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Anttila CK, King RA, Ferris C, Ayres DR, Strong DR. Reciprocal hybrid formation of Spartina in San Francisco Bay. Mol Ecol 9: 765-770
Abstract
Diversity in the tRNALEU1 intron of the chloroplast genome of Spartina was used to study hybridization of native California cordgrass, Spartina foliosa, with S. alterniflora, introduced to San Francisco Bay approximately 25 years ago. We sequenced 544 bases of the tRNALEU1 intron and found three polymorphic sites, a pyrimidine transition at site 126 and transversions at sites 382 and 430. Spartina from outside of San Francisco Bay, where hybridization between these species is impossible, gave cpDNA genotypes of the parental species. S. foliosa had a single chloroplast haplotype, CCT, and this was unique to California cordgrass. S. alterniflora from the native range along the Atlantic coast of North America had three chloroplast haplotypes, CAT, TAA, and TAT. Hybrids were discriminated by random amplified polymorphic DNA (RAPD) phenotypes developed in a previous study. We found one hybrid that contained a cpDNA haplotype unknown in either parental species (TCT). The most significant finding was that hybridization proceeds in both directions, assuming maternal inheritance of cpDNA; 26 of the 36 hybrid Spartina plants from San Francisco Bay contained the S. foliosa haplotype, nine contained haplotypes of the invading S. alterniflora, and one had the cpDNA of unknown origin. Furthermore, cpDNA of both parental species was distributed throughout the broad range of RAPD phenotypes, suggesting ongoing contributions to the hybrid swarm from both. The preponderance of S. foliosa cpDNA has entered the hybrid swarm indirectly, we propose, from F1s that backcross to S. foliosa. Flowering of the native precedes by several weeks that of the invading species, with little overlap between the two. Thus, F1 hybrids would be rare and sired by the last S. foliosa pollen upon the first S. alterniflora stigmas. The native species produces little pollen and this has low viability. An intermediate flowering time of hybrids as well as pollen that is more vigourous and abundant than that of the native species would predispose F1s to high fitness in a vast sea of native ovules. Thus, spread of hybrids to other S. foliosa marshes could be an even greater threat to the native species than introductions of alien S. alterniflora.
... (Daehler and Strong, 1997). Only one chloroplast haplotype, CCT, is documented in S. foliosa populations and little information exists on the level of nuclear genetic diversity (Anttila et al, 2000). The lack of chloroplast diversity found in S. foliosa can be explained by it's geologically younger habitat of this Pacific species and the much smaller geographic range compared to S. alterniflora. ...
... The lack of chloroplast diversity found in S. foliosa can be explained by it's geologically younger habitat of this Pacific species and the much smaller geographic range compared to S. alterniflora. S. alterniflora has much greater molecular diversity in both the nuclear genomes (Perkins et al., 2002) and the chloroplast haplotypes CAT, TAA, and TAT (Anttila et al, 2000). ...
... foliosa begins flowering a few weeks before S. alterniflora, in June, and both species continue flowering through September) (Anttila et al 1998). S. alterniflora has a greater male fitness than S. foliosa, producing 21 times more viable pollen than the native and the S. alterniflora pollen increased the seed set of native plants almost eightfold over native pollen, resulting in hybrid swarms with up to 90% nuclear markers specific to S. alterniflora and displaying the chloroplast haplotype of S. foliosa (Anttila et al, 2000). This hybridization and recurrent backcrossing of closely related sister taxa results in new genotype combinations (Ainouche et al., 2003), but so little is currently known about this new invasive species. ...
... size of the plants), ecological characters such as tolerance to tidal submersion (higher in S. alterniflora ), and slight flowering precocity for S. foliosa (Daehler & Strong, 1997). Although limited information exists about the level of nuclear genetic diversity in S. foliosa populations over its entire range, only one chloroplast haplotype is recorded in this species (Anttila et al ., 2000), contrasting with the greater molecular diversity of S. alterniflora in both the chloroplast (Anttila et al., 2000) and nuclear (Perkins et al., 2002) genomes. ...
... size of the plants), ecological characters such as tolerance to tidal submersion (higher in S. alterniflora ), and slight flowering precocity for S. foliosa (Daehler & Strong, 1997). Although limited information exists about the level of nuclear genetic diversity in S. foliosa populations over its entire range, only one chloroplast haplotype is recorded in this species (Anttila et al ., 2000), contrasting with the greater molecular diversity of S. alterniflora in both the chloroplast (Anttila et al., 2000) and nuclear (Perkins et al., 2002) genomes. ...
... Hybridization has been shown to occur in both directions (Ayres et al., 1999). The greater male fitness of S. alterniflora that produces more viable pollen than the native species and recurrent back-crosses have resulted in hybrid swarms that display most frequently the chloroplast haplotype of S. foliosa and up to 90% nuclear markers specific to S. alterniflora (Anttila et al., 2000), according to the previously well-described 'chloroplast capture' process through pollen swamping (Rieseberg & Wendel, 1993), considered as a conservation threat to the native S. foliosa populations. ...
Hybridization and polyploidy are well illustrated in the genus Spartina. This paper examines how recent molecular approaches have helped our understanding of the past and recent reticulate history of species, with special focus on allopolyploid speciation. Spartina species are tetraploid, hexaploid or dodecaploid perennials, most of them being native to the New World. The molecular phylogeny indicates an ancient split between the tetraploid and the hexaploid species, with S. argentinensis as sister to the hexaploid lineage. Recent hybridization and polyploidization events involved hexaploid species, resulting from introductions of the east-American S. alterniflora. In California, ongoing hybridizations with its sister species S. foliosa result in introgressant hybrid swarms. In Europe, hybridization with S. maritima resulted in S. × neyrautii (France) and S. × townsendii (England), with. S. alterniflora as the maternal parent. The allopolyploid S. anglica resulted from chromosome doubling of S. × townsendii. This young allopolyploid contains divergent homoeologous subgenomes that have not undergone significant changes since their reunion. Hybridization, rather than genome duplication, appears to have shaped the allopolyploid genome at both the structural and epigenetic levels.
... size of the plants), ecological characters such as tolerance to tidal submersion (higher in S. alterniflora ), and slight flowering precocity for S. foliosa (Daehler & Strong, 1997). Although limited information exists about the level of nuclear genetic diversity in S. foliosa populations over its entire range, only one chloroplast haplotype is recorded in this species (Anttila et al ., 2000), contrasting with the greater molecular diversity of S. alterniflora in both the chloroplast (Anttila et al., 2000) and nuclear (Perkins et al., 2002) genomes. ...
... size of the plants), ecological characters such as tolerance to tidal submersion (higher in S. alterniflora ), and slight flowering precocity for S. foliosa (Daehler & Strong, 1997). Although limited information exists about the level of nuclear genetic diversity in S. foliosa populations over its entire range, only one chloroplast haplotype is recorded in this species (Anttila et al ., 2000), contrasting with the greater molecular diversity of S. alterniflora in both the chloroplast (Anttila et al., 2000) and nuclear (Perkins et al., 2002) genomes. ...
... Hybridization has been shown to occur in both directions (Ayres et al., 1999). The greater male fitness of S. alterniflora that produces more viable pollen than the native species and recurrent back-crosses have resulted in hybrid swarms that display most frequently the chloroplast haplotype of S. foliosa and up to 90% nuclear markers specific to S. alterniflora (Anttila et al., 2000), according to the previously well-described 'chloroplast capture' process through pollen swamping (Rieseberg & Wendel, 1993), considered as a conservation threat to the native S. foliosa populations. ...
Hybridization and polyploidy are well illustrated in the genus Spartina. This paper examines how recent molecular approaches have helped our understanding of the past and recent reticulate history of species, with special focus on allopolyploid spe-ciation. Spartina species are tetraploid, hexaploid or dodecaploid perennials, most of them being native to the New World. The molecular phylogeny indicates an ancient split between the tetraploid and the hexaploid species, with S. argentinensis as sister to the hexaploid lineage. Recent hybridization and polyploidization events involved hexaploid species, resulting from introductions of the east-American S. alterniflora. In California, ongoing hybridizations with its sister species S. foliosa result in intro-gressant hybrid swarms. In Europe, hybridization with S. maritima resulted in S. × neyrautii (France) and S. × townsendii (England), with. S. alterniflora as the maternal parent. The allopolyploid S. anglica resulted from chromosome doubling of S. × townsendii. This young allopolyploid contains divergent homoeologous sub-genomes that have not undergone significant changes since their reunion. Hybridi-zation, rather than genome duplication, appears to have shaped the allopolyploid genome at both the structural and epigenetic levels.
... This can be avoided by planting within a species' native range, but that creates another concern that has received far less attention: the genetic effects of large-scale plantings on native populations (Laikre, Schwartz, Waples, & Ryman, 2010). If introduced genotypes or alleles have a fitness advantage and/or they outnumber natives, the natural populations may be threatened by genetic swamping (Anttila, King, Ferris, Ayres, & Strong, 2000;Hufford & Mazer, 2003), or if the advantage is large, simple replacement by a more competitive genotype (Bayms, 2008). Furthermore, gene flow into native populations, especially if all plantings come from a common source, could lead to genetic homogenization across large parts of the planted species' natural range (Olden, Poff, Douglas, Douglas, & Fausch, 2004). ...
... The potential remains for planted material to promote within-region gene flow, especially where plantations are some distance from their source, and hence perhaps also local genetic swamping. However, for genetic swamping to occur, local adaptation of genotypes, and fitness variation between populations, must be small (Anttila et al., 2000;Hufford & Mazer, 2003). To test this possibility, within-region variation in climatic niche needs investigation alongside whether planted material is sometimes nonlocally sourced within a region. ...
Many natural systems are subject to profound and persistent anthropogenic influence. Human‐induced gene movement through afforestation and the selective transportation of genotypes might enhance the potential for intraspecific hybridization, which could lead to outbreeding depression. However, the evolutionary legacy of afforestation on the spatial genetic structure of forest tree species has barely been investigated. To do this properly, the effects of anthropogenic and natural processes must be examined simultaneously. A multidisciplinary approach, integrating phylogeography, population genetics, species distribution modelling and niche divergence would permit evaluation of potential anthropogenic impacts, such as mass planting near native material. Here, these approaches were applied to Pinus armandii, a Chinese endemic coniferous tree species, that has been mass planted across its native range. Population genetic analyses showed that natural populations of P. armandii comprised three lineages that diverged around the late Miocene, during a period of massive uplifts of the Hengduan Mountains, and intensification of Asian Summer Monsoon. Only limited gene flow was detected between lineages, indicating that each largely maintained is genetic integrity. Moreover, most or all planted populations were found to have been sourced within the same region, minimizing disruption of large‐scale spatial genetic structure within P. armandii. This might be because each of the three lineages had a distinct climatic niche, according to ecological niche modeling and niche divergence tests. The current study provides empirical genetic and ecological evidence for the site‐species matching principle in forestry, and will be useful to manage restoration efforts by identifying suitable areas and climates for introducing and planting new forests. Our results also highlight the urgent need to evaluate the genetic impacts of large‐scale afforestation in other native tree species.
... Spartina alterniflora plants spread rapidly with greater tolerance of tidal submersion (Daehler and Strong 1997). Hybridization between the native and the introduced species has been shown to occur in both directions (Antilla et al. 2000), with S. alterniflora exhibiting greater male fitness (Antilla et al. 1998). Recurrent back-crosses and pollen swamping have resulted in hybrid swarms that display most frequently the chloroplast genome of S. foliosa and up to 90% nuclear markers specific to S. alterniflora (Ayres et al. 1999;Antilla et al. 2000). ...
... Hybridization between the native and the introduced species has been shown to occur in both directions (Antilla et al. 2000), with S. alterniflora exhibiting greater male fitness (Antilla et al. 1998). Recurrent back-crosses and pollen swamping have resulted in hybrid swarms that display most frequently the chloroplast genome of S. foliosa and up to 90% nuclear markers specific to S. alterniflora (Ayres et al. 1999;Antilla et al. 2000). The fitness superiority (Ayres et al. 2007) and spread of the hybrids in California marshes ) is considered as a threat to the native S. foliosa populations (Ayres et al. 2003), and recent surveys have confirmed the great ecological impact of these hybrids is in the tidal marshes of the San Francisco Bay in California . ...
In this paper, we examine how the Spartina system has helped our understanding of the genomic aspects of allopolyploid speciation in the context of biological invasion. More specifically the respective roles of hybridization and genome duplication in the success of newly formed allopolyploid species are explored. Hybridization appears to have triggered genetic and epigenetic changes in the two recently formed European homoploid hybrids S. 9 towsendii and S. 9 neyrautii. Deviation from parental structural additivity is observed in both hybrids, with different patterns when considering transposable element insertions or AFLP and methylation alteration. No important changes are observed in the invasive allo-polyploid Spartina anglica that inherited the identical genome to S. 9 townsendii. The repeated rRNA genes are not homogenized in the allopolyploid, and both parental repeats are expressed in the populations examined. Transcriptomic changes suggest possible gene silencing in both hybrids and allopolyploid. In the long-term of evolutionary time, older hexaploid Spar-tina species (Spartina alterniflora, Spartina maritima and Spartina foliosa) appear to have selectively retained differential homeologous copies of nuclear genes. Waxy gene genealogies suggest a hybrid (allopolyploid) origin of this hexaploid lineage of Spartina. Finally, nuclear and chloroplast DNA data indicate a reticulate origin (alloheptaploid) of the invasive Spartina densiflora. All together these studies stress hybridization as a primary stimulus in the invasive success of polyploid Spartina species.
... Genetically diverse, introgressive hybrids Anttila et al ., 2000;Sloop, 2005) between Spartina foliosa Trin. and Spartina alterniflora Loisel. ...
... Hybridization between the two species was first reported in 1994 (Daehler & Strong, 1994). Further genetic work revealed a swarm of genetically diverse hybrids that resulted from back-and inter-hybrid crosses (Anttila et al ., 2000). Artificial F1 hybrids showed an additive pattern of genetic markers specific to each parental species; individuals with this pattern are rare in nature (D.A., unpublished). ...
We genetically analysed cordgrass plants and seedlings throughout the San Francisco, California, USA, estuary and found that hybrids between exotic Spartina alterniflora and native Spartina foliosa are the principal cordgrass invaders and colonizers. We hypothesized that this was due to higher seed set and siring ability by hybrids relative to the native species; too few alien parents remained in San Francisco Bay for our comparative studies. Hybrid seed comprised 91% to 98% of that set in the marsh study plants over the 2 years of the study. Total viable pollen production by hybrid plants was 400 times that of the native plants. Seed and pollen production were highly skewed towards a few hybrid genotypes. In addition to seed produced by hybrid plants, hybrid seed was produced by S. foliosa due to hybrid backcrossing. While the greatest advantage for hybrids was in pollen and seed production, hybrid seeds germinated, and seedlings survived and grew as well or better than the native species. As native S. foliosa becomes increasingly rare, hybrid seed floating on the tides will predominate, overwhelming recruitment sites and resulting in further colonization by hybrids. In an evolutionary context, hybrids with exceptional pollen and seed production will be initially favoured by natural selection, leading to the evolution of even more fertile hybrid genotypes.
... Spartina alterniflora plants spread rapidly with greater tolerance of tidal submersion (Daehler and Strong 1997). Hybridization between the native and the introduced species has been shown to occur in both directions (Antilla et al. 2000), with S. alterniflora exhibiting greater male fitness (Antilla et al. 1998). Recurrent back-crosses and pollen swamping have resulted in hybrid swarms that display most frequently the chloroplast genome of S. foliosa and up to 90% nuclear markers specific to S. alterniflora (Ayres et al. 1999; Antilla et al. 2000). ...
... Hybridization between the native and the introduced species has been shown to occur in both directions (Antilla et al. 2000), with S. alterniflora exhibiting greater male fitness (Antilla et al. 1998). Recurrent back-crosses and pollen swamping have resulted in hybrid swarms that display most frequently the chloroplast genome of S. foliosa and up to 90% nuclear markers specific to S. alterniflora (Ayres et al. 1999; Antilla et al. 2000). The fitness superiority (Ayres et al. 2007) and spread of the hybrids in California marshes (Ayres et al. 2004) is considered as a threat to the native S. foliosa populations (Ayres et al. 2003), and recent surveys have confirmed the great ecological impact of these hybrids is in the tidal marshes of the San Francisco Bay in California (Ayres et al. 2004). ...
In this paper, we examine how the Spartina system has helped our understanding of the genomic aspects of allopolyploid speciation in the context of biological invasion.
More specifically the respective roles of hybridization and genome duplication in the success of newly formed allopolyploid
species are explored. Hybridization appears to have triggered genetic and epigenetic changes in the two recently formed European
homoploid hybrids S.×towsendii and S.×neyrautii. Deviation from parental structural additivity is observed in both hybrids, with different patterns when considering transposable
element insertions or AFLP and methylation alteration. No important changes are observed in the invasive allopolyploid Spartina anglica that inherited the identical genome to S.×townsendii. The repeated rRNA genes are not homogenized in the allopolyploid, and both parental repeats are expressed in the populations
examined. Transcriptomic changes suggest possible gene silencing in both hybrids and allopolyploid. In the long-term of evolutionary
time, older hexaploid Spartina species (Spartinaalterniflora, Spartinamaritima and Spartinafoliosa) appear to have selectively retained differential homeologous copies of nuclear genes. Waxy gene genealogies suggest a hybrid (allopolyploid) origin of this hexaploid lineage of Spartina. Finally, nuclear and chloroplast DNA data indicate a reticulate origin (alloheptaploid) of the invasive Spartina densiflora. All together these studies stress hybridization as a primary stimulus in the invasive success of polyploid Spartina species.
... The present study also focused on the doubling time of Spartina patch growth during the early invasion stages because a solid understanding of the initial growth and spread is extremely important for the removal or control of the invasive species. In terms of the Spartina invasion, it is fairly well known that genetically differentiated hybrid Spartina alterniflora is highly invasive and might have facilitated its rapid range expansion (Anttila et al. 2000;Zhang et al. 2008;Ayres et al. 2008;Strong and Ayres 2013). ...
Spartina alterniflora Loisel, widely recognized as an aggressive invader of estuaries and salt marshes worldwide, was recently reported in Korean waters as rapidly invading intertidal mudflats, growing in circular patches. For more effective control management of the invasive cordgrass, we developed a modified ignition logistic model based on the satellite imageries to estimate the settlement time of the first individual stand and the doubling period of the patch spread in the early colonization state. The present model is designed for estimating the starting time and the doubling period of the patch spread at the growth stage, which is a salient feature different from other logistic models. The importance of estimating the starting time of the invasion may lie in figuring out what ecological changes occurred at that time. A Monte-Carlo simulation is combined with our model to obtain reliable predictions against the noisy data. As a result of applying the model to the Northwest Pacific invading tidal flats of Korea, China, and Japan, it turns out that the doubling periods of the patch spread are generally shorter and similar to each other, which range from 0.6 to 2.1 years. This is probably attributed to the genetically hybridized populations of Spartina alterniflora invading this region.
... Today S. anglica is shortlisted among the 100 most invasive species in the world (Lowe et al. 2000). Spartina alterniflora has a similar history in the San Francisco Bay, California, where repeated hybridization and backcrossing with the native S. foliosa resulted in hybrids (Anttila et al. 2000) that swarmed to the more saline parts of the coastal marshes (Lee et al. 2016) resulting in a loss of open intertidal habitats for birds and tidal communities , and vanished any coastal restoration effort (Lee et al. 2016). Spartina densiflora is a third invasive species within the Spartina complex. ...
Perennial tall-statured grasses are regarded as a sustainable source of renewable energy for their high yields of lignocellulosic biomass, low resource input, wide ecological tolerance and capacity for storing large amounts of atmospheric CO2 in their perennial underground rhizome systems. These same traits, that make such crops agronomically attractive and sustainable, make these species highly competitive and potentially invasive. Several perennial energy crop grasses are outbreeding species that belong to cosmopolitan polyploid species complexes, i.e. groups of interbreeding species with ploidy variation. The cultivation of such highly productive and genetically diverse crops can have unwanted consequences through the evolution of invasive species. The goal of this review is to provide the scientific community, including agronomists, breeders, users and nature managers, with an introduction to the genetic dynamics occurring within the polyploid species complexes of the emerging energy species Arundo donax, Miscanthus × giganteus, Panicum virgatum, Phalaris arundinacea and Phragmites australis, and the broad biogeographical extent of their gene flow impact. Such aspects are difficult to predict, and are not captured by invasion risk assessments and by the sustainability certifications of the bioenergy supply chain. The review integrates literature from the phylogenetic, cytology, population ecology and agronomic research and focuses on the evolutionary processes that shape invasiveness that can be activated post-introduction by the dispersal of pollen, seeds and plant fragments from the energy crops to the environment. Due to the high genetic diversity of the crops, the adverse effects that genetic drift and founder effect can have on the establishment of small populations are very unlikely. On the contrary the data collected suggests that the risk of fostering panmictic continental invasive populations is high. Agronomic measures, regulations and genetic improvements that can contain dispersal from crops are discussed, as well as urgent research needs.
... In California, exotic populations of S. alterniflora and native hexaploid Spartina foliosa Trin. hybridized in both directions (Anttila et al. 2000) that together with recurrent backcrossing resulted in an invasive hybrid swarms (Daehler and Strong 1997;Ayres et al. 1999Ayres et al. , 2008aAinouche et al. 2004). Data on distribution and spread of these swarms support the prediction that if no actions were taken to control the invasion of these hybrids, the extinction of both parental species as distinct genetic entities would occur within several decades to centuries (Hall et al. 2006;Ayres et al. 2004a, b;Strong and Ayres 2013). ...
Interspecific hybridization is a major mechanism for generating evolutionary novelty in plants. Halophytic Spartina species are often abundant in coastal salt marshes around the world, and they frequently hybridize. Despite a large body of knowledge on the ecology and evolution of Spartina species, taxonomic resources for distinction of complex taxa in the genus are scarce. The general aim of this study was to characterize parental species S. maritima (native) and S. densiflora (invasive) and their reciprocal hybrids formed in the Gulf of Cadiz (Southwest Iberian Peninsula). Our approach was to quantify and compare morphological and anatomical characters of tufts, culms, leaves and inlorescences and the limits of their variation in diferent ecological settings. Our results document discriminating trait variables from populations of both hybrids that can be used to distinguish them from their parental species and each other. From these findings and the veriication of type spe cimens, we describe two new nothotaxa, Spartina × onubensis subsp. onubensis andSpartina × onubensis subsp. tartes- siana, and develop taxonomic keys for identiication of the four studied taxa. Floral traits such as the length of the terminal spike, the spikelet and its glumes and lemma allow us to diferentiate S. densiflora (with lower values) from the other three taxa. The presence of exerted stamens, shorter culms and shorter leaf blades separated S. maritima from both hybrids. Each hybrid can also be distinguished since S. × onubensis subsp. onubensis develops more spikes per inlorescence and consistently has longer leaves than S. × onubensis subsp. tartessiana.)
... Considerable attention has been given to the genetics of this system as it existed before the intensive eradication efforts by the ISP (Anttila et al. 2000;Ayres et al. 1999Ayres et al. , 2008Blum et al. 2007;Daehler and Strong 1997;Sloop et al. 2011), but no published study has addressed hybrid Spartina genetics since those efforts began, nor since some of those efforts have been curtailed. An understanding of the changing dynamics of the invasion is critical to continued eradication progress for three reasons. ...
Spartina foliosa is native to the San Francisco, California, Bay and Estuary. Spartina alterniflora was introduced to the Bay in the 1970s and subsequently hybridized with S. foliosa. Backcrossing created an invasive hybrid swarm able to outcompete S. foliosa within its tidal range and spread into higher and lower intertidal zones, drastically altering ecological communities. The San Francisco Estuary Invasive Spartina Project (ISP) has implemented a treatment program that has reduced the net area of invasive Spartina by 96 % from its peak of 323 hectares. An endangered bird, the California Ridgway’s Rail, takes advantage of habitat characteristics supplied by hybrid Spartina, prompting treatment restrictions in some marshes to preserve hybrid Spartina for the benefit of rails. We investigated changes in the population genetics of the invasion after 10 years of eradication efforts and after three subsequent years without eradication in some locations. Our investigation covered three areas: sites surrounding and including the restricted treatment marshes; sites in the southern Bay similar to proposed salt pond restoration projects; and samples collected Baywide. Our results support previous descriptions of a system of self-fertile hybrid plants that supply seed for mostly localized recruitment into available habitat. Compared to published work from 2003 to 2004, before large-scale eradication efforts began, genetic diversity has generally declined and inbreeding within hybrid populations has increased during and after the period of treatment by the ISP. These observations are perhaps due to pollen limitation as a result of the elimination of most hybrid plants by the ISP. We did not detect strong population genetic boundaries Baywide, but a genetically distinguishable localized introduction was detected in 10 of 11 Bay regions, suggesting leptokurtic dispersal. The restricted treatment areas may allow for continued introgression with S. foliosa, recolonization, and longer persistence of these invasive plants in the San Francisco Bay. Proximity to hybrid Spartina threatens the success of major restoration projects.
... Hybrids were detected in the mid-1990s when plants were found that contained genetic material unique to each parent (Daehler and Strong 1997). The highly diverse nuclear and cytoplasmic composition together with chromosome numbers equal to or close to the parents' suggested that these plants comprised a swarm of backcrossing hybrids rather than an allopolyploidization, as per S. anglica (Ayres et al. 1999;Anttila et al. 2000). Much of the shoreline and most creeks flowing into the southern arm of San Francisco Bay were invaded by hybrid cordgrass by 2004 (see fig. 1.4). ...
Maritime Spartina species define and maintain the shoreline along broad expanses of
temperate coasts where they are native. The large Spartina species grow lower on the
tidal plane than other vascular plants; tall, stiff stems reduce waves and currents to
precipitate sediments from turbid estuarine waters. With the right conditions, roots grow
upward through harvested sediments to elevate the marsh. This engineering can alter
the physical, hydrological, and ecological environments of salt marshes and estuaries.
Where native, Spartinas are uniformly valued, mostly for defining and solidifying the
shore. The potential to terrestrialize the shore was the rationale of many of the scores of
Spartina introductions. In a time of rising sea levels, these plants are valued as a barrier
to the sea in native areas and in China and Europe where they have been cultivated.
In contrast, in North America, Australia, Tasmania, and New Zealand, nonnative
Spartinas are seen as a bane both to ecology and to human uses of salt marshes and
estuaries. Four of the seven large-scale invasions involved interspecific hybrids between
introduced and native Spartinas, or intraspecific hybridization between formerly
allopatric populations. Rapid evolution driven by selection of genotypes particularly
adapted for invasive behavior could be the cause of observed high spread rates of hybrid
cordgrass. The study of Spartina introductions is a rich mixture of social and basic
sciences, with interaction of human values, ecology, and evolution.
... However, we have relatively little information regarding the distribution of diversity at these spatial scales in natural marshes. Although there are numerous analyses of the genetic diversity and structure of S. alterniflora (Ayres et al. 1999, O'Brien & Freshwater 1999, Anttila et al. 2000, Richards et al. 2004, Travis et al. 2004, Travis & Hester 2005, Blum et al. 2007), most have focused on variation across sites or geographic regions, intentionally sampling at intervals large enough to minimize collecting multiple samples from the same clone. Only a handful of surveys (Travis et al. 2004) have compared patch diversity at multiple spatial scales (plots of 1, 6, and 12 m diameter) within sites: it is these within-site spatial scales that are likely most relevant to plant-plant and plant-animal ecological inter actions. ...
Fine-scale variation in both the number and relatedness of genetic individuals can influence a range of ecological and evolutionary processes. However, we often have little information from natural populations regarding the fine-scale distribution of genetic diversity. We quantified multiple metrics of genetic diversity in the widespread, dominant marsh plant Spartina alterniflora across 3 spatial scales in 16 natural marshes in the Florida Panhandle. We also examined correlations between genetic diversity and marsh plant species diversity, along with key environmental variables that may influence both. Most of the variance in S. alterniflora genetic structure was explained by variation within sampling areas across sites. S. alterniflora genetic diversity and marsh plant species diversity increased with spatial scale within sites. In addition, diversity was generally higher on mainland marsh sites compared to discrete marsh islands, consistent with theoretical predictions from island biogeography and population genetics. Multiple metrics of genetic diversity increased with island area and with distance to the mainland. Despite significant correlations between diversity and tidal height, sediment organic content, and sediment porewater salinity, site type (mainland or island) and spatial scale were the best predictors of S. alterniflora genetic and marsh plant species diversity. We show that natural populations of S. alterniflora exhibit significant fine-scale spatial genetic structure, and thus continued loss of habitat could lead to a substantial loss of genetic diversity.
... Secondly, evidence of hybrid swarms is mounting. A hybrid of eastern and western Spartina, a cordgrass, spread extremely rapidly in the San Francisco Bay, negatively affecting wildlife habitat [13]. The hybrid females of ...
Science has shown that the introgression or hybridization of modern humans (Homo sapiens) with Neanderthals up to 40,000 YBP may have led to the swarm of modern humans on earth. However, there is little doubt that modern trade and transportation in support of the humans has continued to introduce additional species, genotypes, and hybrids to every country on the globe. We assessed the utility of species distributions modeling of genotypes to assess the risk of current and future invaders. We evaluated 93 locations of the genus Tamarix for which genetic data were available. Maxent models of habitat suitability showed that the hybrid, T. ramosissima x T. chinensis, was slightly greater than the parent taxa (AUCs > 0.83). General linear models of Africanized honey bees, a hybrid cross of Tanzanian Apis mellifera scutellata and a variety of European honey bee including A. m. ligustica, showed that the Africanized bees (AUC = 0.81) may be displacing European honey bees (AUC > 0.76) over large areas of the southwestern U.S. More important, Maxent modeling of sub-populations (A1 and A26 mitotypes based on mDNA) could be accurately modeled (AUC > 0.9), and they responded differently to environmental drivers. This suggests that rapid evolutionary change may be underway in the Africanized bees, allowing the bees to spread into new areas and extending their total range. Protecting native species and ecosystems may benefit from risk maps of harmful invasive species, hybrids, and genotypes.
... Spartina alterniflora x foliosa, and an eradication program to return normal ecosystem function (Anttila et al. 2000;Ayres et al. 2004;Invasive Spartina Project 2003). ...
Invasive species frequently degrade habitats, disturb ecosystem processes, and can increase the likelihood of extinction of imperiled populations. However, novel or enhanced functions provided by invading species may reduce the impact of processes that limit populations. It is important to recognize how invasive species benefit endangered species to determine overall effects on sensitive ecosystems. For example, since the 1990s, hybrid Spartina (Spartina foliosa × alterniflora) has expanded throughout South San Francisco Bay, USA, supplanting native vegetation and invading mudflats. The endangered California clapper rail (Rallus longirostris obsoletus) uses the tall, dense hybrid Spartina for cover and nesting, but the effects of hybrid Spartina on clapper rail survival was unknown. We estimated survival rates of 108 radio-marked California clapper rails in South San Francisco Bay from January 2007 to March 2010, a period of extensive hybrid Spartina eradication, with Kaplan–Meier product limit estimators. Clapper rail survival patterns were consistent with hybrid Spartina providing increased refuge cover from predators during tidal extremes which flood native vegetation, particularly during the winter when the vegetation senesces. Model averaged annual survival rates within hybrid Spartina dominated marshes before eradication (Ŝ = 0.466) were greater than the same marshes posttreatment (Ŝ = 0.275) and a marsh dominated by native vegetation (Ŝ = 0.272). However, models with and without marsh treatment as explanatory factor for survival rates had nearly equivalent support in the observed data, lending ambiguity as to whether hybrid Spartina facilitated greater survival rates than native marshland. Conservation actions to aid in recovery of this endangered species should recognize the importance of available of high tide refugia, particularly in light of invasive species eradication programs and projections of future sea-level rise.
... Reductions in genetically pure Celastrus scandens (American bittersweet) populations have been attributed to hybridization with the non-native C. orbiculatus (oriental bittersweet) (Pooler et al., 2002). Similar reductions have occurred in native Spartina foliosa (California cordgrass), due to hybridization with the introduced Spartina alterniflora ( Anttila et al., 2000). ...
This paper identifies the historical precedents and recent advances in descriptive and analytical aspects of invasive plant ecology. The paper takes a global perspective that focuses primarily on natural and semi-natural systems. The dynamics of plant invasions depend on the unique combination of species and recipient environments in light of short-lived, but highly influential, stochastic events. Spreading from the original point of establishment can be virtually instantaneous or follow a prolonged timelag. Range extension proceeds according to a variety of patterns dependent on the interplay between dispersal modes and landscape characteristics. The impacts of plant invasions are all-encompassing: biodiversity loss, economic impacts and aesthetic impacts occasioned by the loss of traditional cultural or natural landscapes. From the conservation perspective, costs are incalculable, but undoubtedly high. The impacts of invasive plants on natural ecosystems occur across all levels of biotic organization and, in the worst case, result in global extinctions and modification of fundamental ecosystem properties that make restoration practically impossible. Plant invasions occur across all habitat types and have spawned complementary theories, which are briefly presented within particular contexts.
... Groups of plant species were tested at different times depending on when the plants were available and of suitable size. The P. marginata source population for all host-specificity testing was San Francisco Bay, California, where the host plant is the native S. foliosa as well as introduced S. alterniflora and its hybrids with S. foliosa (Anttila et al., 2000 ). This population was chosen because of its proximity to research facilities at Bodega Marine Laboratory and because of prior known impacts of this population of planthoppers on S. alterniflora from Willapa Bay (Daehler and Strong, 1997). ...
Spartina alterniflora (smooth cordgrass) is introduced and invasive in Pacific Coast estuaries of North America. Its invasion transforms unvegetated intertidal mudflats to grass-covered marshes, eliminating habitat for birds, fish, and native and cultivated shellfish that depend on the open mudflats. The delphacid planthopper Prokelisia marginata was recently introduced into Willapa Bay, Washington for biological control of this grass. Prior to its introduction, we demonstrated the narrow host range of P. marginata with no-choice tests in the greenhouse using 23 potential species of nontarget plants, including species of native and otherwise valuable grasses and cranberry, Vaccinium marcrocarpon. P. marginata was capable of completing its life cycle only on S. alterniflora, S. anglica (also a noxious weed in Washington State), and S. foliosa (California cordgrass, native southward from San Francisco). Based on these results, we found no evidence of risk to nontarget plants in Washington State from P. marginata. The first release of P. marginata was made in Willapa Bay in August 2000, and the planthopper survived the winter at all three sites. Following additional releases in early summer of 2001 of 65,000 individuals at each site, population densities increased an average of 4.34±1.71-fold in one generation. The populations had spread 200m from the release area by October, 2001. Macropterous (long-winged) individuals were more common (69%) within 5m of the release center, while brachypters (reduced-winged) were more frequent (71%) at distances greater than 5m from the release area. In field cages, P. marginata reduced S. alterniflora biomass by 50% and plant height by 15% in comparison to planthopper-free controls. These results represented short-term impacts at a localized scale. The ultimate success of this biocontrol program over wider spatial scales will only become known over a longer time period.
... Hybrids were detected in the mid-1990s when plants were found that contained genetic material unique to each parent (Daehler and Strong 1997). The highly diverse nuclear and cytoplasmic composition together with chromosome numbers equal to or close to the parents' suggested that these plants comprised a swarm of backcrossing hybrids rather than an allopolyploidization, as per S. anglica (Ayres et al. 1999;Anttila et al. 2000). Much of the shoreline and most creeks flowing into the southern arm of San Francisco Bay were invaded by hybrid cordgrass by 2004 (see fig. 1.4). ...
Maritime Spartina species define and maintain the shoreline along broad expanses of temperate coasts where they are native. The large Spartina species grow lower on the tidal plane than other vascular plants; tall, stiff stems reduce waves and currents to precipitate sediments from turbid estuarine waters. With the right conditions, roots grow upward through harvested sediments to elevate the marsh. This engineering can alter the physical, hydrological, and ecological environments of salt marshes and estuaries. Where native, Spartinas are uniformly valued, mostly for defining and solidifying the shore. The potential to terrestrialize the shore was the rationale of many of the scores of Spartina introductions. In a time of rising sea levels, these plants are valued as a barrier to the sea in native areas and in China and Europe where they have been cultivated. In contrast, in North America, Australia, Tasmania, and New Zealand, nonnative Spartinas are seen as a bane both to ecology and to human uses of salt marshes and estuaries. Four of the seven large-scale invasions involved interspecific hybrids between introduced and native Spartinas, or intraspecific hybridization between formerly allopatric populations. Rapid evolution driven by selection of genotypes particularly adapted for invasive behavior could be the cause of observed high spread rates of hybrid cordgrass. The study of Spartina introductions is a rich mixture of social and basic sciences, with interaction of human values, ecology, and evolution.
... Hybridization of S. alterniflora and S. foliosa created a backcrossing hybrid swarm that probably began soon after the introductions to Alameda Island (Sloop et al. 2011) and Hayward (Faber 2000). Both species were seed parents of hybrids (Anttila et al. 2000). F 1 hybrids were fewer than 4% of over 1,700 hybrids genetically tested, leading to the conclusion that the initial interspecific hybridization was rare and backcrossing among hybrids and with the parents, rather than frequent F 1 formation, drove the invasion (Ayres et al. 2008b) (see Supplemental Figure 1; follow the Supplemental Material link from the Annual Reviews ...
Spartina species of the mid-low intertidal areas are powerful ecological engineers that are highly valued where they are native. Elsewhere, they overgrow native salt marsh and open intertidal mudflats, diminish biota, increase costs of managing wildlife, and interfere with human uses of estuaries. Huge efforts have been mounted to kill some populations of invading Spartina. All large Spartina invasions are by S. densiflora (2n = 7x = 70) or S. alterniflora (2n = 6x = 62) or hybrids between the hexaploid species (2n = 6x). Hybridization is a recurrent theme in Spartina; the allododecaploid S. anglica (2n = 12x = 120) and the hybrid swarm in San Francisco Bay arose through the introductions of S. alterniflora into the range of native Spartina species. The ancient hybrid S. densiflora also hybridized with native Spartina species. Hybridization promotes the evolution of highly invasive populations and hampers control efforts. Whether Spartina, native and not, would protect the shore as sea levels rise depends upon unimpeded areas for upward marsh growth and sufficient sediment, conditions that are often not satisfied.
... Second, even if hybrids are fit, they may show signs of introgression, changing the population through genetic assimilation, as has occurred for example in Helianthus L. (Carney et al., 2000) and Spartina Schreb. (Anttila et al., 2000). In the case of Ipomopsis hybridization, hybrids with I. tenuituba as the mother and I. aggregata as the father had zero fitness in the home site for I. tenuituba, so hybridization would likely contribute further to decline of the population rather than to genetic assimilation. ...
Pollinators have long been implicated in plant speciation. Peter Raven's earlier work was instrumental in integrating foraging energetics of animals into our understanding of how shifts in major pollinators influence the evolutionary diversification of floral traits. More recently, efforts by Raven and others in the area of conservation have inspired pollination biologists to consider the implications of pollinator shifts and losses due to human activities. This paper uses the shift between hummingbird and hawkmoth pollination as a model for exploring impacts of pollinator shifts on plant populations. Recent studies have quantified the degree of reproductive isolation due to such pollinators in several genera. Data from Ipomopsis Michx. further allow us to consider whether recent changes in pollinator regimes have demographic consequences for plant populations. A majority of plant populations may currently suffer from pollen limitations on seed production, but few data exist on the demographic consequences of poor reproduction. In Ipomopsis, reduced seed production due to pollen limitation can impact the number of individuals surviving to reproduce in the next generation. Some populations of I. tenuituba (Rydb.) V. E. Grant are estimated to have finite rates of increase less than unity, which can be explained in part by current low levels of hawkmoth pollination. In the absence of an increase in hawkmoths, selection for wider corolla tubes and other floral traits could, in principle, attract enough hummingbird pollination to result in a growing population, but models show that such evolution by natural selection may leave the population vulnerable to local extinction. We need more studies of the quantitative demographic consequences of changes in pollinator regimes. Such studies should consider how evolutionary changes influence the risk of extinction.
... Analysis of RAPD nuclear markers , patterns of maternally inherited chloroplast DNA (Anttila et al. 2000), and artificial crossing attempts, taken together (D. R. Ayres, unpublished data), have resulted in the following scenario of Spartina hybridization dynamics. Hybridization has been bidirectional; that is, both species have been seed parents to hybrids. ...
In this article, we summarize previous and ongoing work examining the population and genetical consequences of the introduction of smooth cordgrass, a tidal marsh grass native to the Atlantic, into the range of California cordgrass in San Francisco Bay, CA, in the 1970s. The two species hybridized. A subset of hybrid genotypes outcompetes the native species, overgrows its niche space, produces much seed, and sires the majority of seed on native flowers. The result is the loss of existing native cordgrass plants and the usurpation of cordgrass regeneration sites by copious hybrid seed. These processes could lead to the extinction of the native species. Nomenclature: California cordgrass, Spartina foliosa Trin.; Smooth cordgrass, Spartina alterniflora Lois. Additional index words: Cordgrass, invasive Spartina, Spartina alterniflora, Spartina foliosa. Abbreviations: MLW, mean low water; MSL, mean sea level; RAPD, random amplified polymorphic DNA.
... In Pacific coast ecosystems, there are also examples of intrageneric invasions (Daehler & Strong 1996), as well as intraspecific hybridization events resulting in hybrids with an even greater vigour (Anttila et al. 2000). However, multiple introductions and ⁄ or hybridization events have not always resulted in a physiologically superior genotype (Brodersen, Lavergne & Molofsky 2008). ...
1. Over the last century, native Phragmites australis lineages have been almost completely replaced along the North American Atlantic coast by an aggressive lineage originating from Eurasia. Understanding the mechanisms that facilitate biological invasions is critical to better understand what makes an invasive species successful.
2. Our objective was to determine what makes the introduced lineage so successful in the study area by specifically investigating if morphological and ecophysiological differences exist between native and introduced genetic lineages of P. australis. We hypothesized a priori that due to phenotypic differences and differences in plant nitrogen (N) content between lineages, the introduced lineage would have a greater photosynthetic potential.
3. In situ ecophysiological and morphological data were collected for 2 years in a mid-Atlantic tidal marsh and in a glasshouse experiment. We measured photosynthetic parameters (Amax, water use efficiency, stomatal conductance) using infrared gas analysis, in conjunction with ecophysiological and morphological parameters [specific leaf area (SLA), leaf area, chlorophyll content, N content].
4. Introduced P. australis maintained 51% greater rates of photosynthesis and up to 100% greater rates of stomatal conductance which are magnified by its 38–83% greater photosynthetic canopy compared to the native type. The introduced lineage also had a significantly greater SLA and N content. Glasshouse-grown plants and naturally occurring populations demonstrated similar trends in ecophysiological characteristics, verifying the heritability of these differences. These ecophysiological differences, when combined with an extended growing season, provide the mechanism to explain the success of introduced P. australis in North America.
5. Our findings suggest the native type is a low-nutrient specialist, with a more efficient photosynthetic mechanisms and lower N demand, whereas the introduced type requires nearly four times more N than the native type to be an effective competitor.
6. Synthesis. Our study is the first to combine field and laboratory data to explain a biological invasion attributed to ecophysiological differences between genetic lineages. Our data corroborates earlier work suggesting anthropogenic modification of wetland environments has provided the state change necessary for the success of introduced P. australis. Finally, our results suggest that genotypic differences within species merit further investigations, especially when related to biological invasions.
... resent a novel mechanism of successional invasion and species replacement, in addition to playing an important role in the structuring of these salt marsh communities. Hybridization is well documented in salt marsh dominants of the genus Spartina , both in Europe (Raybould et al . 1991) and North America (Daehler & Strong 1997; Ayres et al . 1999; Anttila et al . 2000) and the best known example, Spartina anglica , has proved extremely invasive (e.g. Gray et al. 1991). However, all of these hybrids have resulted from an inter-or trans-continental alien introduction, rather than being a part of an indigenous succession. ...
We examined salt marsh development over 17 years (1984–2001) in a rapidly accreting, well‐drained lagoon of a macrotidal marsh in south‐west Spain. Scattered, elevated tussocks of colonizing Spartina maritima expanded radially until they either coalesced or were separated only by narrow drainage channels. We recorded changes in elevation of the tussocks and investigated the cover and biomass of successive species invading them.
Sediment accretion produced a mean annual increase in tussock elevation of 3.5 cm. Sarcocornia perennis had begun to invade the raised centres of the tussocks by 1984, subsequently displacing Spartina maritima radially to become dominant by 1990. A hybrid form of Sarcocornia , which appeared on some of the tussocks occupied by S. perennis in 1997, also expanded radially and had achieved dominance on many of the invaded tussocks by 2001, by virtue of its more erect growth form and rapid accumulation of high above‐ and below‐ground biomass.
Tussocks not yet invaded by the hybrid remain dominated by S. perennis and have maintained sediment accretion rates and redox potentials similar to those that have been invaded.
Genetic analysis, using random amplified DNA (RAPD) markers, indicated that the hybrid was a cross between the diploid Sarcocornia perennis (2 n = 18) and the octaploid S. fruticosa (2 n = 72), a high‐marsh species, and suggested that each hybrid individual may have resulted from a separate pollination of an indigenous S. perennis plant.
Invasion by the new hybrid thus probably occurred as a result of pollen flow from high‐marsh S. fruticosa , some 1 km distant, to the stigmas of the established dominant S. perennis . Succession might therefore be facilitated genetically rather than simply by the enhanced sediment accretion, which ameliorated the effects of submersion and low sediment redox potentials that presumably exclude S. fruticosa from lower parts of the marsh.
... Studies of hybrid formation have inferred multiple origins when both parental types of the uniparentally inherited chloroplast have been found in hybrid individuals (e.g., Anttila et al. 2000; see Soltis & Soltis 1993, p. 247). The chloroplasts of P. wawranum and P. oculatum are differentiated by an apparently fixed five-base-pair size difference in the rps4-trnS spacer region (Perrie 2001). ...
Evidence is provided for the first demonstrated example of allopolyploidy in the New Zealand fern flora. Cytological, morphological, and molecular (AFLP-DNA fingerprinting) analyses indicate that the fern previously known as Polystichum richardii constitutes an allopolyploid complex, in which four separate evolutionary lineages are present. These are here recognised as three taxonomic species, with one of these encompassing two subspecies. The two allo-octoploid lineages are accommodated under the reinstated name P. neozelandicum, each as a separate subspecies: P. neozelandicum subsp. neozelandicum and the new combination P. neozelandicum subsp. zerophyllum. The new combination P. wawranum is made for one of the tetraploid lineages, while the name P. oculatum is reinstated for the other. Polystichum richardii is a later synonym of P. neo-zelandicum, and hence is not a legitimate name for any of the taxa recognised here.
... The evidence from natural populations suggests that rare, successful hybrids with extreme phenotypes may play a significant role in plant evolution by causing the ecological expansion of one parental species following introgression (Lewontin & Birch 1966), ecological divergence of the hybrid lineage (Grant & Grant 1996) and speciation (Arnold 1993;Rieseberg et al. 2003). Additionally, several recent studies show hybridization facilitating species invasion (Ellstrand & Schierenbeck 2000) and creating phenotypes with unique enough ecologies to reengineer the ecosystem in which they occur (Daehler & Strong 1997;Anttila et al. 2000;Figueroa et al. 2003). For extreme phenotypic traits to persist in a hybrid lineage, they must have a positive or neutral effect on relative hybrid fitness (Arnold 1997). ...
Recent studies of hybridization have shown that extreme (e.g. transgressive) phenotypes can be generated during massive genetic recombination events such as interspecific hybridization. Extreme hybrid phenotypes appear to have the potential to create new lineages and lead to evolutionary divergence.
Hybrid genotypes of some salt marsh species have proven capable of reengineering their ecosystems, while in other cases, hybridization has transformed plant communities by facilitating species invasion.
Using the Louisiana irises, a naturally hybridizing group of species, we assessed the potential for hybrids of Iris brevicaulis and I. fulva to possess extreme physiological and growth traits. Our glasshouse experiment used two soil water environments (wet and flood) to measure hybrid trait expression at both the genotypic class and individual genotype level.
At the genotypic class level, two of the three hybrid classes (F 1 s, and backcrosses towards each parental species) were transgressive for at least one physiological trait (F 1 s for root mass proportion, specific leaf area and final leaf area; backcrosses to I. brevicaulis for leaf area ratio). Three fitness components (total biomass, ramet production and flower production) were measured on each plant in the experiment. All three hybrid classes had greater clonal fitness (total biomass) but not greater sexual fitness (flower number) than parent species.
At the individual genotype level, two F 1 hybrids showed extreme physiological trait expression: one for specific leaf area, and the other for shallow root allocation. Flower production was the only fitness component that showed some degree of environment‐dependence at the individual genotype level.
Although early generation hybrid classes can contain rare genotypes with extreme phenotypic trait expression, and hybrid fitness components were equal or superior to both parental species in most cases, there was not a strong association between transgressive traits and elevated hybrid fitness.
... Ecotonal hybrid zones in the marine environment are also known, and often involve introduced species; the seagrass Spartina alterniflora native to the Atlantic coast has invaded the Pacific coast of North America, and the hybrids formed with native Spartina spp. are highly fertile and can dominate extensive tracts of coastline (Anttila et al., 2000;Ayres et al., 2004). In each of these cases cited, isolating mechanisms appear to have broken down, allowing hybrids and backcrosses to proliferate within the ecotonal environment; however, neither the extent to which hybrid traits contribute to hybrid success nor the extent to which the ecotonal habitat gradient contributes to the breakdown of species boundaries is clear. ...
In the Upper Oldman River, Alberta, introduced non-native hatchery rainbow trout (Oncorhynchus mykiss) hybridize with native westslope cutthroat trout (O. clarkii), resulting in a hybrid swarm. Rainbow trout dominate at low elevations (< 1250 m) in the river mainstem, cutthroat in high-elevation tributaries (> 1400 m), and hybrids are numerically dominant in the mid-elevation range. We hypothesized that metabolism of rainbow trout would exceed that of cutthroat trout, and that the elevation gradient in genetic makeup would be mirrored by a gradient in metabolic traits, with intermediate traits in the hybrid-dominated ecotone. Metabolic traits were measured and regressed against the genetic makeup of individuals and elevation. Rainbow trout had higher oxygen consumption rates (OCRs), higher white muscle lactate dehydrogenase (LDH), and citrate synthase (CS) activity, and higher plasma acetylcholinesterase (AchE) than cutthroat trout. Hybrids had intermediate OCRs and AchE, but LDH activity as high as rainbow trout. While hybrid zones are usually modelled as a balance between cross species mating and selection against hybrids, ecotonal hybrid zones, where hybrids proliferate in intermediate habitats and have traits that appear well suited to ecotonal conditions, have been proposed for some plants and animals, and may have important implications for resource management and conservation. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 56–72.
... l . 1999a,b; Amsellem et al . 2000), allow transport vectors to be more easily identified (Fonseca et al . 2000), direct efforts for the discovery of appropriate biological control agents (Porter et al . 1995), and illuminate important changes in the biology of the invader during or shortly after introduction (Novak & Mack 1993; Ross et al . 1996; Anttila et al . 2000; Tsutsui et al . 2000; Tsutsui & Case 2001). Because historical human records are often incomplete, inaccurate or simply nonexistent, molecular genetics offers a powerful tool for the identification of putative source populations. Many factors, however, make this seemingly straightforward task difficult. Population bottlenecks during in ...
The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean-type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite-based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.
... The trnL-F and ITS regions were chosen for this study due to their success in evaluating interspecific variation (White et al. 1990; Gielly and Taberlet 1996; Kress et al. 2005). Combined analysis of chloroplast markers and segregation of nuclear markers can be used to determine whether backcrossing or selfing has occurred (Reiseberg et al. 1990; Anttila et al. 2000). Finally, we hope to determine if hybridization is a potential threat to the genetic integrity of L. leptostachya. ...
Natural hybridization is common in the genus Lespedeza. No hybrids between Lespedeza leptostachya Englem. and Lespedeza capitata Michx. are formally recognized in any of the current floras, however observations in the field suggest that hybridization
might occur in many of their shared habitats. Putative hybrids were compared to L. leptostachya and L. capitata using morphological measurements and screened for the presence of species-specific trnL-F gene region (cpDNA) and the ITS gene region (nrDNA). A discriminate analysis of 10 morphological measurements identified
the hybrids as intermediate to both parents with two PCA axes explaining 99% of the variation between taxa. The presence of
hybrids was confirmed by genetic markers with individuals morphologically identified as hybrids having cpDNA trnL-F genotypes identical to L. leptostachya and the ITS (nrDNA) phenotypes in most cases contain the ITS genotype of both parents, however, some putative hybrid individuals
contained the ITS genotype of only one parents. Those individuals with L. leptostachya ITS and trnL-F could be a case of misclassification, but the presence of both L. capitata ITS genotypes and L. leptostachya
trnL-F genotypes suggest segregation has occurred, which may result from either selfing or backcrossing.
Keywords
Lespedeza
-Hybrids-Genetic introgression-Threatened species-Morphological introgression
... Hybridization was shown to occur in both directions, although the introduced species has higher male fitness (Antilla et al. 1998). Rather than suffering from competition with S. alterniflora, S. foliosa is now threatened by introgressant hybrids that result from recurrent backrosses, and that threaten the genetic integrity of the native species (Ayres et al. 2000). Pollen swamping is also thought to have represented an important mechanism allowing hybridization and invasion in oaks (Petit et al. 2004). ...
Invasive species have long raised the interest of evolutionists, as they provide ideal systems to understand the evolutionary processes that determine and accompany the expansion of a species (Baker and Stebbins 1965). Pragmatically, invasions of non-native species or genotypes pose a major biological threat to native biodiversity and ecosystem functioning. While the ecology of these invasions has received considerable international attention, understanding the evolutionary dimensions of this problem remains rudimentary. The observation can be made however, that invasions have two main evolutionary consequences; first, they can result in the rapid evolution of introduced populations in new environments, and secondly, they can promote reticulate gene flow with related taxa at the population, ecotype, or species level (Mooney and Cleland 2001). Both processes will result in new genotype-by-environment interactions and subsequent genetic diversification.
... Molecular markers have been exploited to determine the extent and degree of hybridization between S. alterniflora and S. foliosa, and identity of S. alterniflora ecotypes (Daehler and Strong 1997;Ayres et al. 1999;Daehler et al. 1999;Anttila et al. 2000). Using both RAPD and Inter-Simple Sequence Repeat (ISSR) markers, Ayres and Strong (2001) confirmed multiple times that the hybrid origin of S. × townsendii from the cross between S. alterniflora and S. maritima and accounted for its extensive genetic variation to the hybridization of S. alterniflora with different S. maritima plants. ...
Salinity is one of the most serious abiotic stresses affecting crop productivity worldwide. Improving tolerance to salinity
in field crops is globally important because a majority of the world population relies on salt-sensitive crops such as rice,
corn, and wheat for their daily calories. Although there is no salt stress sensor yet identified, different signaling components
and tolerance mechanisms have been substantiated to a great extent in a glycophyte like Arabidopsis, and more recently in a few halophytes. With the rapid advances in genetics, genomics, and biochemical and transformation
tools, it is now possible to explore the genetic and molecular basis of the unusually high level of salt tolerance in halophilic
plants. We will focus on a halophyte grass, Spartina alterniflora, commonly known as smooth cordgrass, which possesses all known mechanisms of salt tolerance and subsequent exploitation of
its genome information for crop improvement. A number of candidate genes encoding transcription factors, ion transport, osmoprotectants,
antioxidants, detoxifying enzymes, etc. have been identified. Although recent efforts to develop salt tolerant cultivars that could retain the halophytic traits
through transgenesis show some promise, further exploration is needed to test the contribution of single or multiple salt
stress-related genes or regulatory factors from halophilic plants, including S. alterniflora, for possible utilization in crop improvement.
KeywordsAbiotic stress–Crop improvement–Genomics–Grass–Ion homeostasis–Salinity–Smooth cordgrass
Morus L. (Moraceae) is a temperate and subtropical genus of ten to 15 species distributed in Asia, Africa, Europe, North, Central and South America. Despite its broad distribution and economic importance, it has received little attention from systematic botanists. Two species of this genus, the native M. rubra and the exotic M. alba, co-occur in eastern North America including the Flint Hills region of the Central Plains. In this dissertation research, both species level and population level studies were conducted to obtain insights into the diversification of the genus Morus. At the species level, the objectives were to re-evaluate the taxonomy and reconstruct the phylogeny of Morus. Based on herbarium and literature study as well as some field study, 13 species were recognized: eight species occurring in Asia, one in Africa and four in the New World. Sequence data from the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA and the trnL-trnF region of the chloroplast DNA were used to reconstruct the evolutionary history of Morus. The phylogenies were congruent and indicate a) a monophyletic core group of Morus with two well-supported geographical clades (one containing Asian taxa and one of New World taxa); and b) that Morus, as currently circumscribed, is non-monophyletic. At the population level, sex expression pattern variation between the Morus native-exotic pair in the Flint Hills region was studied, and assessed was the hybridization between these species at Konza Prairie Biological Station (KPBS). Both species are subdioecious, and Flint Hills populations exhibit significantly male-biased sex ratios, with sex expression being size independent. Approximately 10% of individuals of each species changed sex annually. In the population study at KPBS, randomly amplified polymorphic DNA (RAPD) markers and microsatellites were applied to genotype individuals and populations. The Morus species were moderately ([Theta]II = 0.079; RAPD data) to highly differentiated genetically (F[subscript]ST = 0.233; microsatellite data). Analysis of genetic structure suggested interspecific gene flow and indicated the presence of later generation hybrids. The presence of the exotic congener may affect the existence and genetic integrity of the native species. Overall, these studies contribute to our understanding of diversity in this interesting plant study system.
Non-native species constitute one of the greatest threats to our indigenous biota. We used data from vegetation surveys of ground, shrub, and canopy strata from 1997-2001 at 399 randomly selected forests, wetlands, and grasslands to elucidate non-native plant pervasiveness throughout Illinois. The dominant non-native species in the ground layer of forests was Lonicera japonica Thunb., in wetlands it was Phalaris arundinacea L., in prairie grasslands it was Poa pratensis L., and across all grasslands it was Festuca arundinacea Schreb. Though rarely recognized as exotic, several of the most prevalent non-natives were introduced ecotypes or cultivars of cosmopolitan species. Conversely, some well-known exotics were surprisingly uncommon. Non-native species were more dominant in the ground cover (9% forests, 33% wetlands, 36% prairie grasslands, 76% secondary grasslands) than in the shrub and canopy strata across the state. Non-native ground cover varied regionally in forests and wetlands while, overall, the southern third of the state had significantly lower non-native cover. These regional patterns may be related to isolation from metropolitan areas, historical disturbances, current land use, unique edaphic features, as well as species-specific distributions for the most successful invaders. Our results show the extent to which non-native species have permeated Illinois habitats and replaced native plant communities.
Over the past decade, mounting evidence has shown the pervasive and escalating harmful impacts of invasive alien species on native species and ecosystems. Thousands of non-native species are established in the United States and many more worldwide. Few areas appear immune to invasions, some of which unfold over decades to centuries as species' ranges slowly expand across the landscape. A plethora of ecological effects have been attributed to invasive species, and other global change processes and widespread habitat destruction will likely multiply these effects. Many conservationists now consider invasive species among the top threats to biological diversity and are grappling with identifying pragmatic, effective solutions. They know that the future will involve mixed assemblages of native and non-native species and have important questions about which invasive species to tackle and which ones to ignore, and even whether to deliberately introduce certain species for restoration. Addressing these issues will push hard against the limits of today's ecological knowledge and our ability to predict the long-term ecological consequences of such decisions. Better envisioning the conservation goals, though, could help to guide the science.
The grass subtribe Sporobolinae contains six genera: Calamovilfa (5 spp. endemic to North America), Crypsis (10 spp. endemic to Asia and Africa), Psilolemma (1 sp. endemic to Africa), Spartina (17 spp. centered in North America), Sporobolus (186 spp. distributed worldwide), and Thellungia (1 sp. endemic to Australia). Most species in this subtribe have spikelets with a single floret, 1-veined (occasionally 3 or more) lemmas, a ciliate membrane or line of hairs for a ligule, and fruits with free pericarps (modified caryopses). Phylogenetic analyses were conducted on 177 species (281 samples), of which 145 species were in the Sporobolinae, using sequence data from four plastid regions (rpl32-trnL spacer, ndhA intron, rps16-trnK spacer, rps16 intron) and the nuclear ribosomal internal transcribed spacer regions (ITS) to infer evolutionary relationships and provide an evolutionary framework on which to revise the classification. The phylogenetic analysis provides weak to moderate support for a paraphyletic Sporobolus that includes Calamovilfa, Crypsis, Spartina, and Thellungia. In the combined plastid tree, Psilolemma jaegeri is sister to a trichotomy that includes an unsupported Urochondra-Zoysia clade (subtr. Zoysiinae), a strongly supported Sporobolus somalensis lineage, and a weakly supported Sporobolus s.l. lineage. In the ITS tree the Zoysiinae is sister to a highly supported Sporobolinae in which a Psilolemma jaegeri–Sporobolus somalensis clade is sister to the remaining species of Sporobolus s.l. Within Sporobolus s.l. the nuclear and plastid analyses identify the same 16 major clades of which 11 are strongly supported in the ITS tree and 12 are strongly supported in the combined plastid tree. The positions of three of these clades representing proposed sections Crypsis, Fimbriatae, and Triachyrum are discordant in the nuclear and plastid trees, indicating their origins may involve hybridization. Seven species fall outside the major clades in both trees, and the placement of ten species of Sporobolus are discordant in the nuclear and plastid trees. We propose incorporating Calamovilfa, Crypsis, Spartina, Thellungia, and Eragrostis megalosperma within Sporobolus, and make the requisite 35 new combinations or new names. The molecular results support the recognition of 11 sections and 11 subsections within Sporobolus s.l.; four sections are new: Airoides, Clandestini, Cryptandri, and Pyramidati; three sections are new combinations: Calamovilfa, Crypsis, and Spartina; four subsections are new combinations: Calamovilfa, Crypsis, Ponceletia, and Spartina; seven subsections are new: Actinocladi, Alterniflori, Floridani, Helvoli, Pyramidati, Spicati, and Subulati; 30 new combinations in Sporobolus: S. aculeatus, S. advenus, S. alopecuroides, S. alterniflorus, S. angelicus, S. arcuatus, S. bakeri, S. borszczowii subsp. acuminatus, S. borszczowii subsp. ambiguus, S. brevipilis, S. coarctatus, S. cynosuroides, S. densiflorus, S. factorovskyi, S. foliosus, S. hadjikyriakou, S. ×longispinus, S. maritimus, S. megalospermus, S. michauxianus, S. minuartioides, S. niliacus, S. pumilus, S. rigidus, S. rigidus var. magnus, S. spartinus, S. schoenoides, S. ×townsendii, S. turkestanicus, and S. vericolor; and five new names in Sporobolus: S. arenicola, S. ×eatonianus, S. hookerianus, S. mobberleyanus, and S. vaseyi are made. Lectotypes are designated for Crypsis factorovskyi, Heleochloa ambigua, and Torgesia minuartioides.
Spartina alterniflora Losiel. is a highly invasive species found on the Chinese coast. To characterize the evolutionary mechanisms underlying the success of S. alterniflora in China, we examined the distribution and structure of genetic variation among three native populations at their source locations and five non-native populations in China, at both the chloroplast and nuclear microsatellite loci. Both cpDNA and microsatellite data revealed comparable genetic diversity and population differentiation between the native and introduced regions, which is consistent with the deliberate historical planting of heterogeneous founding groups in China. Bayesian clustering analysis showed that the best two clusters in the introduced region correspond to populations from the Atlantic coast and Gulf coast, respectively, instead of the three ecotypes proposed by a previous study. Investigating genetic composition of individuals suggested that most of the non-native plants might be of Atlantic origin, while some individuals in the Hong Kong population could be genetic admixtures of Atlantic and Gulf origin. This study indicated varied genetic components among populations in China, which imply different sources for the present Chinese populations.
1.Spartina species (cordgrasses) have been introduced to the estuaries around Europe, USA, Australia, New Zealand and Asia as a coastal management tool to stabilise mud banks, and through accidental introductions. These mainly non-native species are highly aggressive in their new environment, and frequently become the dominant plant species, displacing native flora and fauna.2.The majority of organisations managing estuarine environments within the USA and UK have established integrated control programmes to eradicate Spartina spp. These plans can be expensive to deliver and take many years. For example, the Willapa Bay, WA control programme in 2003 had been implemented for over 3 years, but with an annual cost of approximate $2 m year−1 (∼£1.2 m year−1).3.Through the use of the systematic review process and meta-analysis, the effectiveness of management interventions (chemical, mechanical and biological) used to control or eradicate invasive Spartina spp. were investigated.4.The effectiveness of glyphosate and imazapyr herbicides at reducing S. alterniflora differ greatly depending upon application method and whether surfactant or wetting agents are used. Cutting combined with smothering is highly effective at reducing S. anglica densities, but its efficacy has not been established for other Spartina spp. Cutting alone increases densities of S. anglica and S. townsendii, while reducing S. alterniflora densities. Biological control, while in its infancy, has the potential to significantly reduce S. anglica densities, while against S. alterniflora it might be of benefit as part of an integrated programme.5.The results highlight the need to establish targeted control programmes, dependent upon the Spartina spp., the underlying site characteristics and the available budget. Where the evidence base is limited or absent, recommendations of further experiments are presented, including a call for improved reporting of site characteristics and experimental methodology, allowing managers to establish whether trial results are applicable to their own sites and therefore limiting the implementation of ineffective management interventions. Copyright © 2007 John Wiley & Sons, Ltd.
Spartina anglica arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced East American Spartina alterniflora and following genome duplication of the hybrid (S. × townsendii). This system allows investigations of the early evolutionary changes that accompany stabilization of a new allopolyploid species in natural populations. Various molecular data indicate that S. anglica has resulted from a unique parental genotype. This young species contains two distinctly divergent homoeologous genomes that have not undergone extensive change since their reunion. No burst of retroelements has been encountered in the F1 hybrid or in the allopolyploid, suggesting a ‘structural genomic stasis’ rather than ‘rapid genomic changes’. However, modifications of the methylation patterns in the genomes of S. × townsendii and S. anglica indicate that in this system, epigenetic changes have followed both hybridization and polyploidization. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82, 475–484.
Spartina alterniflora, a native species at the east coast of North America, is currently the focus of increasing management concern due to its rapid expansion in coastal China. To better understand the plant traits associated with the success of invasion, we examined the genetic variation and the possible existence and distribution of ecotype hybrids and ecotype mixtures of the species in China. We collected and analyzed 144 samples from seven populations throughout coastal China (21.6°–38.6°N; 109.7°–121.8°E) using amplified fragment length polymorphisms markers. The results indicated that the genetic diversity of S. alterniflora in China was low, with a percentage of polymorphic bands (PPB) = 23.24%, heterozygosity (HE) = 0.2979 and the average diversity within populations = 0.4427 at the population level, as well as PPB = 28.07%, HE = 0.3743 and the total diversity of the species = 0.5520 at the species level. The majority of the variation was found within rather than among populations, as estimated by Nei's genetic diversity (20.39%), Shannon's index (19.80%) and analysis of molecular variance (23.56%). Results of assignment show that both ecotype hybrids and ecotype mixtures exist in coastal China, especially in southern populations. Therefore, the species’ success in coastal China may be attributable largely to the coexistence of various ecotype hybrids and ecotype mixtures resulting from three independent introductions of the species’ three ecotypes from their native ranges coupled with increasing human disturbances in the region.
Conservation of native biological diversity is one of the major challenges of this century. Invasive alien species (IAS) are a part of this challenge because a small but a significant fraction of IAS contribute to the demise of native species. The present paper reviews current literature on the phenomenon of hybridization in plants supporting the evolution of invasiveness, which is identified as a major threat to the extinction of native species. Further, the phenomenon of hybridization is been reviewed on a Sri Lankan context by citing evidence from a preliminary study between the native Nymphaea nouchali and an introduced violet flowered water-lily that has got established in water-bodies of Sri Lanka. This violet flowered water-lily has been incorrectly identified as the native Nymphaea nouchali, the National flower of Sri Lanka ('Nil Manel'). Revealing the flawed identity of the violet flowered Nymphaea, its hybrid origin and the hybridization with the native N. nouchali, provides an excellent opportunity for initiating studies on a novel area in hybridization between natives and invasive alien species of plants in Sri Lanka.
The literature on biological invasions has principally focused on understanding the ecological controls and consequences of invasions. Invading populations, however, often experience rapid evolutionary changes associated with or soon after their introduction. Ecological and evolutionary processes can, therefore, potentially interact over relatively short timescales. A number of recent studies have begun to document these interactions and their effect on short-term invasion dynamics: (1) The degree to which founder effects, drift, and inbreeding alter the genetic composition of introduced populations is mediated by migration and dispersal patterns, the population dynamics of founding populations, and life history. The genetic changes associated with founding can themselves feed back on population dynamics and life history. (2) Patterns of human-mediated dispersal and landscape change can influence the frequency and pattern of hybridization, which in turn can alter invasion dynamics. These altered invasion dynamics can influence the frequency and pattern of subsequent hybridization and introgression. (3) Strong selection can rapidly generate ecotypic specialization. Dispersal patterns, founder effects, genetic system, and life history influence the rate of local adaptation, its persistence, and its distribution in a landscape. (4) Introduced populations are subject to selection on life history traits and can serve as selective pressure on the life history traits of native populations. Life history evolution in both natives and aliens can influence ecological interactions and population dynamics, which in turn can influence the evolution of life history. Too few studies have investigated these interactions to definitively assess their overall generality or to determine how the relative interaction strength of ecology and evolution varies across taxa or ecosystems. However, the studies that do exist report interactions from a wide breadth of taxa and from all stages in the invasion process. This suggests that ecological-evolutionary interactions may have a more pervasive influence on contemporary invasion dynamics than previously appreciated, and that at least in some situations an explicit understanding of the contemporary co-influence of ecology and evolution can produce more effective and predictive control strategies.
Polyploidy and recurrent interspecific hybridization represent major features of Spartina evolution, resulting in several superimposed divergent genomes that coexist in the currently living species. This chapter summarizes what we presently know about Spartina history, emphasizing the recent hybridization and polyploidization events that have important ecological and evolutionary consequences. Particular attention is devoted to the recent formation of the allododecaploid invasive Spartina anglica, a salt-marsh “ecosystem engineer” that resulted from hybridization between the hexaploid S. alterniflora (introduced from North America) and tetraploid S. maritima (a European native) and subsequent genome duplication of the F1 hybrid S. x townsendii during the nineteenth century in Western Europe. Allopolyploidy was not accompanied by substantial restructuring of the parental genomes, as observed in some other allopolyploid systems. The major evolutionary events affect the regulatory systems controlling gene expression (including epigenetic regulation), which appear to have been profoundly altered by the merger of different genomes. Methodological challenges in exploring non-model, highly redundant genomes resulting from superimposed events of polyploidization (such as those encountered in Spartina) and the contribution of the new massive parallel sequencing technologies are discussed.
Saltmarshes are a major, widely distributed, intertidal habitat. They are dynamic systems, responding to changing environmental conditions. For centuries, saltmarshes have been subject to modification or destruction because of human activity. In this review, the range of factors influencing the survival of saltmarshes is discussed. Of critical importance are changes in relative sea level and in tidal range. Relative sea level is affected by changes in absolute sea level, changes in land level and the capacity of saltmarshes to accumulate and retain sediment. Many saltmarshes are starved of sediment because of catchment modification and coastal engineering, or exposed to erosive forces, which may be of natural origin or reflect human interference. The geographical distribution of individual saltmarsh species reflects climate, so that global climatic change will be reflected by changes in distribution and abundance of species, although the rate of change in communities dominated by perennial plants is difficult to predict. Humans have the ability to create impacts on saltmarshes at a range of scales from individual sites to globally. Pressures on the environment created by the continued increase in the human population, particularly in developing tropical countries, and the likely consequences of the enhanced greenhouse effect on both temperature and sea level give rise to particular concerns. Given the concentration of population growth and development in the coastal zone, and the potential sensitivity of saltmarsh to change in sea level, it is timely to review the present state of saltmarshes and to assess the likelihood of changes in the near (25 years) future. By 2025, global sea level rise and warming will have impacts on saltmarshes. However, the most extensive changes are likely to be the direct result of human actions at local or regional scales. Despite increasing recognition of the ecological value of saltmarsh, major projects involving loss of saltmarshes but deemed to be in the public interest will be approved. Pressures are likely to be particularly severe in the tropics, where very little is known about saltmarshes. At the local scale the cumulative impacts of activities, which individually have minor effects, may be considerable. Managers of saltmarshes will be faced with difficult choices including questions as to whether traditional uses should be retained, whether invasive alien species or native species increasing in abundance should be controlled, whether planned retreat is an appropriate response to rising relative sea level or whether measures can be taken to reduce erosion. Decisions will need to take into account social and economic as well as ecological concerns.
To evaluate the effects of intraspecific hybridization of local and nonlocal genotypes on growth traits of progeny in Abies sachalinensis, we performed reciprocal crossing between nonlocal trees in a high-elevation zone (1,100–1,200 m asl) and local trees in a low-elevation zone (530 m asl) in Hokkaido, northern Japan, in 1979 and established a common garden experiment using local × local (female × male ), local × nonlocal , nonlocal × local , and nonlocal × nonlocal progeny in the low-elevation zone in 1986. Survival, height, diameter at breast height (dbh), needle nitrogen content, specific leaf area (SLA), and needle area per shoot diameter (NA) of 25-year-old progeny were measured in 2005. The survival rate was consistently high (>85% on average). Reductions in height and dbh were apparent in F1 hybrids compared with local × local progeny. Furthermore, outbreeding depression was significant in height growth of nonlocal × local F1 hybrids and in dbh of both F1 hybrids. Reductions in growth traits may be related to morphological needle traits, such as the low values of SLA and NA. Elevation guidelines for A. sachalinensis seed zones are discussed to ensure the long-term viability of both restored and native populations.
Aim To examine the distribution and structure of genetic variation among native Spartina alterniflora and to characterize the evolutionary mechanisms underlying the success of non‐native S. alterniflora .
Location Intertidal marshes along the Atlantic, Gulf and Pacific coasts of North America.
Methods amova , parsimony analysis, haplotype networks of chloroplast DNA (cpDNA) sequences, neighbour‐joining analysis, Bayesian analysis of population structure, and individual assignment testing were used.
Results Low levels of gene flow and geographic patterns of genetic variation were found among native S. alterniflora from the Atlantic and Gulf coasts of North America. The distribution of cpDNA haplotypes indicates that Atlantic coast S. alterniflora are subdivided into ‘northern’ and ‘southern’ groups. Variation observed at microsatellite loci further suggests that mid‐Atlantic S. alterniflora are differentiated from S. alterniflora found in southern Atlantic and New England coastal marshes. Comparisons between native populations on the Atlantic and Gulf coasts and non‐native Pacific coast populations substantiate prior studies demonstrating reciprocal interspecific hybridization in San Francisco Bay. Our results corroborate historical evidence that S. alterniflora was introduced into Willapa Bay from multiple source populations. However, we found that some Willapa Bay S. alterniflora are genetically divergent from putative sources, probably as a result of admixture following secondary contact among previously allopatric native populations. We further recovered evidence in support of models suggesting that S. alterniflora has secondarily spread within Washington State, from Willapa Bay to Grays Harbor.
Main conclusions Underlying genetic structure has often been cited as a factor contributing to ecological variation of native S. alterniflora . Patterns of genetic structure within native S. alterniflora may be the result of environmental differences among biogeographical provinces, of migration barriers, or of responses to historical conditions. Interactions among these factors, rather than one single factor, may best explain the distribution of genetic variation among native S. alterniflora . Comprehensive genetic comparisons of native and introduced populations can illustrate how biological invasions may result from dramatically different underlying factors – some of which might otherwise go unrecognized. Demonstrating that invasions can result from several independent or interacting mechanisms is important for improving risk assessment and future forecasting. Further research on S. alterniflora not only may clarify what forces structure native populations, but also may improve the management of non‐native populations by enabling post‐introduction genetic changes and the rapid evolution of life‐history traits to be more successfully exploited.
Four species of exotic cordgrass (Spartina sp.) occur in the San Francisco estuary in addition to the California native Spartina foliosa. Our goal was to map the location and extent of all non-native Spartina in the estuary. Hybrids of S. alterniflora and S. foliosa are by far the most numerous exotic and are spreading rapidly. Radiating from sites of deliberate introduction, S. alterniflora and hybrids now cover ca. 190 ha, mainly in the South and Central Bay. Estimates of rate of aerial increase range from a constant value to an accelerating rate of increase. This could be due to the proliferation of hybrid clones capable of rapid expansion and having superior seed set and siring abilities. The total coverage of 195 ha by hybrids and other exotic cordgrass species is slightly less than 1% of the Bay's tidal mudflats and marshes. Spartina anglica has not spread beyond its original 1970s introduction site. Spartina densiflora has spread to cover over 5 ha at 3 sites in the Central Bay. Spartina patens has expanded from 2 plants in 1970 to 42 plants at one site in Suisun Bay. Spartina seed floats on the tide, giving it the potential to export this invasion throughout the San Francisco estuary, and to estuaries outside of the Golden Gate. We found isolated plants of S. alterniflora and S. densiflora in outer coast estuaries north of the Bay suggesting the likelihood for the San Francisco Bay populations to found others on the Pacific coast.
Mediterranean red-legged (Alectoris rufa) and rock (Alectoris graeca) partridge populations are affected by genetic pollution. The chukar partridge (Alectoris chukar), a species only partly native to Europe, is the most frequently introgressive taxon detected in the genome of hybrid partridges.
Both theoretical (evolutionary) and practical (resources management) matters spur to get insight into the geographic origin
of the A. chukar hybridizing swarm. The phenotypic A. rufa populations colonizing the easternmost part of the distribution range of this species, the islands of Elba (Italy) and Corsica
(France), were investigated. The analysis of both mitochondrial (mtDNA: Cytochrome-b gene plus Control Region: 2,250 characters) and nuclear (Short Tandem Repeats, STR; Random Amplified Polymorphic DNA, RAPD)
genomes of 25 wild (Elba) and 20 captive (Corsica) partridges, disclosed spread introgression of chukar origin also in these
populations. All mtDNA haplotypes of Elba and Corsica partridges along with those we obtained from other A. rufa (total, n=111: Italy, Spain, France) and A. graeca (n=6, Italy), were compared with the mtDNA haplotypes of chukars (n=205) sampled in 20 countries. It was found that the A. chukar genes detected in red-legged (n=43) and rock partridges (n=4) of Spain, France and Italy as well as in either introduced (Italy) or native (Greece, Turkey) chukars (n=35) were all from East Asia. Hence, a well-defined geographic origin of the exotic chukar genes polluting the genome of
native Mediterranean A. rufa and A. graeca (inter-specific level) as well as A. chukar (intra-specific level), was demonstrated.
This paper examines two assumptions that have formed the basis for much of the past and present work on hybrid zones. These assumptions derive from the observation that crosses between genetically divergent individuals (e. g., from different subspecies, species, etc.) often give rise to genotypes that are less fertile or less viable than those produced from crosses between genetically similar individuals. The first assumption is that natural hybridization will not affect the evolutionary history of the hybridizing forms because there is a low probability of producing novel genotypes with higher relative fitness. The second viewpoint is more extreme in that it assumes that all hybrid genotypes will be less fit. Even if rare gene flow does occur it will thus not contribute to patterns of diversification or adaptation because the hybrids will always be selected against. Examples from both plant and animal hybridization are discussed that are not consistent with these assumptions. Numerous instances of natural hybridization are used to demonstrate that extremely low fertility or viability of early-generation hybrids (e.g., F-1, F-2, B-1) does not necessarily prevent extensive gene flow and the establishment of new evolutionary lineages. In addition, it is demonstrated that various hybrid genotypes have equivalent or higher fitness than their parents in certain habitats.
1. This paper models empirically how habitat loss in winter might affect the size of the European population of oystercatchers Haematopus ostralegus ostralegus. It explores how a density-dependent mortality rate in winter interacts with a density-dependent production rate in summer to determine the total, year-round population size following a loss of winter habitat which itself leads to intensified competition for food and hence increased winter mortality rates. 2. Simulations over a range of probable parameter values show that the density at which winter mortality becomes density-dependent, cW, simply determines the point at which population size is affected as habitat is gradually removed. The population is affected sooner in the more widely fluctuating Continental sub-populations than in the less variable Atlantic subpopulations. 3. Once winter density reaches cW, the consequences depend on the slope, bW, of the density-dependent winter mortality function. In all subpopulations, the reduction in population size increases sharply as bW increases, but only at low values; above a certain level, further increases in bW make less difference. Because of their higher reproductive rate, inland subpopulations are initially less affected by winter habitat loss than coastal subpopulations. These conclusions are robust over a range of assumptions about competition for territories in summer and age difference in mortality in winter. 4. Adding density-dependent fledging success to the basic model reduces the effect of winter habitat loss on population size, but only when low proportions of the habitat are removed. A higher mortality rate in females, whether only in post-fledging young birds or in birds of all ages, makes little additional difference to the population consequences of habitat loss. 5. Field studies on winter habitat loss in migratory bird populations should first test whether density has already reached the critical level, cW; i.e. whether some birds already die of food competition. The parameter bW should then be estimated to determine whether its probable value lies in the range within which predictions are sensitive or insensitive to its precise value. Whether the summer density-dependent functions are linear or curvilinear needs also to be explored, as does the effect of interactions between subpopulations which have different fledgling production rates but share the same winter habitat.
This paper examines two assumptions that have formed the basis for much of the past and present work on hybrid zones. These assumptions derive from the observation that crosses between genetically divergent individuals (e.g., from different subspecies, species, etc.) often give rise to genotypes that are less fertile or less viable than those produced from crosses between genetically similar individuals. The first assumption is that natural hybridization will not affect the evolutionary history of the hybridizing forms because there is a low probability of producing novel genotypes with higher relative fitness. The second viewpoint is more extreme in that it assumes that all hybrid genotypes will be less fit. Even if rare gene flow does occur it will thus not contribute to patterns of diversification or adaptation because the hybrids will always be selected against. Examples from both plant and animal hybridization are discussed that are not consistent with these assumptions. Numerous instances of natural hybridization are used to demonstrate that extremely low fertility or viability of early-generation hybrids (e.g., F 1, F 2, B 1) does not necessarily prevent extensive gene flow and the establishment of new evolutionary lineages. In addition, it is demonstrated that various hybrid genotypes have equivalent or higher fitness than their parents in certain habitats.
Spartina alterniflora, smooth cordgrass, native to the eastern USA, was introduced into south San Francisco Bay ≈ 25 years ago. It has spread by purposeful introduction of rooted plants and dispersal of seeds on the tides. Previous work suggested that S. alterniflora was competitively superior to the native California cordgrass, S. foliosa, and that the two species hybridized. The present study determined the spread of S. alterniflora and S. foliosa × alterniflora hybrids in California and examined the degree of hybridization. We used nuclear DNA markers diagnostic for each species to detect the parental species and nine categories of hybrids. The California coast outside San Francisco Bay contained only the native species. All hybrid categories exist in the Bay, implying that several generations of crossing have occurred and that hybridization is bidirectional. Hybrids were found principally near sites of deliberate introduction of the exotic species. Where S. alterniflora was deliberately planted, we found approximately equal numbers of S. alterniflora and hybrid individuals; S. foliosa was virtually absent. Marshes colonized by water-dispersed seed contained the full gamut of phenotypes with intermediate-type hybrids predominating. The proliferation of hybrids could result in local extinction of S. foliosa. What is more, S. alterniflora has the ability to greatly modify the estuary ecosystem to the detriment of other native species and human uses of the Bay. To the extent that they share these engineering abilities, the proliferation of cordgrass hybrids could grossly alter the character of the San Francisco Bay.
Introduced Spartina alterniflora (smooth cordgrass) is rapidly invading intertidal mudflats in San Francisco Bay, California. At several sites, S. alterniflora co-occurs with native S. foliosa (California cordgrass), a species endemic to California salt marshes. In this study, random amplified polymorphic DNA markers (RAPDs) specific to each Spartina species were identified and used to test for hybridization between the native and introduced Spartina species in the greenhouse and in the field. Greenhouse crosses were made using S. alterniflora as the pollen donor and S. foliosa as the maternal plant, and these crosses produced viable seeds. The hybrid status of the crossed offspring was confirmed with the RAPD markers. Hybrids had low self-fertility but high fertility when back-crossed with S. foliosa pollen. Hybrids were also found established at two field sites in San Francisco Bay; these hybrids appeared vigorous and morphologically intermediate between the parental species. Field observations suggested that hybrids were recruiting more rapidly than the native S. foliosa. Previous work identified competition from introduced S. alterniflora as a threat to native S. foliosa. In this study, we identify introgression and the spread of hybrids as an additional, perhaps even more serious threat to conservation of S. foliosa in San Francisco Bay.
Hybridization with abundant invaders is a well-known threat to rare native species. Our study addresses mechanisms of hybridization between a rare invader, smooth cordgrass (Spartina alterniflora) and the common native California cordgrass (S. foliosa) in the salt marshes of San Francisco Bay. These species are wind-pollinated and flower in summer. The invader produced 21-fold the viable pollen of the native, and 28% of invader pollen germinated on native stigmas (1.5-fold the rate of the native's own pollen). Invader pollen increased the seed set of native plants almost eightfold over that produced with native pollen, while native pollen failed to increase seed set of the invader. This pollen swamping and superior siring ability by the invader could lead to serial genetic assimilation of a very large native population. Unlike California cordgrass, smooth cordgrass can grow into low intertidal habitats and cover open mud necessary to foraging shorebirds, marine life, navigation, and flood control in channels. To the extent that intertidal range of the hybrids is more similar to the invader than to the native parent, introgression will lead to habitat loss for shore birds and marine life as well to genetic pollution of native California cordgrass.
Six primers for the amplification of three non-coding regions of chloroplast DNA via the polymerase chain reaction (PCR) have been designed. In order to find out whether these primers were universal, we used them in an attempt to amplify DNA from various plant species. The primers worked for most species tested including algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The fact that they amplify chloroplast DNA non-coding regions over a wide taxonomic range means that these primers may be used to study the population biology (in supplying markers) and evolution (inter- and probably intraspecific phylogenies) of plants.
Biological invasions are a major global environmental and economic problem. Analysis of the San Francisco Bay and Delta ecosystem revealed a large number of exotic species that dominate many habitats in terms of number of species, number of individuals and biomass, and a high and accelerating rate of invasion. These factors suggest that this may be the most invaded estuary in the world. Possible causes include a large number and variety of transport vectors, a depauperate native biota, and extensive natural and anthropogenic disturbance.
We report the discovery and spread of a dwarf ecotype of Spartina alterniflora in San Francisco Bay. Relative to typical S. alterniflora, this dwarf ecotype has one-fifth the tiller height (∼21 cm), tenfold the tiller density (∼4000 tillers/m(2)), and is restricted to growth in the upper intertidal zone. Chromosome counts of the dwarfs are identical to typical smooth cordgrass (2n = 62), and smooth cordgrass-specific random amplified DNA markers confirm the species identity of the dwarf. Field-collected clonal fragments of the dwarf grown for 2 yr under high-nutrient conditions maintained the dwarf syndrome, as did plants grown from the seed of a dwarf. The dwarf condition is not caused by endophytic fungi. The first dwarf smooth cordgrass patch was discovered in 1991, and by 1996 five separate dwarf patches had appeared within 200 m of the original. Since 1991, total area covered by the dwarf ecotype has increased sixfold to 140 m(2). The ecological range of the dwarf smooth cordgrass ecotype is similar to that of S. patens, a competitor on the Atlantic coast. We suggest that the absence of S. patens from most of San Francisco Bay has allowed the dwarf ecotype of smooth cordgrass to survive and spread.
An example from the genus Eucalyptus is used to argue that hybridization may be of evolutionary significance as a means of gene dispersal where seed dispersal is limited. A previous study of regeneration of E. risdonii and E. amygdalina indicated that the current selective regime was favoring E. risaonii. However, the dispersal of E. risdonii by seeds is shown to be limited (s, = 4.6 m). By comparison, the flow of E. risdonii genes into the range of E. amygdalina by pollen dispersal and F1 hybridization is widespread (sp = 82 m). While the actual level of hybridization is low, interspecific hybridization effectively doubles the dispersal of E. risdonii genes into the range of E. amygdalina. This pollen flow can have a significant genetic impact, since isolated hybrids or patches of abnormal phenotypes have been found 200-300 m from the species boundary. Based on lignotuber size, some of these patches appear to have been founded by F1 hybrids. The frequency of E. risdonii types in the patches appears to increase with patch size suggesting that there is selection for this phenotype in subsequent generations. E. risdonii-like individuals were recovered in the progeny from both intermediate and E. risdonii backcross phenotypes. These results suggest that E. risdonii may invade suitable habitat islands within the E. amygdalina forest, independently of seed migration, by long-distance pollen migration followed by selection for the gene combinations of the pollen parent.
The extent, form and condition of Spartina anglica in the Tamar Estuary changed between the summer of 1971/72 and 1991, probably due to differences in maturity and substrate. This exotic saltmarsh grass stabilised and raised the intertidal mudflats and constrained channel movements, but it has also reduced recreational facilities and its spread is largely uncontrollable. Information is presented on the extent and variations in distribution of Spartina, including results from cross-profile surveys; interrelationships with substrate; variations in area of sward, clumps and shoots, and in area and cross-profile, from 1971/72 to 1991; and tidal, meteorological and human influences on maturing Spartina marsh. -from Author
1. In order to construct a model to predict the effect of winter habitat loss on the migratory population of the European subspecies of the oystercatcher, Haematopus ostralegus ostralegus, data on the reproductive and mortality rates collected throughout Europe over the last 60 years are reviewed. Within the Continental and Atlantic regions, inland-breeding and coastal-breeding subpopulations use the same coastal areas in winter. 2. Census and experimental data suggest pairs compete for territories and that an increasing proportion is excluded from breeding altogether, or nest in poor quality habitats, as the number of pairs attempting to breed increases. This provides a main source of density dependence in the basic model. 3. Mean clutch size, hatching success and fledging success were estimated for each subpopulation in each region. Data from one site suggested that the numbers fledged per breeding pair decreases as the total numbers of territories occupied increases, probably because of a reduction in chick survival. This additional source of density dependence was included in some versions of the model. 4. Most post-hedging mortality occurs in winter. Annual mortality was measured from the annual return rates of adults to the breeding areas and probably gives over-estimates. An additional 7-15% of adults die in severe winters once in 7 years in the Continental region but not in the milder Atlantic region. Oystercatchers in their first and second winter have a 20% higher winter mortality rate than adults. 5. The sometimes quite substantial annual fluctuations in the main production and mortality parameters were generally not correlated across sites within a subpopulation. This allowed the standard deviations of the annual variations in these parameters to be estimated for both subpopulations in each region so that realistic annual variations could also be included in the model.
Length polymorphism in a non-coding spacer (trnLUAA-trnFGAA) in the chloroplast DNA was used in the investigation of the origin of the most common and conspicuous European fern hybrid, Asplenium x alternifolium (Aspleniaceae, Pteridophyta). The origins of A. x alternifolium, the hybrid between A. trichomanes s.l. and A. septentrionale s.l. was studied at three ploidy levels, diploid, triploid and tetraploid. The cpDNA technique allowed us to investigate the mode of hybrid formation between sexual species for the first time over a wide geographic range and with a large sample size. Morphological variation in this hybrid has previously been attributed to different reciprocal parental combinations, and to the influence of chloroplast genes on morphogenesis. Our results demonstrate that one parent, A. septentrionale s.l., acts predominantly as the female parent in these hybrids, with only one population of A. x alternifolium showing reciprocal hybridity. The discovery of predominantly unidirectional hybrid formation in this hybrid may be explained by the different breeding systems of the parental taxa. The role of gametophyte ecology is also assessed.
Little is known about the mode of transmission of chloroplasts in ferns, despite the importance of such knowledge for molecular phylogenetic and biosystematic studies. Andersson-Kötto (1930, 1931) inferred from crossing experiments that chloroplasts are inherited biparentally in Asplenium scolopendrium L. Here we present evidence from artificial hybridisation that demonstrates maternal inheritance of chloroplast DNA in the genus Asplenium (Aspleniaceae, Pteridophyta) using length variation in a non-coding spacer (trnLUAA-trnFGAA) in the chloroplast DNA.
Controlled sheep grazing experiments using a replicated plot technique have been carried out at the upper limits of Spartina marsh in the Bridgewater Nature Reserve, Somerset. Results suggest that a tall marsh vegetation with Phragmites communis and Scirpus maritimus tends to replace ungrazed Spartina marsh in the regions where its upper limits are on soft mud marsh with salinity reduced by land drainage seepage. In such regions sheep grazing favours a significant increase in the spread of Puccinellia maritima and in the tillering of Spartina, which in turn reduces available ground space for the spread of the annual Atriplex hastata. The rate of change of the upper limits of a marsh in which Spartina is dominant, to a predominantly Phragmites marsh (ungrazed) or to a Puccinellia marsh (grazed) is estimated to be of the order of 8 and 10 years respectively in this area. Successional trends towards a Puccinellia marsh originally initiated by open range sheep grazing on a drier, sandy and more saline Spartina marsh in the area, tend to swamp changes brought about by enclosure from sheep grazing. Measurements of the standing crop weight of Spartina and Atriplex marsh give mean values of 7000-8000 kg/ha for both. The distribution and lay of Spartina drift at the top of the marsh depresses the growth of Spartina and favours the growth of Atriplex. This results in an irregular short period sub-seral cycle, and in good seed years Spartina re-establishes on top of the silt-covered drift to complete the cycle. The presence of Spartina drift is important as a means of opening up the dense Spartina sward and allowing the rapid spread of invaders such as Phragmites, Scirpus and Puccinellia into the gaps so formed.
Harris, S. A. & Ingram, R.: Chloroplast DNA and biosystematics: The effects of intraspecific diversity and plastid transmission. ‐ Taxon 40: 393–412. 1991. ‐ ISSN 0040‐0262.
The widespread application of chloroplast DNA (cpDNA) to plant biosystematics is usually based on a number of assumptions. The assumption of low levels of intraspecific cpDNA variation is reviewed and the conclusion reached that far from being rare, intraspecific cpDNA variation is relatively common. The evidence for the assumption of predominantly maternal plastid transmission is also reviewed. In both of the cases some of the possible effects on phylogenetic reconstruction are considered.
The absence of gene flow, genetic isolation, is frequently emphasized in conservation genetics. However, the presence of gene flow can play an equally important role in determining the genetic fate of populations. Here, I first review what is known of patterns of gene flow by pollen. Gene flow by pollen is often substantial among plant populations. I next review the expectations for gene flow patterns in the small populations typical of endangered species. Then, I consider what role gene flow can play in plant conservation genetics. Depending on the specific situation, such gene flow could be either beneficial or detrimental. Geographically disjunct populations might not always be as reproductively isolated as previously thought, and thereby less vulnerable to detrimental drift-based processes. On the other hand, conspecific or heterospecific hybridization may lead to extinction by outbreeding depression or genetic assimilation. Also, the field release of transgenic plants may lead to the escape of engineered genes by crop-wild plant hybridization. Such "genetic pollution" could have profound effects on the fitness of wild species with the potential for disrupting natural communities. Gene flow can be an important force in plant conservation genetics, and its potential role should be considered in any plant conservation management program.
An example from the genus Eucalyptus is used to argue that hybridization may be of evolutionary significance as a means of gene dispersal where seed dispersal is limited. A previous study of regeneration of E. risdonii and E. amygdalina indicated that the current selective regime was favoring E. risaonii. However, the dispersal of E. risdonii by seeds is shown to be limited (ss = 4.6 m). By comparison, the flow of E. risdonii genes into the range of E. amygdalina by pollen dispersal and F1 hybridization is widespread (sp = 82 m). While the actual level of hybridization is low, interspecific hybridization effectively doubles the dispersal of E. risdonii genes into the range of E. amygdalina. This pollen flow can have a significant genetic impact, since isolated hybrids or patches of abnormal phenotypes have been found 200-300 m from the species boundary. Based on lignotuber size, some of these patches appear to have been founded by F1 hybrids. The frequency of E. risdonii types in the patches appears to increase with patch size suggesting that there is selection for this phenotype in subsequent generations. E. risdonii-like individuals were recovered in the progeny from both intermediate and E. risdonii backcross phenotypes. These results suggest that E. risdonii may invade suitable habitat islands within the E. amygdalina forest, independently of seed migration, by long-distance pollen migration followed by selection for the gene combinations of the pollen parent.
The shape of Poole Harbour is due largely to a marine submergence that probably reached its maximum about 6000 years ago. Its branched configuration is typical of a valley system partly drowned by Recent marine submergence with the higher parts persisting as promontories and islands. Sand spits at the harbour entrance are thought to be derived partly from the erosion of Tertiary cliffs to the north, and partly from the floor of Studland Bay to the south of the harbour entrance where Bagshot Beds are believed to outcrop. Longshore drift and counter-drift are checked by tidal currents of up to 5 knots (2.6 m/sec) which maintain the harbour entrance. Shore-line forms and marsh development in Poole Harbour depend largely on exposure to wave action at high tide, cliffs occurring (with beaches associated locally) opposite regions of longest fetch, while marshlands are best developed on more sheltered sections of the shore. Before the marshlands developed, much of the harbour shore was cliffed, the steep bluff behind the marshlands being a degraded cliff with basal relics of sand and gravel beaches partly concealed by marsh soil and vegetation. Topography in the intertidal zone and below depends especially on currents developed in creeks and channels during the ebb. The pattern of tidal channels has remained much as it was since it was first surveyed 178 years ago, although minor changes have been recorded. The sources of sediment delivered to the harbour are discussed and evidence given that significant amounts are still being delivered to the upper reaches of the harbour by the rivers when in spate. The broad lagoon-like form of the harbour acts as a `settling tank' for sediment. The part played by Spartina as a physiographic agent is discussed and estimates of the depth of silt accumulated by this plant vary from 70 to 100 cm in different parts of the harbour. Approximately 2158 ac (873 ha) of Spartina marsh and about 620 ac (251 ha) of Phragmites marsh occur, together occupying about 36% of the intertidal zone. The development of Spartina marsh has probably passed its peak in the area and relatively little active spread is found. Erosion and `die-back' release silt which may be redeposited on areas where Spartina marsh is still actively developing.
Spartina anglica is a grass of saltmarshes and mudflats. It resulted from chromosome doubling in a hybrid between a native grass, S. maritima, and a species accidentally introduced from North America, S. alterniflora. The hybrid, S. x townsendii, was first recorded in 1870 near Hythe in Southampton Water; S. anglica was first recorded in 1892 from Lymington. Spartina anglica arrived in Poole Harbour in the 1890s, and by the early 1920s it covered over 800 ha of mudflats that had previously been largely clear of vegetation. During the late 1920s, S. anglica began to recede, and by 1980 about 350 ha remained. The rapid spread and decline of S. anglica was associated with physical and biological change within the harbour, the most noticeable being changes in the depth of navigation channels and the colonization of pure swards of S. anglica by other saltmarsh plants. The spread of S. anglica is also implicated in local reductions in the populations of wading birds.
Regeneration of a hybrid zone between E. amygdalina and E. risdonii and pure species stands following
wildfire is reported, as well as the reproductive and vegetative fitness of parental and hybrid phenotypes.
E. risdonii phenotypes dominated the seed rain and seedling cohort and there was clearly a marked
fitness differential between E. amygdalina and E. risdonii at their boundary. When the F1 type hybrid
is in competition with both parental types it is generally reproductively the least fit, although frequently
vegetatively vigorous. Reduced fitness appears to extend to advanced generations as hybrid phenotypes
tending.toward either species are, on average, less fit than the corresponding parental type.
The pattern of phenotypic fitness suggests that the species' boundary is in disequilibrium and it is
argued that E. risdonii is invading the range of E. amygdalina by both pollen and seed migration. There
is an asymmetric distribution of F1 type hybrids across the boundary and the hybrid swarm examined
is being invaded by E. risdonii genes. It is suggested that hybridization may be associated with natural
disequilibrium and, where seed migration is limited, boundary movements may be preceded by a wave
of hybridization due partly to pollen swamping of the least fit species. Hybrid swarms may develop but,
at the boundary of large stands, are probably transitory. There is a marked inertia in the population
response to the prevailing selective regime due to the extremely slow population turnover and limited
dispersal potential. This is discussed in the broader context of non-equilibrium models where it is argued
that dispersal may be the factor limiting population response to perturbation of a shallow environmental
gradient. This is due to large geographical shifts in the position of the null point and would be accentuated
in a patchy environment where migration as a front is prevented.
The role of introgression in plant evolution has been the subject of considerable discussion since the publication of Anderson ‘s influential monograph, Introgressive Hybridization (Anderson, 1949). Anderson promoted the view, since widely held by botanists, that interspecific transfer of genes is a potent evolutionary force. He suggested that “the raw material for evolution brought about by introgression must greatly exceed the new genes produced directly by mutation” (1949, p. 102) and reasoned, as have many subsequent authors, that the resulting increases in genetic diversity and number of genetic combinations promote the development or acquisition of novel adaptations (Anderson, 1949, 1953; Stebbins, 1959; Rattenbury, 1962; Lewontin and Birch, 1966; Raven, 1976; Grant, 1981). In contrast to this “adaptationist” perspective, others have accorded little evolutionary significance to introgression, suggesting instead that it should be considered a primarily local phenomenon with only transient effects, a kind of” evolutionary noise” (Barber and Jackson, 1957; Randolph et al., 1967; Wagner, 1969, 1970; Hardin, 1975). One of the vociferous doubters of a significant role of hybridization in plant evolution was Wagner (1969, p. 785), who commented that the “ultimate contributions made by hybrids must be very small or negligible.”
This book summarizes knowledge about invasive species and the problems that they cause in 7 chapters, with the following titles: a framework for the study of invasions; the origins and success and failure of invasions; which communities are invaded by which type of species; the process of spread; ecological consequences of invasions; genetic and evolutionary effects; and implications and communities. Author and subject indexes are provided.
▪ Abstract Nonindigenous species can bring about a form of extinction of native flora and fauna by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially problematic for rare species coming into contact with more abundant ones. Increased use of molecular techniques focuses attention on the extent of this underappreciated problem that is not always apparent from morphological observations alone. Some degree of gene flow is a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.
Spartina anglica is a textbook example of a natural amphiploid, which originated from hybridization between S. alterniflora and S. maritima. Which of these species was the maternal parent has remained a mystery. Inheritance of chloroplast DNA in most angiosperms is strictly maternal and can thus be used to test the parentage of hybrid taxa. The DNA sequence of the chloroplast leucine tRNA gene intron was used to show that the introduced North American S. alterniflora is the female parent of the F1 hybrid S. x townsendii and the amphiploid S. anglica. A possible scenario for their origin is given.
The perennial salt marsh grass Spartina anglica is one of the classic examples of allopolyploid speciation. It originated on the south coast of England at the end of the nineteenth century following chromosome doubling in S. × townsmdii, a hybrid between the native British S. marilima and a species introduced from the United States, S. alterniflora. The nature of the origin of S. anglica is beyond doubt; however, it is not known whether it had a single or multiple origin. In order to address this problem we undertook a survey of the genetic variation in the parental species of S. anglica using isozyme electrophoresis. The results show that S. alterniflora has no detectable variation and that S. maritima has extremely low levels of variation. These results, unfortunately, prevent the question of a single or multiple origin from being answered. Possible reasons for the low levels of variation and its influence on the future of the species are discussed. Another problem concerning the parental species is the rapid decline of S. maritima in Britain. It is often assumed that the major factor in this regression is the invasion of its habitats by S. anglica. We have examined the status of S. marilima throughout its range in Britain and have found that S. anglica rarely co-occurs with S. maritima. We propose that the decline of S. maritima is largely due to the physical erosion of its habitats and that this erosion may produce suitable habitats for colonization by S. anglica.
Traditionally, information on the postglacial history of plant species has been gained from the analysis of fossil pollen data. More recently, surveys of present patterns of genetic variation have given valuable insights into species phylogeography. The genus Alnus, based on fossil data, is known to have had at least four glacial refugia. A survey of chloroplast DNA (cpDNA) diversity in populations of black alder (A. glutinosa) was undertaken in order to gain more insight into its postglacial history. This revealed a high degree of structuring of 13 cpDNA haplotypes on a European scale which indicated that most of northern and central Europe was colonized from a refuge in the Carpathian Mountains. Based on the distribution of two common cpDNA haplotypes, colonization routes from this refuge can be determined. The locations of other previously identified refugia are confirmed and two formerly unconfirmed refugial areas for alder (southern Spain and Turkey) are proposed.
Spartina alterniflora was first introduced into south San Francisco Bay in the 1970’s. Since that time it has spread to new areas within the south
bay and is especially well established at four sites. The spread of this introduced species was evaluated by comparing its
vegetative and reproductive characteristics to the native cordgrass, Spartina foliosa. The characters studied were intertidal distribution, phenology, aboveground and belowground biomass, growth rates, seed
production, and germination rates. Spartina alterniflora has a wider intertidal distribution than S. foliosa and outproduced the native cordgrass in all aspects that were studied. These results indicate that the introduced species
has a much better chance of becoming established in new areas than the native species, and once established, it spreads more
rapidly vegetatively than the native species. Spartina alterniflora is likely to continue to spread to new areas in the bay and displace the native plant. In addition, this introduced species may effect sedimentation
dynamics, available detritus, benthic algal production, wrack deposition and disturbance, habitat structure for native wetland
animals, benthic invertebrate populations, and shorebird and wading bird foraging areas. *** DIRECT SUPPORT *** A01BY058 00013
The magnitude of inbreeding depression in invading plant populations is often presumed to be small and of little consequence. The purpose of this study was to assess the magnitude of inbreeding depression in a pollen-limited, partially self-incompatible, invading plant population. The magnitude and timing of inbreeding depression were compared among ten maternal plants sampled from a population of smooth cordgrass (Spartina alterniflora) invading San Francisco Bay. Selfed and outcrossed progeny were compared for embryo abortion, survival of seedlings, and growth/survival at the end of the first growing season in three greenhouse environments. Estimates of inbreeding depression varied among environments, with competitive environment > high-nutrient environment > low-nutrient environment. Population-level estimates of inbreeding depression ranged from 0.61 to 0.81; however, maternal plants varied significantly in their magnitude of inbreeding depression, ranging from 0.1 to 0.97. The 95% confidence interval for inbreeding depression for some maternal plants included zero. There was a significant negative correlation between the overall magnitude of inbreeding depression and self-fertility rate among maternal plants. The few maternal plants with high self-fertility carried relatively little genetic load, and their selfed progeny are likely to survive on open mudflats. The noncompetitive, pollen-limited growing conditions associated with invasion may allow self-fertility to spread in this population.
The process of natural hybridization may produce genotypes that establish new evolutionary lineages. However, many authors have concluded that natural hybridization is of little evolutionary importance because hybrids, in general, are unfit relative to their progenitors. Deciding between these alternative conclusions requires that fitness be measured for hybrid classes and parental species. Recent analyses have found that hybrids are not uniformly unfit, but rather are genotypic classes that possess lower, equivalent or higher levels of fitness relative to their parental taxa.
Glacial refugia and postglacial migration are major factors responsible for the present patterns of genetic variation we see in natural populations. Traditionally postglacial history has been inferred from fossil data, but new molecular techniques permit historical information to be gleaned from present populations. The chloroplast tRNA(Leu1) intron contains regions which have been highly conserved over a billion years of chloroplast evolution. Surprisingly, in one of these regions which has remained invariant for all photosynthetic organisms so far studied, we have found intraspecific site polymorphism. This polymorphism occurs in two European oaks, Quercus robur and Q. petraea, indicating hybridisation and introgression between them. Two distinct chloroplast types occur and are distributed geographically as eastern and western forms suggesting that these oaks are each derived from at least two separate glacial refugia.
Evolution of a new ecotype of Spartina alterniflora (Poaceae) in
- Daehler Cc
- Ayres Ck Dr Anttila
- Strong
- Dr
- Bailey
- Jp
Daehler CC, Anttila CK, Ayres DR, Strong DR, Bailey JP (1999) Evolution of a new ecotype of Spartina alterniflora (Poaceae) in San Francisco Bay, California, USA. American Journal of Botany, 86, 543 – 546.
Are natural hybrids fit or unfit relat-ive to their parents? Trends in Ecology and Evolution
- Arnold Ml Hodges
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Arnold ML, Hodges SA (1995) Are natural hybrids fit or unfit relat-ive to their parents? Trends in Ecology and Evolution, 10, 67 – 71.
Evolution at the Molecular Level
- Clegg MT
- Learn GH
- Golenberg EM
- M H Williamson
Williamson MH (1996) Biological Invasions. Chapman & Hall,
London.