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Morphology of Acanthochitona crinita trochophore larvae. Anterior faces up. a, b Mid-trochophore larvae. Episphere and hyposphere are separated from each other by the prototroch. In the dorsal hyposphere region, the anlagen of the seven shells are surrounded by spicule-containing cells. c, d Late trochophore larvae. Seven differentiating dorsal shell plates are present. at apical tuft, ds dorsal shell plates, f foot, pt prototroch, sc spicule-forming cell(s)
Source publication
Mollusca is an extremely diverse animal phylum that includes the aculiferans (worm-like aplacophorans and eight-shelled polyplacophorans) and their sister group, the conchiferans, comprising monoplacophorans, bivalves (clams, mussels), gastropods (snails, slugs), scaphopods (tusk shells) and cephalopods (squids, octopuses). Studies on mollusks have...
Contexts in source publication
Context 1
... larvae are oval-shaped and approxi- mately 280 μm in length ( Fig. 3a and b). The hypo- sphere is more elongated, and posterior to the mouth opening the developing anlage of the foot extends in the ventral region. Dorso-laterally, on both sides of the hyposphere, a longitudinal row of epidermal and very prominent spicule-containing cells are discernible. Dor- sally, an anterior transversal row of ...
Context 2
... trochophore larvae are about 360 μm in length and the hyposphere is considerably more elongated than in previous stages ( Fig. 3c and d). On the dorsal side of the hyposphere, seven differentiating shell plates are present. At the end of the entirely lecithotrophic, i.e., non-feeding larval development, late trochophore larvae settle and undergo metamorphosis, which is character- ized by considerable dorso-ventral flattening of the animals as well as loss of the apical ...
Similar publications
Molecular developmental studies of various bilaterians have shown that the identity of the anteroposterior body axis is controlled by Hox and Parahox genes. Detailed Hox and ParaHox gene expression data are available for conchiferan mollusks, such as gastropods (snails and slugs) and cephalopods (squids and octopuses), whereas information on the pu...
Citations
... Within the Mollusca phylum, researchers have identified a total of 11 Hox genes (Albertin et al., 2015;Belcaid et al., 2019;Canapa et al., 2005;Giusti et al., 2000;Samadi and Steiner, 2010). While data on Hox gene expression in the two aplacophoran clades, Neomeniomorpha, and Chaetodermomorpha, remain scarce, a notable study on the polyplacophoran Acanthochitona revealed fascinating spatial expression patterns of 10 Hox genes along the anterior-posterior body axis (Fritsch et al., 2015;Fritsch et al., 2016). This finding contrasts with the situation in conchiferans, where Hox genes exhibit distinct expression during the development of specific morphological features like ganglia, the foot, or the shell field (Wanninger and Wollesen, 2019). ...
... The polyplacophoran shell fields contain distinct cell populations, with different roles in the formation of shell plates and the connective tissues [6,11]. On the molecular level, it has been revealed a number of genes showing striped expression in the shell field, including the wellaccepted molluscan shell-formation gene engrailed [12], key developmental regulators hox genes [13][14][15] and others [16,17]. Nevertheless, it is largely unknown what specific cell types these genes are expressed in. ...
... These facts indicate that shell field morphogenesis occurs during the period between hatching and metamorphosis (Fig. 1b). Indeed, the appearances of the shell field continuously change with larval development in Rhyssoplax olivacea (= Chiton olivaceus) [4], and gene expression in the shell field exhibits dynamic patterns during polyplacophoran larval development [13,14,16]. ...
... Bars represent 50 μm behaviors during shell formation [2,6,11], efforts are required to explore the molecular aspects. Some genes are reported to be expressed in the shell field [13][14][15][16][17], but their correlations with particular cell types remain largely unknown. ...
Background
The polyplacophoran mollusks (chitons) possess serially arranged shell plates. This feature is unique among mollusks and believed to be essential to explore the evolution of mollusks as well as their shells. Previous studies revealed several cell populations in the dorsal epithelium (shell field) of polyplacophoran larvae and their roles in the formation of shell plates. Nevertheless, they provide limited molecular information, and shell field morphogenesis remains largely uninvestigated.
Results
In the present study, we investigated shell field development in the chiton Acanthochitona rubrolineata based on morphological characteristics and molecular patterns. A total of four types of tissue could be recognized from the shell field of A. rubrolineata. The shell field comprised not only the centrally located, alternatively arranged plate fields and ridges, but also the tissues surrounding them, which were the precursors of the girdle and we termed as the girdle field. The girdle field exhibited a concentric organization composed of two circularly arranged tissues, and spicules were only developed in the outer circle. Dynamic engrailed expression and F-actin (filamentous actin) distributions revealed relatively complicated morphogenesis of the shell field. The repeated units (plate fields and ridges) were gradually established in the shell field, seemingly different from the manners used in the segmentation of Drosophila or vertebrates. The seven repeated ridges also experienced different modes of ontogenesis from each other. In the girdle field, the presumptive spicule-formation cells exhibited different patterns of F-actin aggregations as they differentiate.
Conclusions
These results reveal the details concerning the structure of polyplacophoran shell field as well as its morphogenesis. They would contribute to exploring the mechanisms of polyplacophoran shell development and molluscan shell evolution.
... The polyplacophoran shell elds contain distinct cell populations, with different roles in the formation of shell plates and the connective tissues [6,11]. On the molecular level, it has been revealed a number of genes showing striped expression in the shell eld, including the well-accepted molluscan shell-formation gene engrailed [12], the key developmental genes hox [13][14][15] and others [16,17]. Nevertheless, it is largely unknown that which gene is expressed in particular cell types. ...
... These facts indicate that shell eld morphogenesis occurs during the period between hatching and metamorphosis (Fig. 1b). Indeed, the appearances of the shell eld continuously change with larval development in Chiton olivaceus [4], and gene expression in the shell eld exhibits dynamic patterns during polyplacophoran larval development [13,14,16]. ...
... Moreover, while previous research revealed the various cell types inside the shell eld and their behaviors during shell formation [2,6,11], efforts are required to explore the molecular aspects. Some genes are reported to be expressed in the shell eld [13][14][15][16][17], but their correlations with particular cell types remain largely unknown. ...
Background
The polyplacophoran mollusks (chitons) possess serially arranged shell plates. This feature is unique among mollusks and believed to be essential to explore the evolution of mollusks as well as their shells. Previous studies revealed several cell populations in the dorsal epithelium (shell field) of polyplacophoran larvae and their roles in the formation of shell plates. Nevertheless, they provide limited molecular information, and shell field morphogenesis remains largely uninvestigated.
Results
In the present study, we investigated the shell field development in the chiton Acanthochitona rubrolineata based on morphological characteristics and molecular patterns. A total of four types of tissue could be recognized from the shell field of A. rubrolineata. The shell field comprised not only the centrally located, alternatively arranged plate fields and ridges, but also the tissues surrounding them, which were the precursors of the girdle and we termed as the shell field margin. The shell field margin exhibited a concentric organization composed of two imaginary circles, and spicules were only developed in the outer circle. Dynamic engrailed expression and F-actin (filamentous actin) distributions revealed relatively complicated morphogenesis of the shell field. The repeated units (plate fields and ridges) were gradually established in the shell field, seemingly different from the manners used in the segmentation of Drosophila or vertebrates. The seven repeated ridges also experienced different modes of ontogenesis from each other. In the shell field margin, the presumptive spicule-formation cells exhibited different patterns of F-actin aggregations with the ongoing of their specification.
Conclusions
These results reveal the details concerning the structure of polyplacophoran shell field as well as its morphogenesis. They would contribute to exploring the mechanisms of polyplacophoran shell development and molluscan shell evolution.
... However, a majority of other molluscs show only 'hints' of temporal collinearity (Salamanca-Díaz et al., 2021) at best. The presence of spatially staggered expression in chitons (Fritsch et al., 2015) and a decoupling model, which emphasizes the differences between staggered expression on the ventral side and a lineage-specific dorsal side in molluscs (Huan et al., 2020) did not resolve this contradiction, since pseudosegmented polyplacophoran organization cannot be directly descended from the annelid body plan (Nielsen, 2012;Vinther et al., 2017). The latter consideration is important because the majority of molluscs with an entire shell (regardless of their complexity) have a non-collinear Hox gene expression (Lee et al., 2003;Samadi & Steiner, 2010;Salamanca-Díazet al., 2021). ...
... The latter consideration is important because the majority of molluscs with an entire shell (regardless of their complexity) have a non-collinear Hox gene expression (Lee et al., 2003;Samadi & Steiner, 2010;Salamanca-Díazet al., 2021). Therefore, the presence of the collinear Hox-gene expression in chitons (Fritsch et al., 2015) cannot be considered ancestral for all molluscs. Importantly, brachiopods and several annelids also do not show collinear expression (Schiemann et al., 2017;Gąsiorowski & Hejnol, 2020;Hejnol et al., 2021). ...
Various evaluations of the last common bilaterian ancestor ( lcba ) currently suggest that it resembled either a microscopic, non-segmented motile adult; or, on the contrary, a complex segmented adult motile urbilaterian. These fundamental inconsistencies remain largely unexplained. A majority of multidisciplinary data regarding sedentary adult ancestral bilaterian organization is overlooked. The sedentary-pelagic model is supported now by a number of novel developmental, paleontological and molecular phylogenetic data: (1) data in support of sedentary sponges, in the adult stage, as sister to all other Metazoa; (2) a similarity of molecular developmental pathways in both adults and larvae across sedentary sponges, cnidarians, and bilaterians; (3) a cnidarian-bilaterian relationship, including a unique sharing of a bona fide Hox-gene cluster, of which the evolutionary appearance does not connect directly to a bilaterian motile organization; (4) the presence of sedentary and tube-dwelling representatives of the main bilaterian clades in the early Cambrian; (5) an absence of definite taxonomic attribution of Ediacaran taxa reconstructed as motile to any true bilaterian phyla; (6) a similarity of tube morphology (and the clear presence of a protoconch-like apical structure of the Ediacaran sedentary Cloudinidae) among shells of the early Cambrian, and later true bilaterians, such as semi-sedentary hyoliths and motile molluscs; (7) recent data that provide growing evidence for a complex urbilaterian, despite a continuous molecular phylogenetic controversy. The present review compares the main existing models and reconciles the sedentary model of an urbilaterian and the model of a larva-like lcba with a unified sedentary(adult)-pelagic(larva) model of the lcba .
... Hox genes represent one of the most comprehensively studied families of animal transcription factors and their expression has been investigated in numerous Spiralia [25], representing diverse evolutionary lineages, such as flatworms [23,51,52], rotifers [12], chaetognaths [13], dicyemids [53] [59][60][61][62][63][64][65][66]. However, the function of particular Hox genes in those studied species is inferred only from expression patterns, since among spiralians the functional studies of Hox genes has been conducted thus far only on adult planarians [52]. ...
... Lack of Hox gene expression during development of pilidium, actinotrocha and mitraria contrasts with the regular Hox expression in larval brachiopods [7] and trochophores of mollusks and annelids [37,38,40,[61][62][63]65], in which both larval and adult bodies exhibit similar Hox gene patterns. The fact that those larvae are patterned by the conserved Hox system, as well as their phylogenetic distribution, suggest that all of those larval stages are eventually derived from the ancestral larval type present in the last common lophotrochozoan ancestor. ...
The decoding of genomes of a larger number of animal species have provided further insights into the genomic Hox gene organization and with this indicated the evolutionary changes during the radiation of several clades. The expansion of gene expression studies during development and life history stages of more species, complete the picture of the relationship between cluster organisation and temporal and spatial correlation of the Hox activity. Now these results open the opportunity to look deeper into the regulatory pathways that form these patterns and identify what exact changes caused the evolution of the application of this iconical gene set for the evolution of new larval forms and new structures. Here we review recent progress of Hox gene related research in the large clade Spiralia, that comprises Annelida, Mollusca, Lophophorata, Platyhelminthes, Nemertea and others. Albeit their relationship to each other is not resolved yet, there are emerging patterns that indicate that Hox genes are mainly used for patterning late, adult body parts and that Hox genes are often not expressed on the larval stages. Hox genes seem also often recruited for the formation of morphological novelties. Together with the emerging genomic information Hox genes show a much more dynamic evolutionary history than previously assumed.
... This expression is under the control of several cis-regulatory elements (CREs) that cluster together to form cisregulatory modules (CRMs) (Peifer and Bender, 1986;Peifer et al., 1987;Celniker et al., 1989;Martin et al., 1995;Maeda, 2009;Chopra, 2011;Bekiaris et al., 2018). Series of discoveries toward the turn of the 20th century showed the presence of Hox in all bilaterians and even in cnidarians (Ferrier et al., 2000;Kourakis and Martindale, 2001;Ferrier and Minguillón, 2003;Ikuta et al., 2004;Duboule, 2007;Mooi and David, 2008;Mallo et al., 2010;Ikuta, 2011;Janssen et al., 2014;Fritsch et al., 2015;Schiemann et al., 2017;Wanninger and Wollesen, 2019;Nong et al., 2020). The transcription factors coded by these genes have a conserved helix-turn-helix motif-containing DNA binding domain. ...
Hox genes code for transcription factors and are evolutionarily conserved. They regulate a plethora of downstream targets to define the anterior-posterior (AP) body axis of a developing bilaterian embryo. Early work suggested a possible role of clustering and ordering of Hox to regulate their expression in a spatially restricted manner along the AP axis. However, the recent availability of many genome assemblies for different organisms uncovered several examples that defy this constraint. With recent advancements in genomics, the current review discusses the arrangement of Hox in various organisms. Further, we revisit their discovery and regulation in Drosophila melanogaster. We also review their regulation in different arthropods and vertebrates, with a significant focus on Hox expression in the crustacean Parahyale hawaiensis. It is noteworthy that subtle changes in the levels of Hox gene expression can contribute to the development of novel features in an organism. We, therefore, delve into the distinct regulation of these genes during primary axis formation, segment identity, and extra-embryonic roles such as in the formation of hair follicles or misregulation leading to cancer. Toward the end of each section, we emphasize the possibilities of several experiments involving various organisms, owing to the advancements in the field of genomics and CRISPR-based genome engineering. Overall, we present a holistic view of the functioning of Hox in the animal world.
... Gene-specific primers (Data S2, Supplementary Information) were designed using OligoCalc (Kibbe 2007). PCR amplification, cloning, and ligation were performed as previously described (Fritsch et al. 2015). Probes were generated using digoxigenin and fluorescein RNA Labelling kits (#11277073910 and #11685619910, Roche Diagnostics GmbH, Mannheim, Germany). ...
... Probe sequences are provided in (Data S2, Supplementary Information). Whole-mount in situ hybridization was performed as previously described (Fritsch et al. 2015(Fritsch et al. , 2016, but with a probe concentration of 1 ng/μL and a hybridization temperature of 58°C. Larvae were incubated in digoxigenin or fluorescein antibodies conjugated to alkaline phosphatase (#11093274910 and # 11426338910 Roche, 1:3000 dilution) for 24 h at 4°C. ...
... For single in situ hybridization, the color reaction was carried out in alkaline phosphatase buffer (APB, Tris-HCl, 100 mM, pH 9.5) containing 7.5% polyvinyl alcohol and 2% NBT/BCIP (#11681451001, Roche) for 1-6 h. After successful staining, larvae were washed in PBT (0.1 M phosphate buffered saline, 0.1% Tween-20, pH 7), cleared in a 1:1 benzylalcohol:benzylbenzoate solution (Fritsch et al. 2015) and imaged with a Nikon Eclipse E800 microscope and a Nikon Fi2-U3 camera. ...
Ecdysis‐related neuropeptides (ERNs), including eclosion hormone, crustacean cardioactive peptide, myoinhibitory peptide, bursicon alpha and bursicon beta regulate moulting in insects and crustaceans. Recent evidence further revealed that ERNs likely play an ancestral role in invertebrate life cycle transitions, but their tempo‐spatial expression patterns have not been investigated outside Arthopoda. Using RNA‐seq and in situ hybridization, we show that ERNs are broadly expressed in the developing nervous system of a mollusk, the polyplacophoran Acanthochitona fascicularis. While some ERN‐expressing neurons persist from larval to juvenile stages others are only present during settlement and metamorphosis. These transient neurons belong to the “ampullary system”, a polyplacophoran‐specific larval sensory structure. Surprisingly, however, ERN expression is absent from the apical organ, another larval sensory structure that degenerates before settlement is completed in A. fascicularis. Our findings thus support a role of ERNs in A. fascicularis metamorphosis but contradict the common notion that the apical organ‐like structures shared by various aquatic invertebrates (i.e. cnidarians, annelids, mollusks, echinoderms) are of general importance for this process. This article is protected by copyright. All rights reserved
... The plesiomorphic state for hox expression in mollusks is represented by the staggered expression along the anteriorposterior body axis of 1 chiton [70]. Within Conchifera, temporal staggered expression is observed only during the early midstage trochophore larva of 1 scaphopod [71], in the embryo stage 19/20 of 1 cephalopod [72], and for anterior hox genes in the pretorsional veliger of several gastropods [73]. ...
Background
Venoms are deadly weapons to subdue prey or deter predators that have evolved independently in many animal lineages. The genomes of venomous animals are essential to understand the evolutionary mechanisms involved in the origin and diversification of venoms.
Results
Here, we report the chromosome-level genome of the venomous Mediterranean cone snail, Lautoconus ventricosus (Caenogastropoda: Conidae). The total size of the assembly is 3.59 Gb; it has high contiguity (N50 = 93.53 Mb) and 86.6 Mb of the genome assembled into the 35 largest scaffolds or pseudochromosomes. On the basis of venom gland transcriptomes, we annotated 262 complete genes encoding conotoxin precursors, hormones, and other venom-related proteins. These genes were scattered in the different pseudochromosomes and located within repetitive regions. The genes encoding conotoxin precursors were normally structured into 3 exons, which did not necessarily coincide with the 3 structural domains of the corresponding proteins. Additionally, we found evidence in the L. ventricosus genome for a past whole-genome duplication event by means of conserved gene synteny with the Pomacea canaliculata genome, the only one available at the chromosome level within Caenogastropoda. The whole-genome duplication event was further confirmed by the presence of a duplicated hox gene cluster. Key genes for gastropod biology including those encoding proteins related to development, shell formation, and sex were located in the genome.
Conclusions
The new high-quality L. ventricosus genome should become a reference for assembling and analyzing new gastropod genomes and will contribute to future evolutionary genomic studies among venomous animals.
... Interestingly, and strikingly different from any conchiferan investigated to date, the aculiferan polyplacophoran Acanthochitona fascicularis shows almost textbook-like spatial (but not temporal) Hox gene expression along the anterior-posterior axis in the trochophore stage 30,31,38 . Here, Hox genes are not confined to distinct morphological features but instead to well-defined axial territories, thus probably resembling the conserved bilaterian condition 11 . ...
... The Hox complement of Mollusca comprises 11 genes. These are expressed in a staggered fashion in polyplacophorans, a representative of the aculiferan lineage with a number of putative conserved ancestral traits such as a serially arranged dorsoventral musculature, a nervous system with non-ganglionated longitudinal nerve cords, an unpronounced cephalic region, and an elongated foot that extends more than three quarters along the longitudinal body axis 30 . The non-regionalized anterior-posterior distribution of these morphological traits and the conserved mode of www.nature.com/scientificreports/ ...
Hox genes are key developmental regulators that are involved in establishing morphological features during animal ontogeny. They are commonly expressed along the anterior–posterior axis in a staggered, or collinear, fashion. In mollusks, the repertoire of body plans is widely diverse and current data suggest their involvement during development of landmark morphological traits in Conchifera, one of the two major lineages that comprises those taxa that originated from a uni-shelled ancestor (Monoplacophora, Gastropoda, Cephalopoda, Scaphopoda, Bivalvia). For most clades, and bivalves in particular, data on Hox gene expression throughout ontogeny are scarce. We thus investigated Hox expression during development of the quagga mussel, Dreissena rostriformis, to elucidate to which degree they might contribute to specific phenotypic traits as in other conchiferans. The Hox/ParaHox complement of Mollusca typically comprises 14 genes, 13 of which are present in bivalve genomes including Dreissena. We describe here expression of 9 Hox genes and the ParaHox gene Xlox during Dreissena development. Hox expression in Dreissena is first detected in the gastrula stage with widely overlapping expression domains of most genes. In the trochophore stage, Hox gene expression shifts towards more compact, largely mesodermal domains. Only few of these domains can be assigned to specific developing morphological structures such as Hox1 in the shell field and Xlox in the hindgut. We did not find traces of spatial or temporal staggered expression of Hox genes in Dreissena. Our data support the notion that Hox gene expression has been coopted independently, and to varying degrees, into lineage-specific structures in the respective conchiferan clades. The non-collinear mode of Hox expression in Dreissena might be a result of the low degree of body plan regionalization along the bivalve anterior–posterior axis as exemplified by the lack of key morphological traits such as a distinct head, cephalic tentacles, radula apparatus, and a simplified central nervous system.
... Gbx is expressed in the developing shell field and possibly expressed in shell-secreting cells (Wollesen et al., 2015b). Surveys in many species have also shown SoxB1 and Engrailed expressed in the neuroectoderm and Hox genes expressed co-linearly in the developing shell field (Huan et al., 2020;Fritsch et al., 2015;Jacobs et al., 2000;Wollesen et al., 2018). These genes might also play a role in the development of adult sensory structures. ...
For centuries, the eye has fascinated scientists and philosophers alike, and as a result the visual system has always been at the forefront of integrating cutting-edge technology in research. We are again at a turning point at which technical advances have expanded the range of organisms we can study developmentally and deepened what we can learn. In this new era, we are finally able to understand eye development in animals across the phylogenetic tree. In this Review, we highlight six areas in comparative visual system development that address questions that are important for understanding the developmental basis of evolutionary change. We focus on the opportunities now available to biologists to study the developmental genetics, cell biology and morphogenesis that underlie the incredible variation of visual organs found across the Metazoa. Although decades of important work focused on gene expression has suggested homologies and potential evolutionary relationships between the eyes of diverse animals, it is time for developmental biologists to move away from this reductive approach. We now have the opportunity to celebrate the differences and diversity in visual organs found across animal development, and to learn what it can teach us about the fundamental principles of biological systems and how they are built.
