Causes and consequences of extreme variation in reproductive strategy and vegetative growth among invasive populations of a clonal aquatic plant, <Emphasis Type="Italic">Butomus umbellatus</Emphasis> L. (Butomaceae)

The mode of reproduction may influence the spread of invasive species by affecting evolutionary potential and dispersal ability. We sampled 51 introduced North American populations of the clonal aquatic plant Butomus umbellatus L. (flowering rush) and found extreme variation in sexual fertility caused by polyploidy. Populations consisted of diploids that produced thousands of viable seeds or of sexually sterile triploids. Although a trade-off between sexual and clonal reproduction predicts that the sexual sterility of triploids would be compensated for by greater clonal reproduction, a greenhouse experiment involving eight diploid and 10 triploid populations showed that diploid plants not only invest substantially in sexual structures but also make hundreds of tiny clonal bulbils on both rhizomes and inflorescences. In contrast, triploids do not make bulbils and have very limited scope for clonal multiplication and dispersal. Diploid populations were more frequent than triploid populations, especially in the Great Lakes region. This is probably because of the difference between cytotypes in clonal rather than sexual reproduction, as genetic analyses indicate a general lack of sexual recruitment in North America. Although triploids were less common, they have a wider geographical distribution. This could be due to a greater ecological tolerance resulting from polyploidy. However, genetic evidence suggests that triploids have become widespread via their use in and escape from horticulture. North America is being colonized by two distinct forms of B. umbellatus that differ strongly in reproductive strategy as well as the vectors and pathways of invasion.     

Interaction between founder effect and selection during biological invasion in an aquatic plant

Long-distance colonization and rapid range expansion associated with biological invasion may have major evolutionary consequences via both stochastic processes and selection. Using large-scale population genetic surveys, we demonstrate a major shift in the relative frequency of sexually fertile diploid versus sexually sterile triploid populations associated with the invasion of North America by a clonal aquatic plant, Butomus umbellatus. Most populations across the native European range were triploid (84% of 108), whereas most introduced populations were diploid (71% of 136). We evaluated the roles of stochastic processes versus natural selection in causing this shift by surveying predominantly neutral genetic variation at 28 RAPD loci. In Europe (EU) we detected 47 distinct genotypes among 142 plants sampled from 71 populations, whereas in North America (NA) we detected only six genotypes among 138 plants from 69 populations. Of the six NA genotypes, a set of four closely related genotypes were found only in triploid populations and a pair of closely related genotypes were found only in diploid populations, and these were genetically divergent from the triploid genotypes. This result is consistent with severe founder effect. Because sex creates genotypic variation and produces offspring with greater dispersal potential than those produced clonally, we tested the hypothesis that sexual reproduction characteristic of diploids has given them a colonization advantage that accounts for their high frequency in NA. However, we found little or no evidence of sexual recruitment in introduced diploids. One very widespread heterozygous genotype occurred in 95% of 38 introduced diploid populations (i.e., 72 of 76 plants surveyed) suggesting predominant clonal reproduction. Moreover genotypic diversity was not higher within or among diploid than triploid populations in either the native or introduced range. Low genetic diversity in diploid populations was also supported by a comparison of within-population quantitative variation for plant size under a common greenhouse environment. Thus, diploids have not been favored during colonization owing to their sexual fertility. However, concurrent studies have shown that NA diploids exhibit a much higher capacity for clonal reproduction, via small vegetative bulbils, than NA triploids, which almost never produce bulbils. The same difference in clonal capacity is not a consistent feature of the native EU populations. Taken together, these results suggest that strong founder effect has set the stage for a major increase in diploid frequency due to the particular, and possibly idiosyncratic, features of the diploid and triploid lineages introduced to North America.     

Evolutionary increase in sexual and clonal reproductive capacity during biological invasion in an aquatic plant Butomus umbellatus (Butomaceae)

To test the hypothesis that increased allocation to reproduction is selected during biological invasion, we compared germination, survival, growth, and reproduction of native vs. introduced populations of the invasive aquatic plant Butomus umbellatus in a common greenhouse environment. Although seedling emergence and establishment did not differ consistently, survival thereafter was twice as high for eight introduced North American than eight native European populations. As predicted, introduced plants were more likely to produce sexual inflorescences and clonal asexual vegetative bulbils, and they invested much more biomass in both reproductive modes. Higher reproductive investment was due to higher proportional allocation of biomass rather than larger plant size. These results are consistent with selection for increased reproduction during range expansion. However, population genetic surveys indicate that recruitment from seed rarely occurs in introduced populations. Hence increased sexual allocation is not an adaptive response to invasion. Although increased clonal reproduction may be advantageous in expanding populations, genetic evidence from introduced populations of B. umbellatus suggests that increased clonal allocation may have arisen via stochastic processes during long-distance transport or a selective filter right at introduction, rather than incremental natural selection during range expansion.     

Reproductive biology and genetic structure in Lloydia serotina

Concern regarding the conservation status of small, isolated populations of the arctic-alpine plant species Lloydia serotina prompted research to establish the status and performance of this species in Wales, in comparison with large populations in its more typical alpine habitat. Relationships between reproductive strategies and genetic variation were investigated in a number of populations, representing a wide habitat, geographic and population size range. In all populations, vegetative reproduction predominates over sexual reproduction, but seed produced is viable and germinates readily under controlled conditions. Smaller, peripheral populations produced fewer flowers and seeds than the larger ones, but all populations studied supported significant percentages (>30%) of male plants, due to either the occurrence of androdioecy in this species or to a resource limited breeding system. Analysis of allozyme variation in sixteen populations from North America, the European Alps and Wales showed lower levels of genetic variation in smaller populations which averaged 1.1–1.2 alleles per locus and 10–20% of loci polymorphic, whereas larger populations averaged 1.4 alleles per locus and 30–40% polymorphic loci. This applied especially to the most northerly and southerly populations in North America, suggesting the occurrence of genetic drift in these small, peripheral populations. F-statistics suggest relatively high levels of differentiation among smaller populations, even among those closely related geographically, but genetic variation has been retained in all but one population, possibly due to infrequent sexual reproduction by long lived clones. RAPD analysis of four small populations in Wales provided further evidence of clonal growth and possible inbreeding dominating a mixed mating reproductive system with consequent genetic structuring in these populations.     

Population genetic consequences of extreme variation in sexual and clonal reproduction in an aquatic plant

Most plants combine sexual reproduction with asexual clonal reproduction in varying degrees, yet the genetic consequences of reproductive variation remain poorly understood. The aquatic plant Butomus umbellatus exhibits striking reproductive variation related to ploidy. Diploids produce abundant viable seed whereas triploids are sexually sterile. Diploids also produce hundreds of tiny clonal bulbils, whereas triploids exhibit only limited clonal multiplication through rhizome fragmentation. We investigated whether this marked difference in reproductive strategy influences the diversity of genotypes within populations and their movement between populations by performing two large-scale population surveys (n = 58 populations) and assaying genotypic variation using random amplified polymorphic DNA (RAPDs). Contrary to expectations, sexually fertile populations did not exhibit higher genotypic diversity than sterile populations. For each cytotype, we detected one very common and widespread genotype. This would only occur with a very low probability (< 10-7) under regular sexual recombination. Compatibility analysis also indicated that the pattern of genotypic variation largely conformed to that expected with predominant clonal reproduction. The potential for recombination in diploids is not realized, possibly because seeds are outcompeted by bulbils for safe sites during establishment. We also failed to find evidence for more extensive movement of fertile than sterile genotypes. Aside from the few widespread genotypes, most were restricted to single populations. Genotypes in fertile populations were very strongly differentiated from those in sterile populations, suggesting that new triploids have not arisen during the colonization of North America. The colonization of North America involves two distinct forms of B. umbellatus that, despite striking reproductive differences, exhibit largely clonal population genetic structures.     

Reproductive Strategies Explain Genetic Diversity in Atlantic Salmon, Salmo salar

Synopsis We investigated the relationship between conservation status and genetic variability in European and North American Atlantic salmon, Salmo salar, populations, many of which have suffered severe bottlenecks. A negative north--south cline exists for the status of population conservation in this species. A literature review of genetic variability and demographic parameters of wild Atlantic salmon populations resulted in no statistical associations between population conservation status and genetic variation at enzyme or VNTR loci. We found however, a negative relationship between male parr maturation rates and geographical latitude for both American and European populations. The increase in effective population size due to participation by mature male parr and the increased proportions of these males in smaller (southern) populations could explain the lack of expected relationship between genetic variation and conservation status.     

A layover in Europe: Reconstructing the invasion route of asexual lineages of a New Zealand snail to North America

Non‐native invasive species are threatening ecosystems and biodiversity worldwide. High genetic variation is thought to be a critical factor for invasion success. Accordingly, the global invasion of a few clonal lineages of the gastropod Potamopyrgus antipodarum is thus both puzzling and has the potential to help illuminate why some invasions succeed while others fail. Here, we used SNP markers and a geographically broad sampling scheme (N = 1617) including native New Zealand populations and invasive North American and European populations to provide the first widescale population genetic assessment of the relationships between and among native and invasive P. antipodarum. We used a combination of traditional and Bayesian molecular analyses to demonstrate that New Zealand populations harbour very high diversity relative to the invasive populations and are the source of the two main European genetic lineages. One of these two European lineages was in turn the source of at least one of the two main North American genetic clusters of invasive P. antipodarum, located in Lake Ontario. The other widespread North American group had a more complex origin that included the other European lineage and two New Zealand clusters. Altogether, our analyses suggest that just a small handful of clonal lineages of P. antipodarum were responsible for invasion across continents. Our findings provide critical information for prevention of additional invasions and control of existing invasive populations and are of broader relevance towards understanding the establishment and evolution of asexual populations and the forces driving biological invasion.     

Population structure,seasonal genotypic differentiation,and clonal diversity of weedy dandelions in three Boston area populations (Taraxacum sp.)

Weedy dandelions have a worldwide distribution and thrive in urban environments despite a lack of sexual reproduction throughout most of its range. North American dandelions, introduced from Eurasia, are believed to be primarily, if not exclusively, apomictic triploids. In some European populations, apomicts co‐occur with diploid sexual individuals and hybridizations can create genetically unique apomicts, which may subsequently disperse and establish new populations globally. Using six nuclear microsatellite markers and a cpDNA intergenic spacer, we investigate the impact of this unusual natural history on population structure and diversity in three urban Boston area dandelion populations. Our results show high levels of genetic diversity within populations, spatial population structure, and seasonal genotypic differentiation in flowering times. We find evidence that sexual reproduction and recombination, presumably in Europe, and extensive gene flow drive these patterns of diversity and create the appearance of panmixia despite the lack of evidence for local sexual reproduction.     

Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems

Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch-German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian-based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl-mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.     

具混合繁殖策略的草本植物异果舞花姜的居群遗传结构

 居群遗传结构的形成受到各种因素的影响。其中, 繁殖方式可能对居群内遗传变异有极其重要的意义, 而距离隔离也是居群间变异产 生的主要原因之一。异果舞花姜(Globba racemosa)具有混合繁殖策略(以种子进行有性繁殖和以珠芽进行无性克隆繁殖)。调查分布于云南的7 个异果舞花姜居群间有性与无性克隆繁殖的差异。采用ISSR标记研究各个居群的遗传多样性与克隆多样性, 探讨繁殖方式和距离隔离对居群遗 传结构的影响。调查结果表明, 异果舞花姜各个居群存在一定的繁殖差异。ISSR结果显示, 该种在种水平上呈现较高水平的遗传变异 (PPB=71.19%), 大部分的变异来自于居群间(G_ST = 0.590 7)。同时, 异果舞花姜具有较高水平克隆多样性(G/N = 0.88)。遗传多样性和克隆多 样性与繁殖水平的变异间相关性不明显, 说明繁殖方式不是居群遗传结构形成的必要和决定性的因素。居群间的地理距离与遗传距离显著相关 (r = 0.68, p < 0.05), 表明距离隔离是居群间遗传变异形成的重要原因。其它因素(如少量新有性个体的补充、细胞突变、奠基效应等)也对 异果舞花姜居群遗传结构的形成和维持起到了重要作用。     

The loss of sex in clonal plants

Most plants combine sexual and clonal reproduction, and the balance between the two may vary widely between and within species. There are many anecdotal reports of plants that appear to have abandoned sex for clonal reproduction, yet few studies have quantified the degree of sexual variation in clonal plants and fewer still have determined the underlying ecological and/or genetic factors. Recent empirical work has shown that some clonal plants exhibit very wide variation in sexual reproduction that translates into striking variation in genotypic diversity and differentiation of natural populations. Reduced sexual reproduction may be particularly common at the geographical margins of species' ranges. Although seed production and sexual recruitment may often be limited by biotic and abiotic aspects of the environment in marginal populations, genetic factors, including changes in ploidy and sterility mutations, may also play a significant role in causing reduced sexual fertility. Moreover, environmental suppression of sexual recruitment may facilitate the evolution of genetic sterility because natural selection no longer strongly maintains the many traits involved in sex. In addition to the accumulation of neutral sterility mutations in highly clonal populations, the evolution of genetic infertility may be facilitated if sterility is associated with enhanced vegetative growth, clonal propagation or survival through either resource reallocation or pleiotropy. However, there are almost no experimental data with which to distinguish among these possibilities. Ultimately, wide variation in genotypic diversity and gene flow associated with the loss of sex may constrain local adaptation and the evolution of the geographical range limit in clonal plants.     

Genetic and clonal structures of the tree species Tilia cordata mill. in remnants of ancient forests in Denmark

The insect-pollinated forest tree Tilia cordata Mill. grows today in small fragmented populations in Denmark and other western European countries but was, in prehistoric times, a dominating species and is considered an indicator species for ancient forest. The species is known to propagate both sexually and vegetatively, forming clonal groups. Few studies have been made on the species' population genetics and on how clonality affects the population structure. The aim of this study is to evaluate the Danish gene pool by estimating genetic diversity and differentiation, as well as through exhaustive sampling describe clonal structures in some of the populations. Genetic analysis was carried out using nine nuclear microsatellite markers in nine populations, of which four were exhaustively or partly exhaustively sampled. The markers showed a high degree of genetic diversity but low differentiation between populations, with no geographic-related structure. Clonal structures were found in eight out of the nine populations. In the exhaustively sampled populations, recruitment strategies included both sexual and clonal reproduction with indications that clonality may be enhanced by management and other disturbances.     

Changes in genetic structure of North American Bythotrephes populations following invasion from Lake Ladoga, Russia

1. We used allozyme electrophoresis to compare Bythotrephes longimanus (Crustacea: Onychopoda: Cercopagididae) from recently founded North American populations with those from native European populations, and to examine changes in genetic structure of North American populations over time.
2. The genetic structure of North American populations in 1996 was similar to that of European populations, because of the disappearance of founder effects which distinguished North American from European populations in 1989.
3. The Lake Ladoga, Russia population was more closely related to North American populations than to other European populations, consistent with non-genetic evidence implicating Lake Ladoga as the source of North American populations.
4. Our results provide additional evidence of the presence of an invasion corridor that allows Urasian and Ponto-Caspian species to be introduced into North American freshwater ecosystems, and show that founder effects can erode over time following establishment of invasive species.     

Variability in life history traits of the aphid,Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.     

Promiscuity in populations of the cushion plant Saxifraga oppositifolia in the Swiss Alps as inferred from random amplified polymorphic DNA (RAPD)

Overviews on patterns of genetic variation within and among plant populations show that widespread, outcrossing species should have a high proportion of the total genetic variation within populations and a low proportion among populations, which results in little population differentiation. However, in Alpine areas, large–scale distribution barriers as well as small-scale habitat heterogeneity could lead to geographical and temporal isolation, respectively. We investigated the genetic variation of Saxifraga oppositifolia from 10 populations of the Alps in southeastern Switzerland using random amplified polymorphic DNA (RAPD). Based on the banding patterns of four RAPD primers, 84 polymorphic markers identified all 189 sampled individuals as being genetically different. The genetic variation was mainly found within populations (95%), whereas less than 5% was found among populations and among regions. Analyses of molecular variance ( AMOVA ) suggested that population differentiation was highly significant. However, grouping populations differently into regions did not appear to result in a clear correspondence of genetic and geographical relatedness. Genetic variation did not significantly differ between populations of two elevational levels. This coincides with results of former pollination experiments that revealed a breeding system of S. oppositifolia which remains the same irrespective of the elevation. We assume that the high outcrossing rate, rare clonal reproduction, and some long-distance dispersal even among topographically separated populations are the crucial determinants for the pattern of genetic variation found in the investigated area.     

High migration rates shape the postglacial history of amphi‐Atlantic bryophytes

Paleontological evidence and current patterns of angiosperm species richness suggest that European biota experienced more severe bottlenecks than North American ones during the last glacial maximum. How well this pattern fits other plant species is less clear. Bryophytes offer a unique opportunity to contrast the impact of the last glacial maximum in North America and Europe because about 60% of the European bryoflora is shared with North America. Here, we use population genetic analyses based on approximate Bayesian computation on eight amphi‐Atlantic species to test the hypothesis that North American populations were less impacted by the last glacial maximum, exhibiting higher levels of genetic diversity than European ones and ultimately serving as a refugium for the postglacial recolonization of Europe. In contrast with this hypothesis, the best‐fit demographic model involved similar patterns of population size contractions, comparable levels of genetic diversity and balanced migration rates between European and North American populations. Our results thus suggest that bryophytes have experienced comparable demographic glacial histories on both sides of the Atlantic. Although a weak, but significant genetic structure was systematically recovered between European and North American populations, evidence for migration from and towards both continents suggests that amphi‐Atlantic bryophyte population may function as a metapopulation network. Reconstructing the biogeographic history of either North American or European bryophyte populations therefore requires a large, trans‐Atlantic geographic framework.     

Population genetics of the invasive water weed Elodea canadensis in Finnish waterways

Invasions of exotic species often involve a rapid evolutionary change in the introduced populations. Elodea canadensis is an invasive aquatic weed native to North America. Our aims were to reveal the evolutionary consequences of invasion to the population genetic structure of the presumably clonal E. canadensis in Finland and to test the hypothesis that the whole Finnish population originates from the first introduction of the species. We used ten polymorphic microsatellite markers to analyze the genetic characteristics of seven introduced E. canadensis populations in Finland. Despite the species' totally asexual mode of reproduction in Finland, two to five alleles per locus were detected in Finnish populations, and the expected heterozygosities varied between 0.19 and 0.37. The majority of variation was found within populations. Except for one, all pairwise values of population differentiation (F _ST) were significant, indicating restricted gene flow among the Finnish populations. In addition, a Bayesian analysis of population structure revealed the presence of regional population structuring. Genetic analyses indicate that E. canadensis could have been introduced to Finland multiple times. However, the amount of genetic variation and regional clustering detected could also be explained by post-establishment evolution, and based on this study it is not possible to exclude one introduction event followed by rapid evolution. We also tested the usability of the microsatellite markers for native North American samples in order to compare the within-population genetic characteristics of introduced and native populations. However, in native populations only four microsatellite markers amplified reliably, indicating sequence variation within primer-binding regions and, thus, genetic differentiation among populations of E. canadensis.     

Polyploidy and microsatellite variation in the relict tree Prunus lusitanica L.: how effective are refugia in preserving genotypic diversity of clonal taxa?

Refugia are expected to preserve genetic variation of relict taxa, especially in polyploids, because high gene dosages could prevent genetic erosion in small isolated populations. However, other attributes linked to polyploidy, such as asexual reproduction, may strongly limit the levels of genetic variability in relict populations. Here, ploidy levels and patterns of genetic variation at nuclear microsatellite loci were analysed in Prunus lusitanica, a polyploid species with clonal reproduction that is considered a paradigmatic example of a Tertiary relict. Sampling in this study considered a total of 20 populations of three subspecies: mainland lusitanica (Iberian Peninsula and Morocco), and island azorica (Azores) and hixa (Canary Islands and Madeira). Flow cytometry results supported an octoploid genome for lusitanica and hixa, whereas a 16‐ploid level was inferred for azorica. Fixed heterozygosity of a few allele variants at most microsatellite loci resulted in levels of allelic diversity much lower than those expected for a high‐order polyploid. Islands as a whole did not contain higher levels of genetic variation (allelic or genotypic) than mainland refuges, but island populations displayed more private alleles and higher genotypic diversity in old volcanic areas. Patterns of microsatellite variation were compatible with the occurrence of clonal individuals in all but two island populations, and the incidence of clonality within populations negatively correlated with the estimated timing of colonization. Our results also suggest that gene flow has been very rare among populations, and thus population growth following founder events was apparently mediated by clonality rather than seed recruitment, especially in mainland areas. This study extends to clonal taxa the idea of oceanic islands as important refugia for biodiversity, since the conditions for generation and maintenance of clonal diversity (i.e. occasional events of sexual reproduction, mutation and/or seed immigration) appear to have been more frequent in these enclaves than in mainland areas.     

FREQUENT CLONALITY IN FUCOIDS (FUCUS RADICANS AND FUCUS VESICULOSUS; FUCALES,PHAEOPHYCEAE) IN THE BALTIC SEA1

Asexual reproduction by cloning may affect the genetic structure of populations, their potential to evolve, and, among foundation species, contributions to ecosystem functions. Macroalgae of the genus Fucus are known to produce attached plants only by sexual recruitment. Recently, however, clones of attached plants recruited by asexual reproduction were observed in a few populations of Fucus radicans Bergström et L. Kautsky and F. vesiculosus L. inside the Baltic Sea. Herein we assess the distribution and prevalence of clonality in Baltic fucoids using nine polymorphic microsatellite loci and samples of F. radicans and F. vesiculosus from 13 Baltic sites. Clonality was more common in F. radicans than in F. vesiculosus, and in both species it tended to be most common in northern Baltic sites, although variation among close populations was sometimes extensive. Individual clonal lineages were mostly restricted to single or nearby locations, but one clonal lineage of F. radicans dominated five of 10 populations and was widely distributed over 550 × 100 km of coast. Populations dominated by a few clonal lineages were common in F. radicans, and these were less genetically variable than in other populations. As thalli recruited by cloning produced gametes, a possible explanation for this reduced genetic variation is that dominance of one or a few clonal lineages biases the gamete pool resulting in a decreased effective population size and thereby loss of genetic variation by genetic drift. Baltic fucoids are important habitat‐forming species, and genetic structure and presence of clonality have implications for conservation strategies.     

High regional differentiation in a North American crab species throughout its native range and invaded European waters: a phylogeographic analysis

Rhithropanopeus harrisii (Gould 1841) has a native distribution from New Brunswick (Canada) to Veracruz (Mexico) and is considered an invasive species in northwestern North American (Oregon and California), South American (Brazil) and European estuaries and rivers. In Europe, it was observed for the first time in 1874, in The Netherlands. We sequenced and analyzed part of the cytochrome oxidase subunit I gene (mitochondrial DNA) of eight populations, three from the east coast of the United States of America (USA) and five from Europe, in order to assess their genetic diversity and to determine a potential founder population. European populations are characterized by a lower number of haplotypes than the whole native region of the eastern USA, suggesting that genetic bottlenecks occurred during the European colonisation. Along the North American East Coast, there is evidence of clearcut genetic heterogeneity, New Jersey being the most similar population in its genetic structure to the postulated Europe-founding population. Also the different European populations are heterogeneous and there is a tendency of higher genetic diversity in the populations founded earlier. R. harrisii is still in the process of expansion in Europe and may have been introduced once or repeatedly by different invasion mechanisms. The pronounced lack of gene flow among populations is of great ecological significance, since it may facilitate rapid adaptation and specialization to local conditions within single estuarine systems.