins and sea stars mainly. As a comparative approach, the field would benefit from including information on other members of this group, like the sea cucumbers (holothuroids), for which little is known on the molecular basis of their development. Here, we review the spawning and culture methods, the available morphological and molecular information, and the current state of genomic and transcriptomic resources on sea cucumbers. With the goal of making this system accessible to the broader community, we discuss how sea cucumber embryos and larvae can be a powerful system to address the open questions in evo-devo, including understanding the origins of bilaterian structures.
... A study on Antarctic echinoids with deposit feeding showed that the composition of its gut microbiota was mostly driven by the host type and, to a lesser extent, by the population location (20). Ophiuroidea (brittle stars), the largest class of echinoderms, are distributed in various habitats from intertidal zones to the deep sea (21)(22)(23). As a significant epifaunal group, ophiuroids have important ecological functions in material circulation and energy conversion in the benthic boundary layer (24) and are vital components of the marine calcium cycle and calcium reservoir (25). ...
Gastrointestinal microorganisms play a crucial role in host survival and adaptation, but information on host-specific selection or environmental factors that shape the microbiome in natural populations is limited. In this study, we employed 16S rRNA gene amplicon sequencing to investigate the composition and predicted the functions of gut microbiota of two ophiuroid species ( Ophiura sarsii and its subspecies O. sarsii vadicola ) from cold-water habitats across three geographically distant sea areas in the Northern Pacific Ocean. We also explored the potential influence of the host and environment on the microbiota. The two ophiuroids possessed diverse microbial communities, with Proteobacteria being the most dominant phylum in all four groups. Aliivibrio was the predominant genus in O. sarsii from the Bering Sea, while Candidatus Hepatoplasma was the dominant genus in O. sarsii from the Japan Sea and O. sarsii vadicola from the Yellow Sea. Predicted bacterial functions indicated that most of the pathways with significant differences belonged to metabolism and genetic information processing. Notably, no significant difference was observed between the two coexisting ophiuroid species in the Japan Sea. The high similarity in microbial communities between O. sarsii from the Japan Sea and O. sarsii vadicola from the Yellow Sea may be attributed to their analogous ecological niche. The host and the environment jointly shape the composition of the gut microbial community. The presence of specific microorganisms supports the ecological success of ophiuroids across diverse environments, providing a foundation for host adaptation to cold-water habitats.
IMPORTANCE
Gastrointestinal microorganisms are critical to the survival and adaptation of hosts, and there are few studies on the differences and functions of gastrointestinal microbes in widely distributed species. This study investigated the gut microbes of two ophiuroid species ( Ophiura sarsii and its subspecies O. sarsii vadicola ) in cold-water habitats of the Northern Pacific Ocean. The results showed that a combination of host and environmental factors shapes the intestinal microbiota of ophiuroids. There was a high similarity in microbial communities between the two groups living in different regions, which may be related to their similar ecological niches. These microorganisms played a vital role in the ecological success of ophiuroids as the foundation for their adaptation to cold-water environments. This study revealed the complex relationship between hosts and their gut microbes, providing insights into the role they play in the adaptation and survival of marine species.
... Although our study provides an important first step in clarifying the delimitation of Luidia in the northern Gulf of Mexico, sequencing a conserved mitochondrial gene alone lacks sufficient resolution to make definitive conclusions about connectivity within and among Luidia populations or species. An integrative approach of sampling Luidia across the entire species range , incorporating phenotypic variation (Fig. S1, available online) and lifehistory traits (Hart, 2002;Palumbi, 2003) and using population genetic tools (Pante et al., 2015), should be used to determine not only gene flow in this region but also the frequency of hybridization between Luidia in this region. ...
Accurate species delimitation is crucial to understanding biodiversity. In the northern Gulf of Mexico, recent genetic evidence has suggested that the tricolor Luidia lawrencei is not a species distinct from the gray Luidia clathrata. We collected Luidia specimens from Apalachee Bay, Florida, and morphologically identified 11 as L. clathrata and 16 as L. lawrencei. We sequenced 1074 bp of the cytochrome c oxidase subunit I (COI) and found ~14% divergence between L. clathrata and L. lawrencei, suggesting two distinct species (within-species divergence was <1%). Two specimens were phenotypically L. lawrencei (i.e., tricolor morph) but mitochondrially were L. clathrata. Our findings lend support to maintaining L. clathrata and L. lawrencei as distinct species. However, the species boundary between these two taxa may be porous, and ongoing hybridization may occur when the two species are found in sympatry. Future work with nuclear markers is warranted to determine the frequency of hybridization and the extent of introgression. Clarifying the genetic relationship between these species will provide a baseline for assessing ongoing changes in connectivity of these two highly abundant sea stars in the rapidly warming northern Gulf of Mexico.
... Although the precise reasons for the switch from planktotrophy to lecithotrophy are not well understood, nonfeeding development is likely favored when phytoplankton is not a reliable source of energy for larvae (Emlet et al., 1987;Marshall, Krug, Kupriyanova, Byrne, & Emlet, 2012). What is clear is that in many cases, lecithotrophy has evolved quite recently in relation to the long prior history of planktotrophy (Hart, 2002;Hart, Abt, & Emlet, 2011;Hart, Byrne, & Smith, 1997;Jeffery, Emlet, & Littlewood, 2003;Smith, Boom, & Raff, 1990) (Fig. 2). ...
As analyses of developmental mechanisms extend to ever more species, it becomes important to understand not just what is conserved or altered during evolution, but why. Closely related species that exhibit extreme phenotypic divergence can be uniquely informative in this regard. A case in point is the sea urchin genus Heliocidaris, which contains species that recently evolved a life history involving nonfeeding larvae following nearly half a billion years of prior evolution with feeding larvae. The resulting shift in selective regimes produced rapid and surprisingly extensive changes in developmental mechanisms that are otherwise highly conserved among echinoderm species. The magnitude and extent of these changes challenges the notion that conservation of early development in echinoderms is largely due to internal constraints that prohibit modification and instead suggests that natural selection actively maintains stability of inherently malleable trait developmental mechanisms over immense time periods. Knowing how and why natural selection changed during the evolution of nonfeeding larvae can also reveal why developmental mechanisms do and do not change in particular ways.
... Characterizations of the life histories of marine species typically focus on reproductive modes, such as broadcast spawning versus brooding and number of oocytes or larvae generated (c.f., Thorson 1950;Vance 1973;Strathmann 1977;Hadfield and Strathmann 1996;Hart 2002;Kahng et al. 2011). Reef anthozoans tend to broadcast large numbers of eggs that disperse well but have low probabilities of survival and settlement or brood eggs and produce fewer larvae which do not disperse great distances (Szmant 1986). ...
Successful recruitment is critical to the maintenance and resilience of populations and may be at the core of the transition from scleractinian- to octocoral-dominated faunas on some Caribbean reefs. For sessile invertebrates, recruitment incorporates the composite effects of larval supply, settlement and survival. The relative success of these processes differs between species, and successful recruitment may be achieved through different life history strategies. Recruitment of six abundant and widespread Antillogorgia spp. was assessed at six sites on Little Bahama Bank from 2009–2012. Identification of recruits to species level, based on microsatellite analyses, revealed differences in recruitment and survival between species, sites and years. The broadcast spawning species, A. americana and A. acerosa, had low rates of early recruitment and post-settlement survival. Higher levels of recruitment success were achieved among brooding and surface brooding species following somewhat different patterns of early recruitment and survival. The internal brooder A. hystrix had the highest recruitment at five of the sites, but low survival dramatically reduced its abundance and after a year it had similar densities as the surface brooding species, A. elisabethae and A. bipinnata. The brooders have smaller colonies and produce fewer larvae than the broadcast spawning species, but they release competent larvae, which may account for their higher recruitment rates. The Antillogorgia illustrate the diversity of successful reproductive strategies exhibited by octocorals, and differences in the life history strategies among these congeners are best characterized by their mode of larval development.
... Early life stages of broadcast-spawning marine invertebrates from the phylum Echinodermata has been used to assess the effects of increasing surface temperature on marine biota [12,13]. Over the past century, the fertilization of eggs of echinoderms was well categorized due to use in comparative embryological research [14]. Early life stages of echinoderms, especially sea urchins are also recognized to be highly sensitive to a wide range of environmental contaminants and stressors [15], and thus making them as an ideal state for assessing impacts of climatechange. ...
The combined effects of temperature and salinity on percent hatching, normal larval rate at hatching, and survival of fasting larvae after hatching (survival activity index; SAI) of the commercial species of collector sea urchin, Tripneustes gratilla were investigated in a captive laboratory condition. The study was conducted by setting different levels of temperatures (24°C to 36°C) and salinities (38‰ to 23‰). Within the range of temperature from 24 to 36°C and at 32‰ salinity, hatching and normal larval rates, and SAI values were highest at 24 and 27°C. The highest hatching and normal larval rates were found at 35 and 38‰ within the salinity range of 23-38‰; however, SAI value was the highest at 26‰. The results of the experiments in each level of temperature (24, 27 and 30°C) with each salinity (32, 35 and 38‰) indicated interactive effects of temperature and salinity, and within the experimental protocols of 24°C at 38‰ gave an optimal combination for highest hatching and survival of T. gratilla. The findings obtained from the present research would not only be immensely helpful towards the understanding of the suitable temperature-salinity interactions but also facilitate the development of captive breeding, larval raising and mass seed production of this high-valued sea urchin for commercial aquaculture.
... Echinoderms comprise five classes: the most basal Crinoidea and their sister group, Eleutherozoa, consisting of Echinozoa (Echinoid and Holothuria) and Asterozoa (Asteroid and Ophiuroid) [13]. Notably, the larval morphology and the machinery for settlement vary among echinoderm taxa [14][15][16][17]; for example, planktotrophic pluteus larvae of sea urchins and brittle stars settle to the sea bottom using tube feet, while in crinoids, lecithotrophic doliolaria larvae settle using adhesive tufts. Furthermore, it should be noted that the regulation of metamorphosis in sea urchins has been clarified in relatively great detail [18][19][20]; thyroid hormone and histamine signaling modulate larval growth and the acquisition of competency. ...
Many marine invertebrates have a life cycle with planktonic larvae, although the evolution of this type of life cycle remains enigmatic. We recently proposed that the regulatory mechanism of life cycle transition is conserved between jellyfish (Cnidaria) and starfish (Echinoderm); retinoic acid (RA) signaling regulates strobilation and metamorphosis, respectively. However, the function of RA signaling in other animal groups is poorly understood in this context. Here, to determine the ancestral function of RA signaling in echinoderms, we investigated the role of RA signaling during the metamorphosis of the feather star, Antedon serrata (Crinoidea, Echinodermata). Although feather stars have different larval forms from starfish, we found that exogenous RA treatment on doliolaria larvae induced metamorphosis, like in starfish. Furthermore, blocking RA synthesis or binding to the RA receptor suppressed metamorphosis. These results suggested that RA signaling functions as a regulator of metamorphosis in the ancestor of echinoderms. Our data provides insight into the evolution of the animal life cycle from the viewpoint of RA signaling.
... In echinoids, changes associated with the evolution of lecithotrophic larvae include first transitioning from an obligately to facultatively feeding larva that can reach metamorphosis without feeding, followed by loss of feeding ability and reduced larval morphology until finally a highly simplified, nonfeeding larval form is reached (Wray, 1996). Importantly, this switch has occurred numerous times within many marine invertebrate phyla (Haszprunar, von Salvini-Plawen, & Rieger, 1995;Strathmann, 1985), and in Echinodermata alone, lecithotrophy has evolved at least 20 times from the inferred ancestral planktotrophic state (Hart, 2002;McEdward & Miner, 2001;Raff, 1987;Wray, 1996;Wray & Raff, 1991). As a result, comparative analyses of egg provisioning strategies between planktotrophs and lecithotrophs provide insight into the initial evolutionary steps from feeding to nonfeeding larval development in marine invertebrates. ...
A dramatic life history switch that has evolved numerous times in marine invertebrates is the transition from planktotrophic (feeding) to lecithotrophic (nonfeeding) larval development—an evolutionary tradeoff with many important developmental and ecological consequences. To attain a more comprehensive understanding of the molecular basis for this switch, we performed untargeted lipidomic and proteomic liquid chromatography‐tandem mass spectrometry on eggs and larvae from three sea urchin species: the lecithotroph Heliocidaris erythrogramma, the closely related planktotroph Heliocidaris tuberculata, and the distantly related planktotroph Lytechinus variegatus. We identify numerous molecular‐level changes possibly associated with the evolution of lecithotrophy in H. erythrogramma. We find the massive lipid stores of H. erythrogramma eggs are largely composed of low‐density, diacylglycerol ether lipids that, contrary to expectations, appear to support postmetamorphic development and survivorship. Rapid premetamorphic development in this species may instead be powered by upregulated carbohydrate metabolism or triacylglycerol metabolism. We also find proteins involved in oxidative stress regulation are upregulated in H. erythrogramma eggs, and apoB‐like lipid transfer proteins may be important for echinoid oogenic nutrient provisioning. These results demonstrate how mass spectrometry can enrich our understanding of life history evolution and organismal diversity by identifying specific molecules associated with distinct life history strategies and prompt new hypotheses about how and why these adaptations evolve.
... T. gratilla is also important ecologically, especially in sea grass habitats and is a food source with good potential for aquaculture (Juinio-Menez et al., 1998;Dworjanyn et al., 2007;Lawrence et al., 2007;Unsworth, 2010). Early life stages (Hart, 2002) of sea urchins, in particular, are known to be highly sensitive to a wide range of environmental contaminants and stressors (Dinnel et al., 1989) which is making them ideal organisms for assessing impacts of climate change. Its life cycle has a planktonic period of days or weeks in the water column and seawater chemistry and temperature have major impacts on the development. ...
p>The present study investigated the influence of different temperature levels (16, 19, 22, 25, 28, 31 and 34oC) on embryonic and early larval development of the tropical sea urchin, Tripneustes gratilla (Linnaeus, 1758) in a controlled laboratory condition. The critical lower and higher temperature for embryonic development was found to be 16 and 34oC, respectively. Embryos reared in these temperatures exhibited 100% abnormality within 48 h post-insemination. The time required to reach embryonic and larval stages was increased with temperature from 28oC followed by 31, 25, 22 and 19oC. The developmental time of 2-cell to 4-arm pluteus larvae showed significant (p<0.05) differences. The survival (%) of larvae at the prism, 2-arm and 4-arm stages were observed as dissimilar from 22 to 34oC, and the highest values (100% or near 100%) were found at 25 and 28oC. The morphometric measurements from prism to 4-arm pluteus larvae at different temperatures differed significantly (p<0.05). However among the temperatures evaluated, 28oC was found as the best temperature for better growth and development of larvae at all stages. The findings of the study would help to develop captive breeding and seed production programmes for commercial aquaculture of the species.</p
... Long-term transmission of microbiota was studied by comparing the 16S ribosomal gene sequences of bacteria [99], corals [202], echinoderms [203] Via oocytes (vertical) Drosophila/Wolbachia [98], aphid/Buchnera [97], sponge [204], herbs/fungi [205] Coprophagy (vertical and horizontal) Many animals: termites [101], rabbits [206], koala [100], insects [102] Mother's milk (vertical) Mammals [108][109][110] Physical contact starting at birth (Vertical and horizontal) Most animals: fish [207], amphibians [208], mammals [209] Horizontal Grasses/endophytes [120], squid/Vibrio fischeri [119] Adapted from Roughgarden et al. [182]. Vertical transmission is defined as the movement of microbiota from parent to offspring without mixing with microbes in the environment associated with great apes, including humans [124]. ...
The holobiont (host with its endocellular and extracellular microbiome) can function as a distinct biological entity, an additional organismal level to the ones previously considered, on which natural selection operates. The holobiont can function as a whole: anatomically, metabolically, immunologically, developmentally, and during evolution. Consideration of the holobiont with its hologenome as an independent level of selection in evolution has led to a better understanding of underappreciated modes of genetic variation and evolution. The hologenome is comprised of two complimentary parts: host and microbiome genomes. Changes in either genome can result in variations that can be selected for or against. The host genome is highly conserved, and genetic changes within it occur slowly, whereas the microbiome genome is dynamic and can change rapidly in response to the environment by increasing or reducing particular microbes, by acquisition of novel microbes, by horizontal gene transfer, and by mutation. Recent experiments showing that microbiota can play an initial role in speciation have been suggested as an additional mode of enhancing evolution. Some of the genetic variations can be transferred to offspring by a variety of mechanisms. Strain-specific DNA analysis has shown that at least some of the microbiota can be maintained across hundreds of thousands of host generations, implying the existence of a microbial core. We argue that rapid changes in the microbiome genome could allow holobionts to adapt and survive under changing environmental conditions thus providing the time necessary for the host genome to adapt and evolve. As Darwin wrote, "It is not the strongest of the species that survives but the most adaptable".
... Vegetative (asexual) reproduction, another type of vertical transmission, takes place in many animals and plants (Fell 1993;Hart 2002;Vaughn 2010). As a consequence of vegetative reproduction, the microbiome is transferred vertically to offspring. ...
Holobionts, consisting of a host and diverse microbial symbionts, function as distinct biological entities anatomically, metabolically, immunologically, and developmentally. Symbionts can be transmitted from parent to offspring by a variety of vertical and horizontal methods. Holobionts can be considered levels of selection in evolution because they are well-defined interactors, replicators/reproducers, and manifestors of adaptation. An initial mathematical model is presented to help understand how holobionts evolve. The model offered combines the processes of horizontal symbiont transfer, within-host symbiont proliferation, vertical symbiont transmission, and holobiont selection. The model offers equations for the population dynamics and evolution of holobionts whose hologenomes differ in gene copy number, not in allelic or loci identity. The model may readily be extended to include variation among holobionts in the gene identities of both symbionts and host.
... Thus, comparative analyses of echinoid SM cell types are rife for investigation at the genomic level. Even further back in evolutionary time holothuroids and ophiuroids present-in the case of the former-a potentially exemplary case of degeneration of a markedly reduced developmental structure and its GRN (Hart, 2002;McCauley et al., 2012) andin the case of the latter-remarkable convergent evolution of the larval skeleton (Strathmann,'88;Littlewood et al.,'97). Thus, developmental studies on echinoderm skeletogenesis have the potential to inform hypotheses that aim to reveal the evolutionary relationships among the eleutherozoan echinoderms (Smith, '84;Pisani et al., 2012;Telford et al., 2014). ...
Comparative studies of early development in echinoderms are revealing the tempo and mode of alterations to developmental gene regulatory networks and to the cell types they specify. In eu-echinoid sea urchins, skeletogenic mesenchyme (SM) ingresses prior to gastrulation at the vegetal pole and aligns into a ring-like array with two bilateral pockets of cells, the sites where spiculo-genesis will later occur. In cidaroid sea urchins, the anciently diverged sister clade to euechinoid sea urchins, a homologous SM cell type ingresses later in development, after gastrulation has commenced, and consequently at a distinct developmental address. Thus, a heterochronic shift of ingression of the SM cell type occurred in one of the echinoid lineages. In euechinoids, specification and migration of SM are facilitated by vascular endothelial growth factor (VEGF) signaling. We describe spatiotemporal expression of vegf and vegfr and experimental manipulations targeting VEGF signaling in the cidaroid Eucidaris tribuloides. Spatially, vegf and vegfr mRNA localizes similarly as in euechinoids, suggesting conserved deployment in echinoids despite their spatially distinct development addresses of ingression. Inhibition of VEGF signaling in E. tribuloides suggests its role in SM specification is conserved in echinoids. Temporal discrepancies between the onset of vegf expression and SM ingression likely result in previous observations of SM " random wandering " behavior. Our results indicate that, although the SM cell type in echinoids ingresses into distinct developmental landscapes, it retains a signaling mechanism that restricts their
... Recent studies investigating the effects of increased seasurface temperature on marine biota have used early life stages of broadcast-spawning marine invertebrates from the Echinodermata phylum (Byrne 2010(Byrne , 2012. The fertilisation biology of echinoderms is well characterised because of their use in comparative embryological research over the past century (Hart 2002). Early life stages of sea urchins in particular are also known to be highly sensitive to a range of environmental contaminants and stressors (e.g. ...
Global warming has and will continue to warm the world's oceans, which may have detrimental consequences for marine life. Studies assessing the impact of climate-change stressors on early life-stages of marine invertebrates have focussed on immediate fertilisation success or larval development, but have so far not considered gamete longevity. Recent studies have suggested that sea urchin fertilisation can take place for several hours, as dilute spermatozoa can travel to fertilise distant eggs, making gamete longevity an important factor in fertilisation success for some species. The longevity of spermatozoa from Heliocidaris tuberculata was assessed over a 3-h exposure to current ambient (208C), near-future (248C) and future (268C) ocean-temperature scenarios. Sperm mitochondrial activity was also measured throughout the 3-h exposure using the stain Rhodamine 123 (Rh123) and flow cytometry. Sperm longevity, based on fertilisation success, significantly decreased following a 1-h exposure at 268C, or a 3-h exposure at 248C, relative to the 208C treatment. However, sperm mitochondrial activity did not correlate with fertilisation success. Even when fertilisation success was below 20%, Rh123 uptake remained above 80%, indicating the presence of active mitochondria in non-viable spermatozoa. Our results suggested that at projected sea-surface temperatures, the longevity of sea urchin spermatozoa is reduced, which may have consequences for sea urchin population dynamics.
... This morphological variation is thought to be deeply involved in the ecology of pluteus larvae. For example, larvae with longer arms (and hence longer arm skeletons) can feed more effectively than those with shorter arms because larvae feed using cilia onaddition, as representatives of free-spawning marine animals, sea urchin larvae have contributed to the ecological study of dispersal ability, mortality, behavior and locomotion during dispersal, and mechanisms and rates of suspension feeding (McEdward 1995; Lamare and Barker 1999; Hart 2002). The wide range of morphological variation in the larval skeleton is ideal for investigating evolutionary history from both an ecological standpoint (why the variation has been generated) and a developmental standpoint (how the variation is produced). ...
To clarify the morphological variety of larval skeletons, a detailed morphological comparison among the species of the family Echinometridae was performed. Through conspecific comparison of larval skeletons among different ages, we found five skeletal characters of the body skeleton that are stable in the four-armed pluteus and thus useful in homologous comparison among the species. The morphological variation was summarized as the difference in the number of spines and posteroventral transverse rods, and differences in the shape of the body skeleton. Significant correlations were found between some skeletal characters, such as between upper body length and bottom width of body skeleton and between lower body length and the number of spines. We found that the larval skeletons of tropical species tend to have fewer spines and rods than those of temperate species, which is consistent with the hypothesis that a reduction in skeletal elements decreases the specific gravity of larvae as an adaptation to tropical waters.
... Oversimplification of the planktotrophic-lecithotrophic dichotomy with respect to PPD should be made cautiously to avoid biasing our understanding of the complex and varied evolutionary mechanisms operating in marine systems. The dispersal biology of echinoderms is likely to be far more complex than their comparative embryology might suggest (Hart, 2002) and the same may be true of other widely distributed marine phyla with diverse larval modes. ...
Aim
As the main or sole motile stage of many aquatic taxa, propagules play a central role in their population dynamics, macroevolution and biogeography. The premises of studies concerned with dispersal, on issues as diverse as marine protected areas, sustainable fisheries and invasive species, commonly make simplified assumptions linking larval development mode (planktotrophic, lecithotrophic) and pelagic propagule duration ( PPD ) or pelagic larval duration. Because general empirical tests of these connections have yielded equivocal results, we critically examined the relationship between larval mode and PPD in light of regional and taxon‐specific criteria.
Location
Oceans world‐wide.
Methods
We collated data from 190 species from the phylum Echinodermata and used two‐way and one‐way ANOVA to compare PPD among larval modes, taxonomic classes, geographic regions and climate zones. We also assessed the contribution of egg size, temperature and other continuous predictors using multiple regressions and ANCOVA , and examined whether geographic range size differed among larval modes.
Results
Our results depart from the common assumption that planktotrophs have longer PPDs than lecithotrophs. Instead, we detected a synergistic influence of phylogeny and climate. No significant difference in PPD between planktotrophs and lecithotrophs occurred in two of the four classes and only four out of nine major ocean basins contained species with significantly different PPDs on the basis of larval mode. Species from colder climates displayed relatively constant PPDs regardless of larval mode, whereas temperate and warm‐water species displayed a clearer dichotomy. PPD did not correlate with geographic range size nor did planktotrophs and lecithotrophs exhibit significantly different range sizes.
Main conclusions
Our findings indicate that larval mode is not a reliable predictor of PPD across broad geographic and taxonomic scales. Instead developmental strategies may instigate shorter or longer PPDs in schemes that are influenced by evolutionary and environmental pressures.
... Planktotrophic larvae exist and feed in the plankton, whereas lecithotrophic larvae may be planktonic, benthic or develop via external or internal brood protection. Within the Echinodermata, there is good evidence that planktotrophic development is the ancestral state from which lecithotrophic development has repeatedly evolved (McEdward and Miner, 2001;Hart, 2002;Raff and Byrne, 2006). ...
Question: How is maternal investment of energy storage lipids linked to the evolution of development for echinoderms with larval phases? Hypotheses: Egg nutrients sustain development to the exotrophic larval stage in echinoderms with feeding (planktotrophic) larvae and to the exotrophic juvenile stage in species with non-feeding (lecithotrophic) larvae. Whereas planktotrophic echinoderm development requires egg lipid reserves that are readily metabolized, lipids suitable for long-term energy storage might be more appropriate fuels for lecithotrophic development. Organisms: We considered closely related asteroid and ophiuroid species that possess a range of egg sizes and represent three modes of larval development (planktotrophy, planktonic lecithotrophy, benthic lecithotrophy). Methods: We used Iatroscan TLC-FID to quantify maternal investment of lipids on a per egg basis for each species and focused on egg content of the two dominant classes of energy storage lipid, triacylglycerol (TAG) and diacylglycerol ether (DAGE). Results: Energetic lipids in the small eggs of echinoderms with feeding larvae are primarily TAG, a class of short-term storage lipids. DAGE, which is metabolized more slowly than TAG, dominates the large eggs of echinoderms with non-feeding larvae. Increased deposition of DAGE lipids in the eggs of planktotrophic species may facilitate the transition to lecithotrophy.
... Strathmann 1985; Duda and Palumbi 1999; McEdward and Miner 2001). The regaining of a feeding larva after the evolution of nonfeeding development appears far less likely (Wray 1995; Hart 2002). The Echinodermata have proven extremely useful for the study of developmental evolution due to the presence of developmental diversity among closely related species (RaV and Byrne 2006 ). ...
For marine invertebrates, larval developmental mode is inseparably linked to the nutritional content of the egg. Within the
asterinid family of sea stars there have been multiple, independent, evolutionary transitions to lecithotrophic development
from the ancestral, planktotrophic state. To investigate the evolution of maternal investment and development within the Asterinidae,
we quantified individual lipid classes and total protein for eggs and larval stages of closely related species representing
three developmental modes (planktotrophy, planktonic lecithotrophy and benthic lecithotrophy). Within species, maternal provisioning
differed between females indicating that egg quality varied with parentage. Maternal investment was related to egg size but,
after correcting for egg volume, we identified two major oogenic modifications associated with the evolution of lecithotrophic
development: (1) a reduction in protein deposition that probably reflects the reduced structural requirements of nonfeeding
larvae, (2) an increase in deposition of a single class of energetic lipid, triglyceride (TG). The exception was Parvulastra exigua, which has benthic, lecithotrophic development and lays eggs with a lipid to protein ratio close to that of planktotrophs.
This oogenic strategy may provide P. exigua larvae with a protein “weight-belt” that assists in maintaining a benthic existence. Asterinids with planktotrophic development
used a significant portion of egg TG to build a feeding bipinnaria larva. For Meridiastra mortenseni, female-specific differences in egg TG were still evident at the bipinnaria stage indicating that egg quality has flow-on
effects for larval fitness. In lecithotrophic asterinids, TG reserves were not depleted in development to the larval stage
whereas protein stores may help fuel early larval development. Available data indicate that there may be two evolutionarily
stable egg lipid profiles for free-spawning, temperate echinoderms.
... Ex05: exposed sites within 5 km of the coast, Ex15: exposed sites 5 < x < 15 km off the coast, Ex30: exposed sites 15 < x < 30 km off the coast, Ex >30: exposed sites >30 km off the coast, Sh05: sheltered sites within 5 km of the coast, Sh15: sheltered sites 5 < x < 15 km off the coast, Sh30: sheltered sites 15 < x < 30 km off the coast, Sh >30: sheltered sites >30 km off the coast. disperse greater distances in ocean currents (Hart 2002;Shanks et al. 2003;Siegel et al. 2003). Furthermore, sea urchins, in contrast to sponges, corals and foraminifera, are frequently collected by people at reef flats for consumption, which undoubtedly has strong effects on the distribution (W. ...
Abstract In order to preserve diversity it is essential to understand how assemblages change across space. Despite this fact, we still know very little about how marine diversity is spatially distributed, especially among lesser-studied invertebrate taxa. In the present study beta-diversity patterns of sea urchins, sponges, mushroom corals and larger foraminifera were assessed in the Spermonde Archipelago (Indonesia). Using ordinations we showed that the inshore zone (30 km offshore) all contained distinct assemblages of sponges and corals, while only foraminifera assemblages from the inshore (c. 1600 km2). The analyses of the corals and foraminifera were additionally tested at two spatial scales of sampling. Both taxa were primarily associated with local-scale environmental variables at the local scale and larger-scale variables at the larger scale. Mean inter-plot similarity was also higher and variation lower at the larger scale. The results suggest that substantial variation in similarity can be predicted using simple locally assessed environmental variables combined with remotely sensed parameters.
... Important considerations are that the levels of oxidative stress incurred by organisms are not constant but vary with developmental stage, environmental conditions and levels of activity (Monaghan et al., 2009). Echinoderms have been used extensively to theorise on life-history strategies (Hart, 2002) and could provide ideal model systems to address questions relating to the role of UVR and oxidative stress in shaping developmental patterns. ...
There is general consensus that solar ultraviolet radiation (UVR) negatively impacts many marine species. Echinoderms are ubiquitous within the marine environment, with members of the phyla often long-lived and numerically dominant within the benthic macrofauna, consequently the impact of UVR on the population dynamics of these organisms will influence marine communities and ecosystems. Research to date has shown that exposure of echinoderms to solar UVR can, affect reproduction and development, change behaviour, cause numerous biochemical and physiological changes and potentially cause increased mutation rates, by causing DNA damage. There is also considerable evidence that echinoderms utilise several different mechanisms to protect themselves against excessive UVR and subsequent UVR-induced damage. However, these protective mechanisms may pose conflicting selection pressures on echinoderms, as UVR is an additional stressor in oceans subjected to anthropogenic-induced climate change. This review summarises our knowledge of the effects of UVR on the Echinodermata. We outline the research conducted to date, highlight key studies on UVR that have utilised echinoderms and look to the future of UVR research in a rapidly changing ocean.
... Important considerations are that the levels of oxidative stress incurred by organisms are not constant but vary with developmental stage, environmental conditions and levels of activity (Monaghan et al., 2009). Echinoderms have been used extensively to theorise on life-history strategies (Hart, 2002) and could provide ideal model systems to address questions relating to the role of UVR and oxidative stress in shaping developmental patterns. ...
The physiological mechanisms that regulate adaptive plasticity of clonal organisms are key to their success in changing environments. Here, we review the mechanisms that regulate morphological plasticity of colonial hydrozoans. There is a heritable, genetic basis to colony form, but environmentally-induced plasticity and self-reinforcing developmental physiology explain much of total phenotypic variance. Morphological development of colonial hydrozoans emerges from interactions among (1) behaviors which drive gastrovascular transport, (2) architecture of the gastrovascular system that determines hydrodynamic characteristics of vascular flow, and, (3) gene products that vary in response to physiological signals provided by gastrovascular transport. Several morphogenetic signaling mechanisms have been identified, including, reactive oxygen species and nutrient concentrations in the hydroplasm, and hydromechanical forces associated with gastrovascular transport. We present a conceptual model of the interacting forces that drive hydrozoan morphological development. Several avenues for future research are suggested by the synthesis of information from prior studies of hydrozoans. Elucidating the morphogenetic signaling pathways responsive to metabolites or hydromechanical forces and the epigenetic effect of vascular architecture on colony form may give new insight into the self-maintenance of indeterminately growing and continuously developing vascular systems.
... Given the unique morphological evolution, a complete fossil record [11], and readily feasible developmental studies [12], clypeasteroids can be a valuable group for evolutionary analyses. Furthermore, C. subdepressus has been used in recent studies related to development [13][14][15][16], posing the need for a detailed developmental table in order to comprehend the evolution of echinoid morphology. ...
Sea biscuits and sand dollars diverged from other irregular echinoids approximately 55 million years ago and rapidly dispersed to oceans worldwide. A series of morphological changes were associated with the occupation of sand beds such as flattening of the body, shortening of primary spines, multiplication of podia, and retention of the lantern of Aristotle into adulthood. To investigate the developmental basis of such morphological changes we documented the ontogeny of Clypeaster subdepressus. We obtained gametes from adult specimens by KCl injection and raised the embryos at 26 degrees C. Ciliated blastulae hatched 7.5 h after sperm entry. During gastrulation the archenteron elongated continuously while ectodermal red-pigmented cells migrated synchronously to the apical plate. Pluteus larvae began to feed in 3 d and were 20 d old at metamorphosis; starved larvae died 17 d after fertilization. Postlarval juveniles had neither mouth nor anus nor plates on the aboral side, except for the remnants of larval spicules, but their bilateral symmetry became evident after the resorption of larval tissues. Ossicles of the lantern were present and organized in 5 groups. Each group had 1 tooth, 2 demipyramids, and 2 epiphyses with a rotula in between. Early appendages consisted of 15 spines, 15 podia (2 types), and 5 sphaeridia. Podial types were distributed in accordance to Lovén's rule and the first podium of each ambulacrum was not encircled by the skeleton. Seven days after metamorphosis juveniles began to feed by rasping sand grains with the lantern. Juveniles survived in laboratory cultures for 9 months and died with wide, a single open sphaeridium per ambulacrum, aboral anus, and no differentiated food grooves or petaloids. Tracking the morphogenesis of early juveniles is a necessary step to elucidate the developmental mechanisms of echinoid growth and important groundwork to clarify homologies between irregular urchins.
... One might expect that the short minimum time to metamorphosis in C. rosaceus would lead to genetic divergence across the range of the species, as has been observed in other echinoderms with short larval lives (reviewed in Hart, 2002). This, however, is not the case, as we found no evidence of divergence between Panama and Florida. ...
Larvae of marine invertebrates either arise from small eggs and feed during their development or arise from large eggs that proceed to metamorphosis sustained only from maternal provisioning. Only a few species are known to possess facultatively feeding larvae. Of about 250 echinoid species with known mode of development, only two, Brisaster latifrons and Clypeaster rosaceus, are known to develop through facultatively planktotrophic larvae. To obtain more information on this form of development and its consequences, we determined egg size and egg energetic and protein content of these two species. We found that eggs of B. latifrons resemble those of species with nonfeeding larvae in these characteristics more than those of C. rosaceus. We also compared DNA sequences of the cytochrome oxidase (COI) gene from the Caribbean C. rosaceus to those of the sympatric planktotrophic developer C. subdepressus and also to those of the eastern Pacific species C. europacificus to estimate the degree of divergence between species with different developmental modes. Comparison of COI sequences of C. rosaceus from Panama and Florida revealed that there is no geographic differentiation in this species. Cross-fertilization experiments between C. rosaceus and C. subdepressus indicated that bidirectional gametic incompatibility has evolved between the two species.
... Cell lineages, cell-cell interactions, genes, and genetic cascades for larval skeletogenesis have been investigated extensively (Okazaki 1960;Ettensohn and Malinda 1993;Guss and Ettensohn 1997;Zhu et al. 2001;Wilt 2002;Livingston et al. 2006;Ro¨ttinger et al. 2008). In addition, as representatives of free-spawning marine animals, sea urchin larvae have contributed to the ecological study of dispersal ability, mortality, behavior and locomotion during dispersal, and mechanisms and rates of suspension feeding (McEdward 1995;Lamare and Barker 1999;Hart 2002). The wide range of morphological variation in the larval skeleton is ideal for investigating evolutionary history from both an ecological standpoint (why the variation has been generated) and a developmental standpoint (how the variation is produced). ...
The larval skeletons of sea urchins show considerable morphological diversity, even between closely related species, although the evolutionary history and functional significance of this diversity are poorly understood. To infer the evolutionary history of the skeletal morphology, we focused on echinometrid species for which the morphological variation in larval skeletons had been investigated qualitatively and quantitatively. We reconstructed the phylogenetic relationships among 14 echinometrid species based on mitochondrial ND1 and ND2 genes and mapped the morphological characters onto the resultant trees. The monophyly of each genus in the Echinometridae was well supported by our results, as was the close affinity between Colobocentrotus, Heterocentrotus, and Echinometra. The mapping of the morphological characters of the larval skeletons indicated that the length, direction, and density of spines on the postoral rods was well conserved in each group of Echinometridae and that the abundance of spines and the size and shape of the body skeleton changed relatively frequently and hence were less conserved. In Echinometrid species, morphological variation in relatively unconserved features tends to be associated with latitudinal distributions, rather than phylogenetic relationships, indicating that the morphological diversity of larval skeletons could have been caused by adaptation to the habitat environment. Some morphological differences, however, seem to be nonfunctional and generated by the constraints on larval skeletogenesis. Thus, echinometrid species can be a good model with which to study the evolutionary history from both ecological and developmental standpoints.
... The effect of reproductive mode on relative rates of non-synonymous substitutions between species observed inTable 1 is probably due to reduced N e in brooding lineages, a result consistent with previous studies on x B values in three gastropod genera (Hydrobia, Crepidula and Littorina: Foltz, 2003). Population genetic subdivision in marine invertebrate species generally increases as the length of the pelagic larval period decreases (reviews: Bohonak, 1999; Hart, 2002; Kinlan & Gaines, 2003). An analysis of genetic subdivision for allozyme genes in the genera included in Table 1 is complicated by differences in the loci examined and in the geographic scale of sampling. ...
When most amino acid substitutions in protein-coding genes are slightly deleterious rather than selectively neutral, life history differences can potentially modify the effective population size or the selective regime, resulting in altered ratios of non-synonymous to synonymous substitutions among taxa. We studied substitution patterns for the mitochondrial cytochrome oxidase subunit I (COI) gene in a sea star genus (Leptasterias spp.) with an obligate brood-protecting mode of reproduction and small-scale population genetic subdivision, and compared the results to available COI sequences in nine other genera of echinoderms with pelagic larvae: three sea stars, five sea urchins and one brittle star. We predicted that this life history difference would be associated with differences in the ratio of non-synonymous (d
N) to synonymous (d
S) substitution rates. Leptasterias had a significantly greater d
N/d
S ratio (both between species and within species), a significantly smaller transition/transversion rate ratio, and a significantly lower average nucleotide diversity within species, than did the non-brooding genera. Other explanations for the results, such as altered mutation rates or selective sweeps, were not supported by the data analysis. These findings highlight the potential influence of reproductive traits and other life history factors on patterns of nucleotide substitution within and between species.
... Echinoderms are the only organisms that display a pentaradial symmetry of their body organization and their internal organs. This pentaradial arrangement develops ontogenetically from a bilaterally symmetrical larva through well-known growth and differentiation processes (David and Mooi, 1998;Hart, 2002;McCain and McClay, 1994;Morris, 1999;Wray, 1997). The evolutionary origin of the pentaradial organization, however, is less well understood. ...
The early evolutionary history of echinoderms was reconstructed on the basis of structural-functional considerations and application of the quasi-engineering approach of 'Konstruktions-Morphologie'. According to the presented evolutionary scenario, a bilaterally symmetrical ancestor, such as an enteropneust-like organism, became gradually modified into a pentaradial echinoderm by passing through an intermediate pterobranch-like stage. The arms of a pentaradial echinoderm are identified as hydraulic outgrowths from the central coelomic cavity of the bilateral ancestor which developed due to a shortening of the body in length but widening in the diameter. The resulting pentaradial symmetry is a consequence of mechanical laws that dictate minimal contact surface areas among hydraulic pneumatic entities. These developed in the coelomic cavity (metacoel) in the bilaterally symmetrical ancestor, when from the already U-shaped mesentery with the intestinal tract two additional U-shaped bows developed directly or subsequently. During the subsequent development tensile chords of the mesentery 'sewed' the gut with the body wall first in three and secondly in five 'seams'. During the direct development five 'seams' between tensile chords and body wall developed straightly. These internal tensile chords subdivide the body coelom into five hydraulic subsystems ('pneus'), which eventually arrange in a pentaradial pattern. The body could then enlarge only between the tensile chords, which means that five hydraulic bulges developed. These bulges initially supported the tentacles and finally each of them enclosed the tentacle until only the feather-like appendages of the tentacles projected over the surface. The tentacles with their feathers were transformed into the ambulacral system, and the bulges become the arms. These morphological transformations were accompanied and partly determined by specific histological modifications, such as the development of mutable connective tissues and skeletal elements that fused to ossicles and provided shape stabilization in form of a calcareous skeleton in the body wall. The organism resulted was an ancestral echinoderm ('Ur-Echinoderm') with an enlarged metacoel, stabilized by hydraulic pressure working against a capsule of mutable connective tissue, skeletal elements and longitudinal muscles. In regard to these reconstructions, the body structure of echinoderms can be understood as a hydraulic skeletal capsule.
... Nonfeeding larvae have evolved many times independently from feeding larvae in several marine invertebrate groups such as annelids (i.e., Pernet 2003) and echinoids (sand dollars, sea urchins, and sea biscuits) (i.e., Strathmann 1985; Wray 1995). Ecological considerations such as trade offs between egg size and egg number, differences in survival in the plankton, limitations to dispersal, as well as differences in juvenile growth and mortality have been discussed as important factors shaping the evolution of nonfeeding development (reviewed in Hart 2002). Moreover, dramatic differences in morphogenesis between closely related species developing from small and large eggs have been identified (Raff and Wray 1989; Wray and Raff 1991a, b; Raff et al. 1999). ...
Critical roles of hormones in metamorphic life history transitions are well documented in amphibians, lampreys, insects, and many plant species. Recent evidence suggests that thyroid hormones (TH) or TH-like compounds can regulate development to metamorphosis in echinoids (sea urchins, sand dollars, and their relatives). Moreover, previous research has provided evidence for endogenous hormone synthesis in both feeding and nonfeeding echinoderm larvae. However, the mechanisms for endogenous synthesis remain largely unknown. Here, we show that facultatively planktotrophic larvae (larvae that reach metamorphosis in the absence of food but have the ability to feed) from the subtropical sea biscuit Clypeaster rosaceus can synthesize thyroxine endogenously from incorporated iodine (I(125)). When treated with the goitrogen thiourea (a peroxidase inhibitor), iodine incorporation, thyroxine synthesis, and metamorphosis are all blocked in a dose-dependent manner. The inhibitory effect on metamorphosis can be rescued by administration of exogenous thyroxine. Finally, we demonstrate that thiourea induces morphological changes in feeding structures comparable to the phenotypic plastic response of larval structures to low food conditions, further supporting a signaling role of thyroxine in regulating larval morphogenesis and phenotypic plasticity. We conclude that upregulation of endogenous hormone synthesis might have been associated with the evolution of nonfeeding development, subsequently leading to morphological changes characteristic of nonfeeding development.
... Recent eleutherozoan echinoderms show a pentaradial symmetry of their body shape and of their internal organ systems. This symmetric arrangement develops ontogenetically by a specific growth process that is well known from embryological investigations (David & Mooi 1998;Hart 2002;McCain & McClay 1994;Morris 1999;Wray 1997). However, the evolutionary origin of the pentaradial organisation is almost unknown. ...
When echinoderms are conceptualized as hydraulic entities, the early evolution of this group can be presented in a scenario which describes how a bilateral ancestor (an enteropneust-like organism) gradually evolved into a pentaradial echinoderm. According to this scenario, the arms are outgrowths from the anterior/posterior body axis of the bilateral pterobranchia-like intermediate. These outgrowths developed when the originally U-shaped mesentery of the intestinal tract formed loops, and correspondingly, the tensile chords of the mesentery were attached to the body wall in five loops. The wall faces between these regions of tensile chords could bulge out under the hydraulic pressure of the body coelom. The originally more or less round body cavity was deformed into a pneu with five bulges. The loops of the gut forced a roughly symmetric arrangement, which was enhanced by a physical fact: five pneus as well as one pneu with five internal tethers, naturally adopt a pentaradial pattern of "minimum contact surfaces", as the most economic arrangement. These evolutionary transformations were accompanied by certain histological modifications, such as the development of mutable connective tissues and skeletal elements that fused to ossicles and provided shape stabilization in the form of a calcareous skeleton in the tissues of the body wall. The resultant organism was an ancestral eleutherozoan echinoderm (Ur-Echinoderm).
Chromatin configuration is highly dynamic during embryonic development in animals, exerting an important point of control in transcriptional regulation. Yet there exists remarkably little information about the role of evolutionary changes in chromatin configuration to the evolution of gene expression and organismal traits. Genome-wide assays of chromatin configuration, coupled with whole-genome alignments, can help address this gap in knowledge in several ways. In this study we present a comparative analysis of regulatory element sequences and accessibility throughout embryogenesis in three sea urchin species with divergent life histories: a lecithotroph Heliocidaris erythrogramma, a closely related planktotroph H. tuberculata, and a distantly related planktotroph Lytechinus variegatus. We identified distinct epigenetic and mutational signatures of evolutionary modifications to the function of putative cis-regulatory elements in H. erythrogramma that have accumulated non-uniformly throughout the genome, suggesting selection, rather than drift, underlies many modifications associated with the derived life history. Specifically, regulatory elements composing the sea urchin developmental gene regulatory network are enriched for signatures of positive selection and accessibility changes which may function to alter binding affinity and access of developmental transcription factors to these sites. Furthermore, regulatory element changes often correlate with divergent expression patterns of genes involved in cell type specification, morphogenesis, and development of other derived traits, suggesting these evolutionary modifications have been consequential for phenotypic evolution in H. erythrogramma. Collectively, our results demonstrate that selective pressures imposed by changes in developmental life history rapidly reshape the cis-regulatory landscape of core developmental genes to generate novel traits and embryonic programs.
Microbiomes are transmitted between generations by a variety of different vertical and/or horizontal modes, including vegetative reproduction (vertical), via female germ cells (vertical), coprophagy and regurgitation (vertical and horizontal), physical contact starting at birth (vertical and horizontal), breast-feeding (vertical), and via the environment (horizontal). Analyses of vertical transmission can result in false negatives (failure to detect rare microbes) and false positives (strain variants). In humans, offspring receive most of their initial gut microbiota vertically from mothers during birth, via breast-feeding and close contact. Horizontal transmission is common in marine organisms and involves selectivity in determining which environmental microbes can colonize the organism’s microbiome. The following arguments are put forth concerning accurate microbial transmission: First, the transmission may be of functions, not necessarily of species; second, horizontal transmission may be as accurate as vertical transmission; third, detection techniques may fail to detect rare microbes; lastly, microbiomes develop and reach maturity with their hosts. In spite of the great variation in means of transmission discussed in this paper, microbiomes and their functions are transferred from one generation of holobionts to the next with fidelity. This provides a strong basis for each holobiont to be considered a unique biological entity and a level of selection in evolution, largely maintaining the uniqueness of the entity and conserving the species from one generation to the next.
To consider holobionts with their hologenomes as levels of selection in evolution, there must be a continuity of partnerships between holobiont throughout generations. Microbiotas are transmitted from parent to offspring by a variety of methods, including cytoplasmic inheritance, via eggs and seeds, coprophagy (consumption of feces), close contact during and after birth, via insect vectors, and via the environment. Vertical transmission is defined as the movement of microbiota from parent to offspring without mixing with microbes in the environment. In humans, colonization of the newborn gut occurs initially via inoculation with maternal vaginal and fecal microbes when the baby transits the birth channel (vertical transmission). Breastfeeding provides an additional route of maternal vertical microbial transmission. Individuals can acquire and transfer symbionts throughout their lives, and not just during their reproductive phase. Using animal systems, including humans, it has been shown that a large fraction of bacterial species and specific strains in the microbiome are transmitted to offspring over short and long periods. It has been suggested that transmission of mutualistic symbionts and group living (animal sociality) coevolved. The large varieties in modes of transmission have an interesting implication: The fidelity of transmission of the microbiome lends a strong basis for each holobiont to be a unique biological entity, largely maintaining the uniqueness of the entity and conserving the species, from one generation to the next.
The marine environment around Japan experienced significant changes during the Cenozoic Era. In this study, we report findings suggesting that this dynamic history left behind traces in the genome of the Japanese sand dollar species Peronella japonica and P. rubra. Although mitochondrial Cytochrome C Oxidase I sequences did not indicate fragmentation of the current local populations of P. japonica around Japan, two different types of intron sequence were found in the Alx1 locus. We inferred that past fragmentation of the populations account for the presence of two types of nuclear sequences as alleles in the Alx1 intron of P. japonica. It is likely that the split populations have intermixed in recent times; hence, we did not detect polymorphisms in the sequences reflecting the current localization of the species. In addition, we found two allelic sequences of theAlx1 intron in the sister species P. rubra. The divergence times of the two types of Alx1 intron sequences were estimated at approximately 14.9 and 4.0 million years ago for P. japonica and P. rubra, respectively. Our study indicates that information from the intron sequences of nuclear genes can enhance our understanding of past genetic events in organisms.
In Rarotonga, Linckia multifora (Lamarck) exists in two forms: a blue gray type that is found on the reef intertidally and a red form that is found subtidally. Both types reproduce asexually by regeneration of autotomized arms, as well as sexually, but the relative potential for sexual reproduction varies greatly between these different sites. In the laboratory, reciprocal crosses of the blue gray intertidal form and the red subtidal form developed as successfully as the controls and were indistinguishable in morphology. In addition, both the blue gray intertidal form and the red subtidal form contain two different classes of haplotypes of the mitochondrial gene cytochrome oxidase subunit I (COI), which exhibit 12 fixed differences. These results suggest that L. multifora of Rarotonga has a dual origin and that the two different forms seen in the two environments belong to a single interbreeding population and may represent ecophenotypes.
In Rarotonga, Linckia multifora (Lamarck) exists in two forms: a blue gray type that is found on the reef intertidally and a red form that is found sub-tidally. Both types reproduce asexually by regeneration of autotomized arms, as well as sexually, but the relative potential for sexual reproduction varies greatly between these different sites. In the laboratory, reciprocal crosses of the blue gray intertidal form and the red subtidal form developed as successfully as the controls and were indistinguishable in morphology. In addition, both the blue gray intertidal form and the red subtidal form contain two different classes of haplotypes of the mitochondrial gene cytochrome oxidase subunit I (COI), which exhibit 12 fixed differences. These results suggest that L. multifora of Rarotonga has a dual origin and that the two different forms seen in the two environments belong to a single interbreeding population and may represent ecophenotypes. Linckia multifora (Lamarck) is a small sea star, a member of the Ophidiasteridae, that is found on coral reefs throughout the Indo-Pacific Ocean (Clark and Rowe 1971). It generally has arms of uneven lengths be-cause it reproduces asexually by autotomizing arms, which can then regenerate into whole starfish (Edmondson 1935). It is identified by the arrangement of spines along the am-bulacral grooves, the presence of two madre-porites, the arrangement of papular openings, the granularity of the epidermis, and the gen-eral shape of the arms (Clark and Rowe 1971). On the island of Rarotonga, Cook Islands, South Pacific, there appear to be two differ-ent types of L. multifora: a red-spotted form that is found in the subtidal zone and a gray blue form that is found on the top of the reef in the intertidal zone. To confirm that these are both L. multifora and to explore the relationship between them we have sequenced a mitochondria gene, cytochrome oxidase subunit I (COI). To understand aspects of the ecology and evolutionary biology of an organism, it is im-portant to understand its mechanism(s) of re-production (Hart 2002). Linckia multifora is generally considered to reproduce primarily by asexual means (Rideout 1978), and it has been referred to in the literature as an asexual organism (Ebert 1996). However, it produces eggs and sperm (Mortensen 1938, Rideout 1978), and viable cultures of embryos have been produced in the laboratory (Mortensen 1938). In this study we set up breeding ex-periments with both types of L. multifora to see if viable embryos would result from both types and from both crosses. In addition, we studied the population size structure at sev-eral different sites to determine whether sex-ual as well as asexual reproduction occurs in nature and to determine whether the relative importance of larval recruitment varies be-tween different sites.
Ceranka, T. 2007. Symmetry disorders of the test of the Miocene echinoid Echinocyamus from Poland. Acta Palaeonto− logica Polonica 52 (3): 503–518. This paper presents cases of disorders in the test symmetry in three species of Echinocyamus (E. pusillus, E. calariensis, and E. apicatus) from the Miocene deposits of Poland. It is the first study of this topic based on fossil material. The large collection (ca. 60 000 specimens) allowed distinction of several types of these rare disorders (14 specimens), which are il− lustrated by SEM and explanatory drawings. An example of a deformity formed on the pentamerous system is also pre− sented. The deformations develop in the rudiment; consequently the apical system and the test of the mature individual which is formed at its margin display disorders. The anomalies most commonly appear as an additional growth zone (6−ray symmetry), lack of one growth zone (4−ray symmetry), or both deformations simultaneously (changed 5−ray sym− metry). The changes appear in the apical system (number of ocular pores), test (number of petals), and peristome outline. The anomaly linked with the incorrect position of the periproct on the test surface, resulting from the delay of its displace− ment during early ontogenic stages, is characteristic and unique for such deformations.
Convergence is a significant evolutionary phenomenon. Arrival at similar morphologies from different starting points indicates a strong role for natural selection in shaping morphological phenotypes. There is no evidence yet of convergence in the developmental mechanisms that underlie the evolution of convergent developmental phenotypes. Here we report the expression domains in sea urchins of two important developmental regulatory genes ( Orthodenticle and Runt), and show evidence of molecular convergence in the evolution of direct-developing sea urchins. Indirect development is ancestral in sea urchins. Evolutionary loss of the feeding pluteus stage and precocious formation of the radially symmetric juvenile has evolved independently in numerous sea urchin lineages, thus direct development is an evolutionary convergence. Indirect-developing species do not express Otx during the formation of their five primordial tube feet, the ancestral condition. However, each direct-developing urchin examined does express Otx in the tube feet. Otx expression in the radial arms of direct-developing sea urchins is thus convergent, and may indicate a specific need for Otx use in direct development, a constraint that would make direct development less able to evolve than if there were multiple molecular means for it to evolve. In contrast, Runt is expressed in tube feet in both direct- and indirect-developing species. Because echinoderms are closely related to chordates and postdate the protostome/deuterostome divergence, they must have evolved from bilaterally symmetrical ancestors. Arthropods and chordates use Otx in patterning their anterior axis, and Runt has multiple roles including embryonic patterning in arthropods, and blood and bone cell differentiation in vertebrates. Runt has apparently been co-opted in echinoderms for patterning of pentamery, and Otx in pentameral patterning among direct-developing echinoids. The surprisingly dynamic nature of Otx evolution reinvigorates debate on the role of natural selection vs shared ancestry in the evolution of novel features.
Cooption and modularity are informative concepts in evolutionary developmental biology. Genes function within complex networks that act as modules in development. These modules can then be coopted in various functional and evolutionary contexts. Hormonal signaling, the main focus of this review, has a modular character. By regulating the activities of genes, proteins and other cellular molecules, a hormonal signal can have major effects on physiological and ontogenetic processes within and across tissues over a wide spatial and temporal scale. Because of this property, we argue that hormones are frequently involved in the coordination of life history transitions (LHTs) and their evolution (LHE). Finally, we promote the usefulness of a comparative, non-model system approach towards understanding how hormones function and guide development and evolution, highlighting thyroid hormone function in echinoids as an example.
Comprehending the origin of marine invertebrate larvae remains a key domain of research for evolutionary biologists, including the repeated origin of direct developmental modes in echinoids. In order to address the latter question, we surveyed existing evidence on relationships of homology between the ectoderm territories of two closely related sea urchin species in the genus Heliocidaris that differ in their developmental mode. Additionally, we explored a recently articulated idea about homology called 'organizational homology' (Müller 2003. In: Müller GB, Newman SA, editors. Origination of organismal form: beyond the gene in developmental and evolutionary biology. Cambridge, MA: A Bradford Book, The MIT Press. p 51-69. ) in the context of this specific empirical case study. Applying the perspective of organizational homology to our experimental system of congeneric echinoids has led us to a new hypothesis concerning the ectoderm evolution in these species. The extravestibular ectoderm of the direct developer Heliocidaris erythrogramma is a novel developmental territory that arose as a fusion of the oral and aboral ectoderm territories found in indirect developing echinoids such as Heliocidaris tuberculata. This hypothesis instantiates a theoretical principle concerning the origin of developmental modules, 'integration', which has been neglected because the opposite theoretical principle, 'parcellation', is more readily observable in events such as gene duplication and divergence (Wagner 1996. Am Zool 36:36-43).
The larval arms of echinoid plutei are used for locomotion and feeding. They are composed of internal calcite skeletal rods covered by an ectoderm layer bearing a ciliary band. Skeletogenesis includes an autonomous molecular differentiation program in primary mesenchyme cells (PMCs), initiated when PMCs leave the vegetal plate for the blastocoel, and a patterning of the differentiated skeletal units that requires molecular cues from the overlaying ectoderm. The arms represent a larval feature that arose in the echinoid lineage during the Paleozoic and offers a subject for the study of gene co-option in the evolution of novel larval features. We isolated new molecular markers in two closely related but differently developing species, Heliocidaris tuberculata and Heliocidaris erythrogramma. We report the expression of a larval arm-associated ectoderm gene tetraspanin, as well as two new PMC markers, advillin and carbonic anhydrase. Tetraspanin localizes to the animal half of blastula stage H. tuberculata and then undergoes a restriction into the putative oral ectoderm and future location of the postoral arms, where it continues to be expressed at the leading edge of both the postoral and anterolateral arms. In H. erythrogramma, its expression initiates in the animal half of blastulae and expands over the entire ectoderm from gastrulation onward. Advillin and carbonic anhydrase are upregulated in the PMCs postgastrulation and localized to the leading edge of the growing larval arms of H. tuberculata but do not exhibit coordinated expression in H. erythrogramma larvae. The tight spatiotemporal regulation of these genes in H. tuberculata along with other ontogenetic and phylogenetic evidence suggest that pluteus arms are novel larval organs, distinguishable from the processes of skeletogenesis per se. The dissociation of expression control in H. erythrogramma suggest that coordinate gene expression in H. tuberculata evolved as part of the evolution of pluteus arms, and is not required for larval or adult development.
Observations of a sea urchin larvae show that most species adopt one of two life history strategies. One strategy is to make numerous small eggs, which develop into a larva with a required feeding period in the water column before metamorphosis. In contrast, the second strategy is to make fewer large eggs with a larva that does not feed, which reduces the time to metamorphosis and thus the time spent in the water column. The larvae associated with each strategy have distinct morphologies and developmental processes that reflect their feeding requirements, so that those that feed exhibit indirect development with a complex larva, and those that do not feed form a morphologically simplified larva and exhibit direct development. Phylogenetic studies show that, in sea urchins, a feeding larva, the pluteus, is the ancestral form and the morphologically simplified direct-developing larva is derived. The current hypothesis for evolution of the direct-developing larval form in sea urchins suggests that major developmental changes occur by neutral loss of larval features after the crucial transition to a nonfeeding life history strategy. We present evidence from Clypeaster rosaceus, a sea urchin with a life history intermediate to the two strategies, which indicates that major developmental changes for accelerated development have been selected for in a larva that can still feed and maintains an outward, pluteus morphology. We suggest that transformation of larval form has resulted from strong selection on early initiation and acceleration of adult development.
Traits from early development mapped onto phylogenetic trees can potentially offer insight into the evolutionary history of development by inferring the states of those characters among ancestors at nodes in the phylogeny. A key and often-overlooked aspect of such mapping is the underlying model of character evolution. Without a well-supported and realistic model ("nothing"), character mapping of ancestral traits onto phylogenetic trees might often return results ("something") that lack a sound basis. Here we reconsider a challenging case study in this area of evolutionary developmental biology: the inference of ancestral states for ecological and morphological characters in the reproduction and larval development of asterinid sea stars. We apply improved analytical methods to an expanded set of asterinid phylogenetic data and developmental character states. This analysis shows that the new methods might generally offer some independent insight into choice of a model of character evolution, but that in the specific case of asterinid sea stars the quantitative features of the model (especially the relative probabilities of different directions of change) have an important effect on the results. We suggest caution in applying ancestral state reconstructions in the absence of an independently corroborated model of character evolution, and highlight the need for such modeling in evolutionary developmental biology.
For more than a century, evolutionary biologists, ecologists, and oceanographers alike have been intellectually stimulated by marine invertebrate larval forms. In 1995, Ecology of Marine Invertebrate Larvae, edited by the late Dr. Larry McEdward, captured the fundamental phenomenon and tremendous diversity of reproductive, biological, and oceanographic aspects of larval ecology. Now, more than twenty years later, this current edited volume provides an update to many of the original 13 chapters, while also reviewing several braches of larval ecology and evolution that have developed since. In Evolutionary Ecology of Marine Invertebrate Larvae, authors review the origins of marine invertebrate larvae and the developmental mechanisms and ecological factors that may generate this great diversity, and how these microscopic organisms feed, develop, and behave in the pelagic environment. Whether actively swimming in the coastal seas or the deep abyss, larvae are often in motion and must settle on the seafloor; however, if delayed, they are susceptible to certain consequences in subsequent stages later in life. Now, in an age of climate change, larvae face a warmer, more acidic, and more toxic ocean than ever before. Responses to these plus many other stressors and facets of larval biology can be broadly profiled, thanks to current technological advances. This edited volume provides a major synthesis of an important interdisciplinary field, as it aims to foster stimulating discussions centered on the evolution and ecology of marine invertebrate larvae. [https://global.oup.com/academic/product/evolutionary-ecology-of-marine-invertebrate-larvae-9780198786962?facet_narrowbyproducttype_facet=Digital&lang=en&cc=no]
The pantropical sea urchin genus Eucidaris contains four currently recognized species, all of them allopatric: E. metularia in the Indo-West Pacific, E. thouarsi in the eastern Pacific, E. tribuloides in both the western and eastern Atlantic, and E. clavata at the central Atlantic islands of Ascension and St. Helena. We sequenced a 640-bp region of the cytochrome oxidase I (COI) gene of mitochondrial DNA to determine whether this division of the genus into species was confirmed by molecular markers, to ascertain their phylogenetic relations, and to reconstruct the history of possible dispersal and vicariance events that led to present-day patterns of species distribution. We found that E. metularia split first from the rest of the extant species of the genus. If COI divergence is calibrated by the emergence of the Isthmus of Panama, the estimated date of the separation of the Indo-West Pacific species is 4.7-6.4 million years ago. This date suggests that the last available route of genetic contact between the Indo-Pacific and the rest of the tropics was from west to east through the Eastern Pacific Barrier, rather than through the Tethyan Sea or around the southern tip of Africa. The second cladogenic event was the separation of eastern Pacific and Atlantic populations by the Isthmus of Panama. Eucidaris at the outer eastern Pacific islands (Galapagos, Isla del Coco, Clipperton Atoll) belong to a separate clade, so distinct from mainland E. thouarsi as to suggest that this is a different species, for which the name E. galapagensis is revived from the older taxonomic literature. Complete lack of shared alleles in three allozyme loci between island and mainland populations support their separate specific status. Eucidaris galapagensis and E. thouarsi are estimated from their COI divergence to have split at about the same time that E. thouarsi and E. tribuloides were being separated by the Isthmus of Panama. Even though currents could easily convey larvae between the eastern Pacific islands and the American mainland, the two species do not appear to have invaded each other's ranges. Conversely, the central Atlantic E. clavata at St. Helena and Ascension is genetically similar to E. tribuloides from the American and African coasts. Populations on these islands are either genetically connected to the coasts of the Atlantic or have been colonized by extant mitochondrial DNA lineages of Eucidaris within the last 200,000 years. Although it is hard to explain how larvae can cross the entire width of the Atlantic within their competent lifetimes, COI sequences of Eucidaris from the west coast of Africa are very similar to those of E. tribuloides from the Caribbean. FST statistics indicate that gene flow between E. metularia from the Indian Ocean and from the western and central Pacific is restricted. Low gene flow is also evident between populations of E. clavata from Ascension and St. Helena. Rates of intraspecific exchange of genes in E. thouarsi, E. galapagensis, and E. tribuloides, on the other hand, are high. The phylogeny of Eucidaris confirms Ernst Mayr's conclusions that major barriers to the dispersal of tropical echinoids have been the wide stretch of deep water between central and eastern Pacific, the cold water off the southwest coast of Africa, and the Isthmus of Panama. It also suggests that a colonization event in the eastern Pacific has led to speciation between mainland and island populations.
The diminutive asterinid sea stars Patiriella vivipara and P. parvivipara incubate their embryos in the gonads to the advanced juvenile stage. Despite the small size of their eggs (135-150 μm diameter), development is lecithotrophic. Development proceeds through the wrinkled blastula, gastrula, and brachiolaria larval stages. The gastrulae and larvae are uniformly ciliated and swim, propelled by the cilia, in the gonadal fluid. The brachiolaria is pear-shaped and has a vestigial brachiolar attachment complex composed of three small brachia. At no stage in development are the embryos attached to the gonad. Metamorphosis occurs as the larvae swim in the gonadal lumen. Internal development involves formation of one large enterocoel at the anterior end of the archenteron and one small posterior enterocoel on the left side of the archenteron. The archenteron closes to form the rudiment for the adult gut. As a result of the small size of the egg and the nonfeeding mode of development, the larvae of P. vivipara and P. parvivipara are minute, about 270 μm and 210 μm in length, respectively. Newly metamorphosed juveniles are about 240 μm and 310 μm in diameter, respectively. Post-metamorphic development involves substantial growth of the juveniles, which leave the parent at a diameter between 1.0 and 5.0 mm. The presence of a vestigial brachiolar complex and lecithotrophic development indicates that these species had a free-living lecithotrophic brachiolaria in their ancestry. We suggest that the evolution of viviparity in Patiriella sp. involved retention of a large egg by an ancestor that had a lecithotrophic brachiolaria followed by a secondary reduction in the size of the ovum and simplification of the larva. The range of life histories seen in Patiriella is atypical of asteroid genera and supports the contention that the evolution of viviparity and other modes of modified development in the Asteroidea follows phylogenetic lineages.
Normal reproductive charateristics in echinoderms include non-maternal nutritions with a planktonic feeding developmental state, sexual reproduction with gonochorism, seasonal reproduction, and no protection of young by secondary metabolites. Deviant reproductive characteristics include maternal nutrition of the developmental stage that may be planktonic, demersal, or brooded; hermaphroditic sexual reproduction; intermittent or continuous low-level reproduction; protection of young by secondary metabolites; and asexual reproduction. We tested the hypothesis that stress, which causes a decrease in capacity for production, was a factor responsible for deviant reproductive characteristics by comparing taxa from the subphylum to species levels. In the examples used, deviant reproduction occurred in the taxa for which stress could be predicted. Although other factors undoubtably affect reproductive characteristics, the analysis indicates that stress must be considered an important one.
The ‘Eastern Pacific Barrier’ (EPB), 5400 km of uninterrupted deep water between the central and eastern Pacific, constitute the greatest marine obstacle to the dispersal of shallow–water organisms. However, some species are found on both sides o the EPB. These ‘transpacific’ species are considered by ‘dispersal’ biogeographers as evidence of invasions through the barrier.
‘Vicariance’ biogeographers, on the other hand, think that transpacific species are morphologically conservative remnant of previously continuous distributions. We compared nucleotide sequences in a 642 bp region of mitochondrial DNA, and electrophoreticall detected alleles in 17 enzymatic loci of central and eastern Pacific populations of Echinothrix diadema, an Indo–Pacific sea urchin recently reported from the eastern Pacific. Both types of molecules produced clear evidence o massive, recent gene flow across the EPB. Thus, rather than being isolated relicts of Tethyan distributions, conspecific population from the eastern and central Pacific are genetically connected. Though the EPB is biogeographically important as a cause o speciation in many groups, it allows genetic connections in others, possibly through larval transport during El Niño events.
Differences in genetic composition among samples of larvae produced during a single spawning season by a semi-isolated population of Pacific oysters (Crassostrea gigas) in Dabob Bay, Washington, confirm a specific prediction of the hypothesis that this and other marine animals have large variances in reproductive success. To study the genetics of single larvae, we cloned and sequenced part of the mitochondrial genome and developed polymerase chain reaction (PCR) primers to amplify four segments totaling nearly 2300 base pairs, or 13% of the genome. PCR products were digested with restriction enzymes into smaller fragments, which were then screened for single-strand conformational polymorphisms (SSCP). Seven plankton samples (total N = 877), taken between 10 and 21 August 1993, showed a common composite PCR-SSCP haplotype that comprised from 53 to 85% of samples. Nevertheless, exact probability and permutation tests reveal that early and late samples from north Dabob Bay differed significantly from the rest. These differences cannot be ascribed to spatial variation and are consistent with the hypothesis that larvae are produced by relatively few adults, in accord with previous observations of substantial genetic drift in this large population.
Results of a first-stage Sea Urchin Genome Project are summarized here. The species chosen was Strongylocentrotus purpuratus, a research model of major importance in developmental and molecular biology. A virtual map of the genome was constructed
by sequencing the ends of 76,020 bacterial artificial chromosome (BAC) recombinants (average length, 125 kb). The BAC-end
sequence tag connectors (STCs) occur an average of 10 kb apart, and, together with restriction digest patterns recorded for
the same BAC clones, they provide immediate access to contigs of several hundred kilobases surrounding any gene of interest.
The STCs survey >5% of the genome and provide the estimate that this genome contains ≈27,350 protein-coding genes. The frequency
distribution and canonical sequences of all middle and highly repetitive sequence families in the genome were obtained from
the STCs as well. The 500-kb Hox gene complex of this species is being sequenced in its entirety. In addition, arrayed cDNA libraries of >105 clones each were constructed from every major stage of embryogenesis, several individual cell types, and adult tissues and
are available to the community. The accumulated STC data and an expanding expressed sequence tag database (at present including
>12,000 sequences) have been reported to GenBank and are accessible on public web sites.
Six species of echinoderms were observed spawning (1 holothuroid, 1 echinoid, 4 asteroids) during a single 40 min dive to 10 to 18 m depth in Barkley Sound, Vancouver Island, on the afternoon of 9 August 1987, the day after the full moon. Most of the spawning animals were not close to conspecific individuals. None were observed spawning the following day. Published reports indicate that it is not uncommon for widely scattered individuals of echinoderms to broadcast spawn in the field. Fertilization success often may be low. Moreover, simultaneous spawning by different species, as we and others have observed, indicates that the incidence of hybridization (either lethal or viable) could be high and have important consequences to the biology of the species involved.
Larvae of two species of sea urchins (Strongylocentrotus droebachiensis and S. purpuratus) differ in initial form and in the rate of development. To determine whether these differences are attributable to the large interspecific difference in egg size, we experimentally reduced egg size by isolating blastomeres from embryos. The rate of development of feeding larvae derived from isolated blastomeres was quantified using a novel morphometric method. If the differences early in the life histories of these two species are due strictly to differences in egg size, then experimental reduction of the size of S. droebachiensis eggs should yield an initial larval form and rate of development similar to that of S. purpuratus. Our experimental manipulations of egg size produced three clear results: 1) smaller eggs yielded larvae that were smaller and had simpler body forms, 2) smaller eggs resulted in slower development through the early feeding larval stages, and 3) effects of egg size were restricted to early larval stages. Larvae from experimentally reduced eggs of the larger species had rates of development similar to those of the smaller species. Thus, cytoplasmic volumes of the eggs, not genetic differences expressed during development, account for differences in larval form and the rate of form change. This is the first definitive demonstration of the causal relationship between egg size (parental investment per offspring) and life-history characteristics in marine benthic invertebrates.
Both mtDNA variation and allozyme data demonstrate that geographic groupings of different color morphs of the starfish Linckia laevigata are congruent with a genetic discontinuity between the Indian and Pacific Oceans. Populations of L. laevigata sampled from Thailand and South Africa, where an orange color morph predominates, were surveyed using seven polymorphic enzyme loci and restriction fragment analysis of a portion of the mtDNA including the control region. Both allozyme and DNA data demonstrated that these populations were significantly genetically differentiated from each other and to a greater degree from 23 populations throughout the West Pacific Ocean, where a blue color morph is predominant. The genetic structure observed in L. laevigata is consistent with traditional ideas of a biogeographic boundary between the Indian and Pacific Oceans except that populations several hundreds kilometers off the coast of north Western Australia (Indian Ocean) were genetically similar to and had the same color morphs as Pacific populations. It is suggested that gene flow may have continued (possibly at a reduced rate) between these offshore reefs in Western Australia and the West Pacific during Pleistocene falls in sea level, but at the same time gene flow was restricted between these Western Australian populations and those in both Thailand and South Africa, possibly by upwellings. The molecular data in this study suggest that vicariant events have played an important role in shaping the broadscale genetic structure of L. laevigata. Additionally, greater genetic structure was observed among Indian Ocean populations than among Pacific Ocean populations, probably because there are fewer reefs and island archipelagos in the Indian Ocean than in the Pacific, and because present-day surface ocean currents do not facilitate long-distance dispersal.
The Pacific marine biota, particularly species with long planktonic larval stages, are thought to disperse widely throughout the Pacific via ocean currents. The little genetic data available to date has supported this view in that little or no significant regional differentiation of populations has been found over large geographical distances. However, recent data from giant clams has demonstrated not only significant regional differentiation of populations, but routes of gene flow that run perpendicular to the main present-day ocean currents. Extensive surveys of genetic variation at eight polymorphic loci in 19 populations of the giant clam Tridacna maxima, sampled throughout the West and Central Pacific, confirmed that the patterns of variation seen so far in T. gigas were not unique to that species, and may reflect a fundamental genetic structuring of shallow-water marine taxa. Populations of T. maxima within highly connected reef systems like the Great Barrier Reef were panmictic (average FST < 0.003), but highly significant genetic differences between reef groups on different archipelagos (average FST = 0.084) and between West and Central Pacific regions (average FST = 0.156) were found. Inferred gene flow was high (Ne m usually > 5) between the Philippines and the Great Barrier Reef, between the Philippines and Melanesia (the Solomon Islands and Fiji), and between the Philippines and the Central Pacific island groups (Marshall Islands, Kiribati, Tuvalu and Cook Islands). Gene flow was low between these three sets of island chains (Ne m < 2). These routes of gene flow are perpendicular to present-day ocean currents. It is suggested that the spatial patterns of gene frequencies reflect past episodes of dispersal at times of lower sea levels which have not been erased by subsequent dispersal by present-day circulation. The patterns are consistent with extensive dispersal of marine species in the Pacific, and with traditional views of dispersal from the Indo-Malay region. However, they demonstrate that dispersal along present-day ocean surface currents cannot be assumed, that other mechanisms may operate today or that major dispersal events are intermittent (perhaps separated by several thousands of years), and that the nature and timing of dispersal of Pacific marine species is more complex than has been thought.
Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene flow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in taxa with high dispersal potential. To understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene flow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects. These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene flow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.
Within the sea urchin genus Heliocidaris, changes in early embryonic and larval development have resulted in dramatic differences in the length of time larvae spend in the plankton before settling. The larvae of one species, H. tuberculata, spend several weeks feeding in the plankton before settling and metamorphosing into juveniles. The other species, H. erythrogramma, has modified this extended planktonic larval stage and develops into a juvenile within 3-4 days after fertilization. We used restriction site polymorphisms in mitochondrial DNA to examine the population genetic consequences of these developmental changes. Ten restriction enzymes were used to assay the mitochondrial genome of 29 individuals from 2 localities for H. tuberculata and 62 individuals from 5 localities for H. erythrogramma. Within H. tuberculata, 11 mitochondrial genotypes were identified. A GST analysis showed high levels of genetic exchange between populations separated by 1,000 kilometers of open ocean. In contrast, in H. erythrogramma, 13 mitochondrial genotypes differing by up to 2.33% were geographically partitioned over spatial scales ranging from 800 to 3,400 kilometers. Between distant localities, there was complete mitochondrial lineage sorting and large sequence divergence between resulting clades. Over much smaller spatial scales (< 1,000 km), genetic differentiation was due to the differential sorting of very similar genotypes. This pattern of mitochondrial variation suggests that these population differences have arisen recently and may reflect the historical interplay between the restricted dispersal capabilities of H. erythrogramma and the climatic and geological changes associated with Pleistocene Ice Ages.
We analyzed phylogenetic relationships among 12 nominal species of starfish in the genera Patiriella and Asterina (Order Valvatida, Family Asterinidae), based on complete sequences for a mitochondrial protein coding gene (cytochrome oxidase subunit I) and five mitochondrial transfer RNA genes (alanine, leucine, asparagine, glutamine, and proline) (1923 bp total). The resulting phylogeny was used to test a series of hypotheses about the evolution of life-history traits. (1) A complex, feeding, planktonic larva is probably ancestral for these starfish, but this is not the most parsimonious reconstruction of ancestral larval states. (2) The feeding larval form was lost at least four times among these species, and three of these losses occurred among members of a single clade. (3) Small adult size evolved before both cases of hermaphroditism and viviparous brooding, but viviparity was not always preceded by an intermediate form of external brooding. (4) An ordered transformation series from feeding planktonic development to viviparous brooding has been predicted for starfish, but we could not find an example of this transformation series. (5) Viviparity evolved recently (< 2 Mya). (6) Both species selection and transformation of lineages may have contributed to the accumulation of species with nonfeeding development among these starfish. (7) Neither Asterina nor Patiriella are monophyletic genera. Larval forms and life-history traits of these starfish have evolved freely under no obvious constraints, contrary to the widely assumed evolutionary conservatism of early development.
This study documents evolutionary modifications in mechanisms of gastrulation in Heliocidaris erythrogramma, an echinoid with lecithotrophic larvae. Radially symmetrical cell rearrangements and changes in cell shape drive elongation of the archenteron in the ancestral mode of echinoid gastrulation. Cell marking experiments indicate that in H. erythrogramma, however, prolonged movement of cells over the ventral lip of the blastopore accompanies extension of the archenteron. Evolutionary modifications to archenteron extension in H. erythrogramma thus include utilization of a different type of cellular movement as well as the imposition of dorsoventral asymmetry in cellular movements. The conservation of gastrulation mechanisms among phylogenetically divergent echinoids with planktotrophic development suggests that the plesiomorphic condition has persisted at least 250 million years and perhaps much longer. Yet H. erythrogramma diverged from an ancestor with planktotrophic development only about 10 mya, indicating that morphogenetic mechanisms of early development can undergo substantial evolutionary changes, even after long periods of stasis.
Spatial variation in allelic frequencies at nine allozyme loci were assayed in 20 populations of the crown-of-thorns starfish, Acanthaster planci, collected throughout the Pacific and Indian Oceans. These data were analyzed together with published data, for the same loci, from an additional 19 populations, giving a total sample size of approximately 1800 individuals. There was a marked discontinuity between the Indian and Pacific Ocean populations, but those off Western Australia and from the Southeast Asian region had a strong Pacific affinity. The genetic groups were congruent with the distributions of two color morph groups: gray-green to red-brown forms in the Pacific and a blue to pale red form in the Indian Ocean. These patterns of genetic structure are similar to those described for the starfish Linckia laevigata, which has similar life-history characteristics. Vicariant events may have influenced some populations within the Pacific, but the allozyme data cannot resolve the effects of these events clearly. Patterns of variation within regions were consistent with isolation by distance, but, at larger scales, were obscured by regional vicariance and some outliers, particularly by apparently high levels of gene flow between Japan and the Great Barrier Reef, Australia. Apparent gene flow between population pairs was not closely related to present-day ocean currents. The results demonstrate a strong influence of allopatric separation on genetic divergence at large geographic scales, but also show evidence of slow rates of change in gene frequencies consistent with the large population sizes of this species. Low levels of divergence between groups demonstrate the genetic structure is recent (Pleistocene) and are likely responses to changes in climate and sea level.
MtDNAs from 2 protein coding regions comprising 576 base pairs were sequenced from 17 individual sea urchins of the species Strongylocentrotus pallidus collected from the north Pacific and north Atlantic oceans. Twelve of 17 individual sequences were identical. Two of these were further sequenced in a third, 441 base pair region, and were also found to be identical. We show how to interpret these results using a simplified Markov model of mtDNA evolution at silent sites. The model was calibrated using 3 urchin species with a published fossil record, and shows that identical S. pallidus mtDNAs in different oceans shared a common ancestor at most 90,000-150,000 years ago (95% confidence interval of upper limit of divergence). The Markov model, used to examine patterns of genetic distance within and between species, shows unexpected variation in the rate of base substitution. The rate of change of G's at fourfold sites is nearly 20 times greater than the rate of change of C's. At twofold sites, this range is less extreme, although purines consistently have a higher rate of change than pyrimidines. Striking genetic similarity and recent genetic exchange between oceans for these urchins is in marked contrast to most other trans-Arctic marine populations, which usually show morphological and genetic differentiation at the species or subspecies level. Recent fossil evidence shows that the north Atlantic and northeastern Pacific have been the scene of radical faunal changes during the Pliocene and Pleistocene. The genetic results presented here extend this conclusion to intraspecific patterns of genetic variability, and direct attention to the northwest Pacific where higher productivity and less environmental change may have left a heritage of greater marine genetic diversity.
Restriction-fragment analysis was used to measure mitochondrial DNA (mtDNA) variability in 79 individuals of two species of temperate sea urchins. For the purple urchin Strongylocentrotus purpuratus, individuals were collected 1,500 km apart in 1985 and again from the same localities in 1988 (about one urchin generation). Twenty mtDNA genotypes belonging to four clades were found among 38 individuals. All four clades were found in both localities and in both years. Genetic structure was further tested by calculating the degree of interdeme genetic variation (GST ) and comparing this value to the GST 's from randomly shuffled data. No geographic structure was found. For S. droebachiensis, only six mtDNA genotypes were found among 41 individuals collected from the Pacific and Atlantic oceans. More than 80% of the individuals belonged to two genotypes. The genotype that dominated collections in the Pacific also occurred in the Atlantic; however, a common Atlantic genotype was never found in the Pacific. These two genotypes were identical at 64 of 65 restriction sites, and were only 0.2% divergent from each other. GST analysis confirmed that there were significant genetic differences between Atlantic and Pacific populations. The small divergence between genotypes suggests recent, but not continuous, migration. These marine species show smaller genotypic differences than terrestrial species over similar spatial and temporal scales. Both recruitment of adults from planktonic larval pools and the spread of sibling larvae over large distances from parents probably act as buffers to genetic differences in species with planktonic life-history phases.
In this study, we integrate information from phylogeny, comparative ontogeny, and experimental embryology in an attempt to elucidate the mechanisms controlling evolutionary trends towards digital reduction and loss observed in amphibians. Frogs and salamanders that have lost phalanges and even whole toes have done so in a very ordered manner, i.e., certain skeletal elements are lost prior to others. This pattern of morphological diversity is described and trends elucidated. It is concluded that the process is characterized by striking intraordinal convergences coupled with substantial differences between the trends observed in frogs as compared to urodeles. We argue that this pattern is essentially a reflection of the differences in the ontogenies of the two orders. Similarly, the convergences within urodeles and within anurans can be explained as the result of regulation of developmental parameters in a resilient developmental program. We further explore this hypothesis by experimentally perturbing the number of cells in the embryonic limb primordium to show that reduction in the number of mesenchymal cells secondarily affects the developmental process of pattern formation causing a rearrangement of the skeletal morphology of the foot. The same experimental manipulation has different effects in frogs as compared to salamanders. However, in both cases, the experimentally generated morphologies tend to parallel the phenotypes and trends observed in nature. Our conclusion is that most of the patterns of diversity in the digital morphology of amphibians can be explained as a reflection of developmental properties. In general, we present a methodology that attempts to empirically address the issue of identifying developmental constraint in morphological evolution.
The pantropical sea urchin genus Eucidaris contains four currently recognized species, all of them allopatric: E. metularia in the Indo-West Pacific, E. thouarsi in the eastern Pacific, E. tribuloides in both the western and eastern Atlantic, and E. clavata at the central Atlantic islands of Ascension and St. Helena. We sequenced a 640-bp region of the cytochrome oxidase I (COI) gene of mitochondrial DNA to determine whether this division of the genus into species was confirmed by molecular markers, to ascertain their phylogenetic relations, and to reconstruct the history of possible dispersal and vicariance events that led to present-day patterns of species distribution. We found that E. metularia split first from the rest of the extant species of the genus. If COI divergence is calibrated by the emergence of the Isthmus of Panama, the estimated date of the separation of the Indo-West Pacific species is 4.7-6.4 million years ago. This date suggests that the last available route of genetic contact between the Indo-Pacific and the rest of the tropics was from west to east through the Eastern Pacific Barrier, rather than through the Tethyan Sea or around the southern tip of Africa. The second cladogenic event was the separation of eastern Pacific and Atlantic populations by the Isthmus of Panama. Eucidaris at the outer eastern Pacific islands (Galapagos, Isla del Coco, Clipperton Atoll) belong to a separate clade, so distinct from mainland E. thouarsi as to suggest that this is a different species, for which the name E. galapagensis is revived from the older taxonomic literature. Complete lack of shared alleles in three allozyme loci between island and mainland populations support their separate specific status. Eucidaris galapagensis and E. thouarsi are estimated from their COI divergence to have split at about the same time that E. thouarsi and E. tribuloides were being separated by the Isthmus of Panama. Even though currents could easily convey larvae between the eastern Pacific islands and the American mainland, the two species do not appear to have invaded each other's ranges. Conversely, the central Atlantic E. clavata at St. Helena and Ascension is genetically similar to E. tribuloides from the American and African coasts. Populations on these islands are either genetically connected to the coasts of the Atlantic or have been colonized by extant mitochondrial DNA lineages of Eucidaris within the last 200,000 years. Although it is hard to explain how larvae can cross the entire width of the Atlantic within their competent lifetimes, COI sequences of Eucidaris from the west coast of Africa are very similar to those of E. tribuloides from the Caribbean. FST statistics indicate that gene flow between E. metularia from the Indian Ocean and from the western and central Pacific is restricted. Low gene flow is also evident between populations of E. clavata from Ascension and St. Helena. Rates of intraspecific exchange of genes in E. thouarsi, E. galapagensis, and E. tribuloides, on the other hand, are high. The phylogeny of Eucidaris confirms Ernst Mayr's conclusions that major barriers to the dispersal of tropical echinoids have been the wide stretch of deep water between central and eastern Pacific, the cold water off the southwest coast of Africa, and the Isthmus of Panama. It also suggests that a colonization event in the eastern Pacific has led to speciation between mainland and island populations.
Larvae of many echinoderms and other benthic marine invertebrates depend on ciliary suspension feeding for growth and development, but some larvae may be superior feeding devices (with consequences for variation in growth, size, and fitness). I measured differences in feeding performance among larvae of 9 echinoderm species from 4 taxonomic classes (Asteroidea, Ophiuroidea, Holothuroidea, Echinoidea). Maximum clearance rates (relative to size of the feeding structure) of some larvae were much higher than the rates of others, indicating substantial variation in feeding capabilities among larvae of similar size. In order to interpret these differences in feeding capability, I also give some preliminary data on larval form, development, and organic content. Although incomplete in several respects, these data do not indicate a simple relation between larval feeding and growth. This surprising result indicates that functional morphology is not always the most appropriate perspective from which to examine variation in growth and development of planktonic larvae.
This study documents evolutionary modifications in mechanisms of gastrulation in Heliocidaris erythrogramma, an echinoid with lecithotrophic larvae. Radially symmetrical cell rearrangements and changes in cell shape drive elongation of the archenteron in the ancestral mode of echinoid gastrulation. Cell marking experiments indicate that in H. erythrogramma, however, prolonged movement of cells over the ventral lip of the blastopore accompanies extension of the archenteron. Evolutionary modifications to archenteron extension in H. erythrogramma thus include utilization of a different type of cellular movement as well as the imposition of dorsoventral asymmetry in cellular movements. The conservation of gastrulation mechanisms among phylogenetically divergent echinoids with planktotrophic development suggests that the plesiomorphic condition has persisted at least 250 million years and perhaps much longer. Yet H. erythrogramma diverged from an ancestor with planktotrophic development only about 10 mya, indicating that morphogenetic mechanisms of early development can undergo substantial evolutionary changes, even after long periods of stasis.
Within the sea urchin genus Heliocidaris, changes in early embryonic and larval development have resulted in dramatic differences in the length of time larvae spend in the plankton before settling. The larvae of one species, H. tuberculata, spend several weeks feeding in the plankton before settling and metamorphosing into juveniles. The other species, H. erythrogramma, has modified this extended planktonic larval stage and develops into a juvenile within 3-4 days after fertilization. We used restriction site polymorphisms in mitochondrial DNA to examine the population genetic consequences of these developmental changes. Ten restriction enzymes were used to assay the mitochondrial genome of 29 individuals from 2 localities for H. tuberculata and 62 individuals from 5 localities for H. erythrogramma. Within H. tuberculata, 11 mitochondrial genotypes were identified. A GST analysis showed high levels of genetic exchange between populations separated by 1,000 kilometers of open ocean. In contrast, in H. erythrogramma, 13 mitochondrial genotypes differing by up to 2.33% were geographically partitioned over spatial scales ranging from 800 to 3,400 kilometers. Between distant localities, there was complete mitochondrial lineage sorting and large sequence divergence between resulting clades. Over much smaller spatial scales (<1,000 km), genetic differentiation was due to the differential sorting of very similar genotypes. This pattern of mitochondrial variation suggests that these population differences have arisen recently and may reflect the historical interplay between the restricted dispersal capabilities of H. erythrogramma and the climatic and geological changes associated with Pleistocene Ice Ages.
Larvae of two species of sea urchins (Strongylocentrotus droebachiensis and S. purpuratus) differ in initial form and in the rate of development. To determine whether these differences are attributable to the large interspecific difference in egg size, we experimentally reduced egg size by isolating blastomeres from embryos. The rate of development of feeding larvae derived from isolated blastomeres was quantified using a novel morphometric method. If the differences early in the life histories of these two species are due strictly to differences in egg size, then experimental reduction of the size of S. droebachiensis eggs should yield an initial larval form and rate of development similar to that of S. purpuratus. Our experimental manipulations of egg size produced three clear results: 1) smaller eggs yielded larvae that were smaller and had simpler body forms, 2) smaller eggs resulted in slower development through the early feeding larval stages, and 3) effects of egg size were restricted to early larval stages. Larvae from experimentally reduced eggs of the larger species had rates of development similar to those of the smaller species. Thus, cytoplasmic volumes of the eggs, not genetic differences expressed during development, account for differences in larval form and the rate of form change. This is the first definitive demonstration of the causal relationship between egg size (parental investment per offspring) and life-history characteristics in marine benthic invertebrates. Because larval form influences feeding capability, the epigenetic effects of egg size on larval form are likely to have important functional consequences. Adaptive evolution of egg size may be constrained by the developmental relationships between egg size and larval form: evolutionary changes in egg size alone can result in concerted changes in larval form and function; likewise evolutionary changes in larval form and function can be achieved through changes in egg size. These findings may have broader implications for other taxa in which larval morphology and, consequently, performance may be influenced by changes in egg size.
MtDNAs from 2 protein coding regions comprising 576 base pairs were sequenced from 17 individual sea urchins of the species Strongylocentrotus pallidus collected from the north Pacific and north Atlantic oceans. Twelve of 17 individual sequences were identical. Two of these were further sequenced in a third, 441 base pair region, and were also found to be identical. We show how to interpret these results using a simplified Markov model of mtDNA evolution at silent sites. The model was calibrated using 3 urchin species with a published fossil record, and shows that identical S. pallidus mtDNAs in different oceans shared a common ancestor at most 90,000-150,000 years ago (95% confidence interval of upper limit of divergence). The Markov model, used to examine patterns of genetic distance within and between species, shows unexpected variation in the rate of base substitution. The rate of change of G's at fourfold sites is nearly 20 times greater than the rate of change of C's. At twofold sites, this range is less extreme, although purines consistently have a higher rate of change than pyrimidines. Striking genetic similarity and recent genetic exchange between oceans for these urchins is in marked contrast to most other trans-Arctic marine populations, which usually show morphological and genetic differentiation at the species or subspecies level. Recent fossil evidence shows that the north Atlantic and northeastern Pacific have been the scene of radical faunal changes during the Pliocene and Pleistocene. The genetic results presented here extend this conclusion to intraspecific patterns of genetic variability, and direct attention to the northwest Pacific where higher productivity and less environmental change may have left a heritage of greater marine genetic diversity.
Spatial variation in allelic frequencies at nine allozyme loci were assayed in 20 populations of the crown-of-thorns starfish, Acanthaster planci, collected throughout the Pacific and Indian Oceans. These data were analyzed together with published data, for the same loci, from an additional 19 populations, giving a total sample size of approximately 1800 individuals. There was a marked discontinuity between the Indian and Pacific Ocean populations, but those off Western Australia and from the Southeast Asian region had a strong Pacific affinity. The genetic groups were congruent with the distributions of two color morph groups: gray-green to red-brown forms in the Pacific and a blue to pale red form in the Indian Ocean. These patterns of genetic structure are similar to those described for the starfish Linckia laevigata, which has similar life-history characteristics. Vicariant events may have influenced some populations within the Pacific, but the allozyme data cannot resolve the effects of these events clearly. Patterns of variation within regions were consistent with isolation by distance, but, at larger scales, were obscured by regional vicariance and some outliers, particularly by apparently high levels of gene flow between Japan and the Great Barrier Reef, Australia. Apparent gene flow between population pairs was not closely related to present-day ocean currents. The results demonstrate a strong influence of allopatric separation on genetic divergence at large geographic scales, but also show evidence of slow rates of change in gene frequencies consistent with the large population sizes of this species. Low levels of divergence between groups demonstrate the genetic structure is recent (Pleistocene) and are likely responses to changes in climate and sea level.
We analyzed phylogenetic relationships among 12 nominal species of starfish in the genera Patiriella and Asterina (Order Valvatida, Family Asterinidae), based on complete sequences for a mitochondrial protein coding gene (cytochrome oxidase subunit I) and five mitochondrial transfer RNA genes (alanine, leucine, asparagine, glutamine, and proline) (1923 bp total). The resulting phylogeny was used to test a series of hypotheses about the evolution of life-history traits. (1) A complex, feeding, planktonic larva is probably ancestral for these starfish, but this is not the most parsimonious reconstruction of ancestral larval states. (2) The feeding larval form was lost at least four times among these species, and three of these losses occurred among members of a single clade. (3) Small adult size evolved before both cases of hermaphroditism and viviparous brooding, but viviparity was not always preceded by an intermediate form of external brooding. (4) An ordered transformation series from feeding planktonic development to viviparous brooding has been predicted for starfish, but we could not find an example of this transformation series. (5) Viviparity evolved recently (< 2 Mya). (6) Both species selection and transformation of lineages may have contributed to the accumulation of species with nonfeeding development among these starfish. (7) Neither Asterina nor Patiriella are monophyletic genera. Larval forms and life-history traits of these starfish have evolved freely under no obvious constraints, contrary to the widely assumed evolutionary conservatism of early development.
Restriction-fragment analysis was used to measure mitochondrial DNA (mtDNA) variability in 79 individuals of two species of temperate sea urchins. For the purple urchin Strongylocentrotus purpuratus, individuals were collected 1,500 km apart in 1985 and again from the same localities in 1988 (about one urchin generation). Twenty mtDNA genotypes belonging to four clades were found among 38 individuals. All four clades were found in both localities and in both years. Genetic structure was further tested by calculating the degree of interdeme genetic variation (GST) and comparing this value to the GST's from randomly shuffled data. No geographic structure was found. For S. droebachiensis, only six mtDNA genotypes were found among 41 individuals collected from the Pacific and Atlantic oceans. More than 80% of the individuals belonged to two genotypes. The genotype that dominated collections in the Pacific also occurred in the Atlantic; however, a common Atlantic genotype was never found in the Pacific. These two genotypes were identical at 64 of 65 restriction sites, and were only 0.2% divergent from each other. GST analysis confirmed that there were significant genetic differences between Atlantic and Pacific populations. The small divergence between genotypes suggests recent, but not continuous, migration. These marine species show smaller genotypic differences than terrestrial species over similar spatial and temporal scales. Both recruitment of adults from planktonic larval pools and the spread of sibling larvae over large distances from parents probably act as buffers to genetic differences in species with planktonic life-history phases.
The-Pacific marine biota, particularly species with long planktonic larval stages, are thought to disperse widely throughout the Pacific via ocean currents. The little genetic data available to date has supported this view in that little or no significant regional differentiation of populations has been found over large geographical distances. However, recent data from giant clams has demonstrated not only significant regional differentiation of populations, but routes of gene flow that run perpendicular to the main present-day ocean currents. Extensive surveys of genetic variation at eight polymorphic loci in 19 populations of the giant clam Tridacna maxima, sampled throughout the West and Central Pacific, confirmed that the patterns of variation seen so far in T. gigas were not unique to that species, and may reflect a fundamental genetic structuring of shallow-water marine taxa. Populations of T. maxima within highly connected reef systems like the Great Barrier Reef were panmictic (average FST 5) between the Philippines and the Great Barrier Reef, between the Philippines and Melanesia (the Solomon Islands and Fiji), and between the Philippines and the Central Pacific island groups (Marshall Islands, Kiribati, Tuvalu and Cook Islands). Gene flow was low between these three sets of island chains (Nem
The relationship between the five echinoderm classes has perplexed phylogeneticists for some time. Although each of the crinoids (C), asteroids (A), ophiuroids (O), echinoids (E) and holothuroids (H) are morphologically distinct, evidence from larval and adult morphology, molecular biology, and stratigraphy have failed to provide a single consensus solution. We have reviewed all available morphological and molecular data, added new data and reanalysed independent data sets individually and in combination, in order to resolve echinoderm class relationships. In total, we present 21 larval and 50 adult morphological characters, partial 28S-like large subunit rRNA gene data for 39 taxa and complete 18S-like small subunit rRNA gene data for 37 taxa. For a 5 taxon problem there are 105 possible rooted tree topologies, and yet we were consistently presented with three competing hypotheses when data sets were analysed both individually and in combination. The total evidence solution favoured (outgroup(C(A(O(E,H))))) although the alternative tree topology, (outgroup(C(O(A(E,H))))) was only one step longer and (outgroup(C((A,O),(E,H)))) was two steps longer. Only these three trees are serious contenders and the distribution of morphological characters suggests we should discount the solution placing ophiuroids as sister group to an asteroid+echinoid+holothurian clade. Thus we are left with (outgroup(C(A(O(E,H))))) and (outgroup(C((A,O),(E,H)))) as the two most plausible phylogenetic hypotheses. Our data showed high levels of phylogenetic signal and these trees best fit the available data.
Percent fertilization of eggs of the echinoid Strongylocentrotus droebachiensis (0. F. Miiller) was determined both in laboratory and field experiments. In the labo- ratory, over 50% of the eggs were fertilized only in relatively dense sperm suspensions (> lo6 sperm/l); such suspensions retained their potency for less than 20 minutes. In the field, divers induced individual S, droebachiensis to spawn with KC1 injections. Along five meter transects running directly downcurrent from spawning males, fixed volumes of seawater presumably containing sperm were drawn into syringes already containing eggs. Within 20 cm of spawning males 60-95% fertilization usually occurred; at distances greater than 20 cm less than 15% of the eggs were fertilized. Higher per- centages of eggs were fertilized when current speeds were low (
The sea star genus Patiriella has the greatest diversity of life histories known for the Asteroidea. P. regularis has small eggs (150 μm diameter) and the ancestral planktotrophic larvae. P. calcar, P. gunnii, P. exigua and P. pseudoexigua have large eggs (390–440 μm diameter) and non-feeding lecithotrophic larvae. Two species with lecithotrophic larvae P. vivipara and P. parvivipara have secondarily evolved a small egg (135–150 μm diameter). We examined the oogenic strategies involved with evolution of egg size in these sea stars. Comparison of protein profiles, histochemistry and ultrastructure of the eggs of Patiriella indicated that the major changes underlying acquisition of a large egg involved enhanced deposition of lipid in some species and an increase in yolk reserves in others. The eggs of the planktotroph, P. regularis, and the benthic lecithotroph, P. exigua, contained an abundance of major yolk protein MYP. By contrast, the eggs of the planktonic lecithotrophs P. gunnii and P. calcar were dominated by lipid and the MYP appeared to be greatly reduced. The eggs of P. calcar contained an abundant protein which may be a truncated form of vitellogen. The small eggs of the viviparous species P. vivipara and P. parvivipara appear to be miniature versions of the eggs of the closely related P. exigua. Comparison of the eggs of Patiriella species with lecithotrophic development revealed among species variation in oogenesis. Depending on the species, the evolutionary modification of oogenesis appeared to be influenced by phylogenetic history and selection for egg characteristics with respect to the planktonic or benthic location of development.
The closely related intertidal starfish Patiriella calcar and P exigua have markedly different dispersal capabilities. P calcar is considered to have sexually produced planktonic larvae and hence the potential for wide dispersal. If this is the case local populations should be strongly interconnected and there should be little or no accumulation of genetic variation. Conversely, P exigua has no known means of dispersal between local populations since development proceeds directly from egg masses laid on the rocky shore. Therefore, local populations of P exigua may be less genotypically diverse than P calcar, but should display marked genetic heterogeneity among local populations resulting from the accumulating effects of genetic drift and localised natural selection. To test these predictions genetic data were used to assess the genotypic structure and apparent connectedness of local populations of both species along 230 km of the southeast coast of Australia. Electrophoretic variation was surveyed for 5 loci. Single-locus genotype frequencies within all local populations of both species closely matched expectations for Hardy-Weinberg equilibria. This implies that local populations are maintained by recruitment of outcrossed sexually produced offspring, but as predicted, P exigua had fewer alleles at each locus and fewer heterozygous individuals than P calcar. Furthermore, it appears that these 2 species have vastly different levels of gene flow among local populations. The 6 local populations of P calcar were genetically homogeneous [F(ST) = 0 +/- 0.001 (SE)] and these populations were thus inferred to be strongly connected by larval dispersal. In sharp contrast, the 11 local populations of the directly developing P exigua were highly genetically heterogeneous which is reflected in consistently high levels of genetic variance (F(ST) = 0.462 +/- 0.048).
Egg sizes of free-spawning invertebrates, fishes, amphibians, and reptiles can reflect an evolutionary compromise between offspring size and number. One explanation for this widespread trade-off in marine invertebrates is the effect of egg size on performance of the planktonic larva: smaller larvae from smaller eggs might incur greater risk in the plankton if they feed and grow more slowly. I tested this hypothesis by experimental manipulation of egg size and measurement of changes in larval feeding performance and growth in the laboratory. In a sea urchin, experimentally halved ''eggs'' became smaller larvae with lower feeding rates. Preliminary experiments suggested that larvae that obtain less food (either because they develop from halved eggs or because they are provided less food) may have longer larval periods or become smaller juvenile sea urchins. Either cost of reduced egg size might limit the evolution of large clutches of small eggs. However, halving egg size did not measurably affect time from fertilization to metamorphosis and had a significant but small effect on juvenile size. Though larval feeding capability is widely supposed to limit the evolution of very small eggs, these experiments suggest that other selective forces must also govern the evolution of egg size in species with feeding planktonic larvae.
Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene Bow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in tars with high dispersal potential. Tn understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene Bow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects, These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene Bow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.
An account is given of the comparative morphology of the family Littorinidae, based on examination of 122 species, grouped into 32 subgenera. The shell, operculum and principal organ systems are described, and their phylogenetic significance assessed. A total of 53 characters, coded as 131 character states, were chosen for inclusion in a cladistic analysis of the phylogenetic relationships of the subgenera. The analysis supports the monophyly of the Littorinidae, and the family can be formally defined by the two synapomorphies of a spiral pallial oviduct and an anterior bursa copulatrix. Three principal clades are identified and given subfamilial rank; the Lacuninae, the Laevilitorininae and the Littorininae. The reconstruction of character states on the cladogram provides hypotheses about the evolution of individual characters. The poor fossil record of the family is reviewed, and its biogeography discussed in the light of the phylogenetic hypothesis. Some ecological implications of the phylogenetic hypothesis are considered, with special reference to the diverse types of spawn and life-history strategies. -from Author
In this study, we integrate information from phylogeny, comparative ontogeny, and experimental embryology in an attempt to elucidate the mechanisms controlling evolutionary trends towards digital reduction and loss observed in amphibians. Frogs and salamanders that have lost phalanges and even whole toes have done so in a very ordered manner, i.e., certain skeletal elements are lost prior to others. This pattern of morphological diversity is described and trends elucidated. It is concluded that the process is characterized by striking intraordinal convergences coupled with substantial differences between the trends observed in frogs as compared to urodeles. We argue that this pattern is essentially a reflection of the differences in the ontogenies of the two orders. Similarly, the convergences within urodeles and within anurans can be explained as the result of regulation of developmental parameters in a resilient developmental program. We further explore this hypothesis by experimentally perturbing the number of cells in the embryonic limb primordium to show that reduction in the number of mesenchymal cells secondarily affects the developmental process of pattern formation causing a rearrangement of the skeletal morphology of the foot. The same experimental manipulation has different effects in frogs as compared to salamanders. However, in both cases, the experimentally generated morphologies tend to parallel the phenotypes and trends observed in nature. Our conclusion is that most of the patterns of diversity in the digital morphology of amphibians can be explained as a reflection of developmental properties. In general, we present a methodology that attempts to empirically address the issue of identifying developmental constraint in morphological evolution.
Both mtDNA variation and allozyme data demonstrate that geographic groupings of different color morphs of the starfish Linckia laevigata are congruent with a genetic discontinuity between the Indian and Pacific Oceans. Populations of L. laevigata sampled from Thailand and South Africa, where an orange color morph predominates, were surveyed using seven polymorphic enzyme loci and restriction fragment analysis of a portion of the mtDNA including the control region. Both allozyme and DNA data demonstrated that these populations were significantly genetically differentiated from each other and to a greater degree from 23 populations throughout the West Pacific Ocean, where a blue color morph is predominant. The genetic structure observed in L. laevigata is consistent with traditional ideas of a biogeographic boundary between the Indian and Pacific Oceans except that populations several hundreds kilometers off the coast of north Western Australia (Indian Ocean) were genetically similar to and had the same color morphs as Pacific populations. It is suggested that gene flow may have continued (possibly at a reduced rate) between these offshore reefs in Western Australia and the West Pacific during Pleistocene falls in sea level, but at the same time gene flow was restricted between these Western Australian populations and those in both Thailand and South Africa, possibly by upwellings. The molecular data in this study suggest that vicariant events have played an important role in shaping the broadscale genetic structure of L. laevigata. Additionally, greater genetic structure was observed among Indian Ocean populations than among Pacific Ocean populations, probably because there are fewer reefs and island archipelagos in the Indian Ocean than in the Pacific, and because present-day surface ocean currents do not facilitate long-distance dispersal.
Although larval ecology has roots as far back as Aristotle, the earliest accurate ideas about larval recruitment, dispersal, and behavior arose about 150 years ago, during the time of J. Vaughn Thompson and Edward Forbes. In this review, the history of larval ecology is traced from the initial discovery of larvae and the early formulation of ideas in the nineteenth century through the development of methodology for addressing hard-to-study field processes in the 1980's. A survey of the literature in major marine biology journals reveals the overall trends in larval research and the temporal changes in the proportion of effort devoted to various kinds of studies. Many recent studies of larval processes resemble seldom-cited studies that were done more than a half century earlier.
Genetic variation in four natural populations of the starfish Linckia laevigata from the Indo-West Pacific was examined using restriction fragment analysis of a portion of the mtDNA including the control region. Digestion with seven restriction enzymes identified 47 haplotypes in a sample of 326 individuals. Samples collected from reef sites within each location were not significantly differentiated based on ΦST or spatial distribution of haplotypes, indicating that dispersal is high over short to moderate distances. Evidence of gene flow is further supported by the low divergence among haplotypes and the lack of any clear geographical structuring among different haplotypes in the gene phylogeny. However, analysis of molecular variance (AMOVA), ΦST and contingency χ2 analyses of the spatial distribution of haplotypes demonstrate the presence of significant broad scale population genetic structure among the four widespread locations examined. RFLP data are consistent with high gene flow between the Philippines and Western Australia and moderate gene flow between the Great Barrier Reef (GBR) and Fiji, but only limited gene flow between either the Philippines or Western Australia and either the GBR or Fiji. The presence of mtDNA structure contrasts with previous allozyme data which suggest that dispersal among widely separated locations is equivalent to dispersal among populations within the highly connected GBR studies. This discordance between patterns of gene flow inferred from these two markers cannot be fully accounted for by differences in effective population size for mtDNA. This might suggest that while mtDNA variation may represent contemporary patterns of gene flow, allozyme variation among populations is yet to reach equilibrium between drift and migration over the range surveyed.
Traditionally, broadcast spawning and planktonic larvae have been considered the plesiomorphic ‘ground plan’ for the Polychaeta and other metazoan groups. To assess whether this reproductive mode is in fact ‘primitive’, the study of monophyletic groups with various reproductive modes should be informative. A large range of body sizes would allow testing the ideas that aspects of reproductive mode may be functionally constrained. The family Sabellidac is one such group, with sexual reproductive modes ranging from broadcast spawning to intratubular brooding to ovovivi-parity, and a body size range over more than five orders of magnitude. Sabellids have previously been the subject of detailed cladistic analyses (Fitzhugh 1989, 1991); here we introduce several new characters based on morphology of reproductive structures. Larval development in four brooding sabellid species is also described with the aim of introducing new characters for future systematic analyses. Our cladistic analysis of sabellid genera suggests that gonochorism and brooding of direct-developing larvae are plesiomorphic in the Sabellidae, with external fertilization and swimming larvae limited to apomorphie clades in the subfamily Sabellinae. The presence of sperm with elongate heads may be correlated with the presence of intratubular brooding, though an adequate causal explanation for this relationship can not yet be presented. The concept that ‘modified’ sperm must be derived from ‘primitive’ sperm is shown to be false, with ‘modified’ sperm being plesiomorphic for the Sabellidae, from which ‘primitive’ sperm is derived in apomorphic Sabellinae. All sabellids have lecithotrophic development and appear to be phylogenetically constrained in this regard. Data gathered on body size and reproductive variables in the Sabellidac suggests the following (when phylogenetic effects are not controlled): (1) egg number and total egg volume are significantly correlated with body size, with small animals having fewer, larger eggs than large animals; (2) individual egg volume is not correlated with body size; (3) reproductive mode is significantly correlated with body size; intratubular brooders tend to be small-bodied, whereas broadcast spawners are large. However when the effect of body size is controlled for, then (4) egg number, egg volume and total egg volume all vary significantly with reproductive mode. Broadcast spawners expel a large number of small eggs for a high total egg volurne. Intratubular brooders have a few relatively large eggs for a small total egg volume. When statistics arc performed using phylogenetically independent contrasts there is a significant correlation between total egg volume and body size but not for egg number and body size. The effect of non-independence (due to phylogeny) of our data needs to be more fully controlled in future analyses but methods of incorporating continuous data into cladistic analyses should also be investigated. We show that some predictions can be made about reproductive mode based on body size but ad hoc patterns of reproductive character-state transformation should not be made independent of empirical hypotheses of phylogenetic relationship. Further studies of this kind throughout the Annelida are needed to determine the plesiomorphic reproductive mode for the phylum.
The sperm of starfish (Asteroidea), sea cucumbers (Holothuroidea), and brittle stars (Ophiuroidea) are chemotactically attracted to the tip of a micropipette releasing very small volumes of egg water or seawater solution of an alcoholic extract of ovaries or spawned eggs. The sperm are unresponsive to injections of seawater or a weak solution of ammonium chloride in seawater. No other echinoderm tissue tested yields an extract with sperm-attracting activity. A complex set of species-specificities has been demonstrated between many of the genera and species used in these experiments. Plotting of the paths of chemotactic sperm reveals that they approach the tip of the pipette along a path consisting of small loops alternating with straight segments orientated directly up the gradient. This is similar to the chemotactic behavior previously reported for the sperm of hydrozoans, molluscs, and urochordates. Sperm velocity does not change in response to the sperm attractant. Attracted sperm remain motile long after they have been attracted and do not agglutinate. In at least some cases the attraction response is reversible. This is the first direct evidence of sperm chemotaxis in echinoderms.
Larvae of the brittle star Ophiopholis aculeata, common to the North Pacific coast of the United States, and an unidentified species of ophiuroid, collected from waters off the eastern coast of Florida, undergo asexual reproduction of the primary larva to produce a secondary larval clone. Generation of a secondary larva begins with the release of the larval posterolateral arms, which are initially retained by the settled juvenile. In O. aculeata, the released arms regenerate all the structures typical of the primary ophiopluteus. Tissue and energy reserves required for formation of the secondary feeding larva appear to be supplied by the absorption and reorganization of part of the posterolateral arms. Various developmental stages of the unidentified ophiopluteus were collected from plankton samples taken off the coast of Florida. These included just-released posterolateral arms, plutei, and metamorphosed juveniles with the posterolateral arms still attached. The identification of regenerating arms from the plankton demonstrates that asexual reproduction by ophiuroid larvae is not restricted to a single, laboratory-cultured species. In both O. aculeata and the unidentified Atlantic ophiopluteus, cloning involves the dedifferentiation of primary larval tissue and a developmental progression similar to that followed by the zygote, although it is not known whether the formation of the secondary larva follows the same pathway utilized by the primary larva or a novel developmental program. Asexually produced secondary larvae of O. aculeata undergo metamorphosis, settle to the benthos, and initiate a tertiary larval generation, indicating that cloned larvae could be added to the population as long as environmental conditions could support a planktonic existence. This phenomenon represents an unusual potential to increase the geographic range and the number of juveniles of a given parentage in future generations without additional reproductive input from the adult.
The relationship between the five echinoderm classes has perplexed phylogeneticists for some time. Although each of the crinoids (C), asteroids (A), ophiuroids (O), echinoids (E) and holothuroids (H) are morphologically distinct, evidence from larval and adult morphology, molecular biology, and stratigraphy have failed to provide a single consensus solution. We have reviewed all available morphological and molecular data, added new data and reanalysed independent data sets individually and in combination, in order to resolve echinoderm class relationships. In total, we present 21 larval and 50 adult morphological characters, partial 28S-like large subunit rRNA gene data for 39 taxa and complete 18S-like small subunit rRNA gene data for 37 taxa. For a 5 taxon problem there are 105 possible rooted tree topologies, and yet we were consistently presented with three competing hypotheses when data sets were analysed both individually and in combination. The total evidence solution favoured (outgroup(C(A(O(E, H))))) although the alternative tree topology, (outgroup(C(O(A(E, H))))) was only one step longer and (outgroup(C((A, O),(E, H)))) was two steps longer. Only these three trees are serious contenders and the distribution of morphological characters suggests we should discount the solution placing ophiuroids as sister group to an asteroid+echinoid + holothurian clade. Thus we are left with (outgroup(C(A(O(E, H))))) and (outgroup(C((A, O),(E, H)))) as the two most plausible phylogenetic hypotheses. Our data showed high levels of phylogenetic signal and these trees best fit the available data.