Genetic diversity and colony breeding structure in native and introduced ranges of the Formosan subterranean termite, Coptotermes formosanus

The Formosan subterranean termite, Coptotermes formosanus is recognized as one of the most important invasive pest species. Originating from China, C. formosanus has spread to many tropical and subtropical regions around the globe in the last 400 years, including Japan, Hawaii and the continental USA. Although the current distribution is well documented, information about the patterns of invasion and effects of introduction on the population genetics of this species is largely lacking. We analyzed the genetic structure of populations from two native populations (Guangdong and Hunan provinces, China) and two introduced populations (Maui and Kauai, Hawaii) using microsatellite genotyping. We also reanalyzed published data of additional populations from China, Japan, Hawaii, and the continental USA. The population from Hunan, the earliest introduction outside of China (Japan) and the first introduction to the continental USA (South Carolina) showed little genetic similarity with any of the native or introduced populations investigated. However, populations from Oahu (HI), New Orleans (LA) and Rutherford County (NC) showed close similarity. In general, genetic patterns suggest multiple introductions to the USA, with, for example, two separate introductions to the island of Maui. Bottleneck effects were detected in almost all recent introductions (after 1940). All populations in the introduced range showed lower genetic diversity than those in the native range. However, this low genetic diversity did not result in the formation of polygynous supercolonies as has been described for other invasive termite and ant species.     

Understanding invasion history: genetic structure and diversity of two globally invasive plants and implications for their management

Aim Resolving the origin of invasive plant species is important for understanding the introduction histories of successful invaders and aiding strategies aimed at their management. This study aimed to infer the number and origin(s) of introduction for the globally invasive species, Macfadyena unguis‐cati and Jatropha gossypiifolia using molecular data. Location Native range: Neotropics; Invaded range: North America, Africa, Europe, Asia, Pacific Islands and Australia. Methods We used chloroplast microsatellites (cpSSRs) to elucidate the origin(s) of introduced populations and calculated the genetic diversity in native and introduced regions. Results Strong genetic structure was found within the native range of M. unguis‐cati, but no genetic structuring was evident in the native range of J. gossypiifolia. Overall, 27 haplotypes were found in the native range of M. unguis‐cati. Only four haplotypes were found in the introduced range, with more than 96% of introduced specimens matching a haplotype from Paraguay. In contrast, 15 haplotypes were found in the introduced range of J. gossypiifolia, with all invasive populations, except New Caledonia, comprising multiple haplotypes. Main conclusions These data show that two invasive plant species from the same native range have had vastly different introduction histories in their non‐native ranges. Invasive populations of M. unguis‐cati probably came from a single or few independent introductions, whereas most invasive J. gossypiifolia populations arose from multiple introductions or alternatively from a representative sample of genetic diversity from a panmictic native range. As introduced M. unguis‐cati populations are dominated by a single haplotype, locally adapted natural enemies should make the best control agents. However, invasive populations of J. gossypiifolia are genetically diverse and the selection of bio‐control agents will be considerably more complex.     

Standing variation boosted by multiple sources of introduction contributes to the success of the introduced species, Lotus corniculatus

Although ecological differences between native and introduced ranges have been considered to drive rapid expansion of invasive species, recent studies suggest that rapid evolutionary responses of invasive species to local environments may also be common. Such expansion across heterogeneous environments by adaptation to local habitats requires genetic variation. In this study, we investigated the source and role of standing variation in successful invasion of heterogeneous abiotic environments in a self-incompatible species, Lotus corniculatus. We compared phenotypic and genetic variation among cultivars, natives, and introduced genotypes, and found substantial genetic variation within both native and introduced populations. Introduced populations possessed genotypes derived from both cultivars and native populations, and had lower population differentiation, indicating multiple sources of introduction and population admixture among the sources in the introduced range. Both cultivars and introduced populations had similarly outperforming phenotypes on average, with increased biomass and earlier flowering compared with native populations, but those phenotypes were within the range of the variation in phenotypes of the native populations. In addition, clinal variation within introduced populations was detected along a climatic gradient. Multiple introductions from different sources, including cultivars, may have contributed to pre-adaptive standing variation in the current introduced populations. We conclude that both introduction of cultivar genotypes and natural selection in local environments contributed to current patterns of genetic and phenotypic variation observed in the introduced populations.     

Bayesian inference of a complex invasion history revealed by nuclear and chloroplast genetic diversity in the colonizing plant,Silene latifolia

Species invading new ranges are subject to a series of demographic events that can strongly shape genetic diversity. Describing this demographic history is important for understanding where invasive species come from and how they spread, and is critical to testing hypotheses of postinvasion adaptation. Here, we analyse nuclear and chloroplast genetic diversity to study the invasion history of the widespread colonizing weed, Silene latifolia (Caryophyllaceae). Bayesian clustering and PCA revealed strong population structure in the native range of Europe, and although genotypes from multiple native sources were present in the introduced range of North America, the spatial distribution of genetic variance was dramatically reorganized. Using approximate Bayesian computation (ABC), we compared support for different invasion scenarios, including the number and size of independent introduction events and the amount of admixture occurring between sources of introduced genotypes. Our results supported independent introductions into eastern and western North America, with the latter forming a bridgehead for a secondary invasion into the Great Lakes region of central North America. Despite small estimated founder population sizes, the duration of the demographic bottleneck after the initial introduction appeared extremely short‐lived. This pattern of repeated colonization and rapid expansion has effectively eroded the strong population structure and cytonuclear associations present in Europe, but has retained overall high genetic diversity since invasion. Our results highlight the flexibility of the ABC approach for constructing a narrative of the demographic history of species invasions and provide baseline for future studies of evolutionary changes in introduced S. latifolia populations.     

Patterns of genetic diversity reveal multiple introductions and recurrent founder effects during range expansion in invasive populations of Geranium carolinianum (Geraniaceae)

Genetic diversity, and thus the adaptive potential of invasive populations, is largely based on three factors: patterns of genetic diversity in the species'' native range, the number and location of introductions and the number of founding individuals per introduction. Specifically, reductions in genetic diversity (‘founder effects'') should be stronger for species with low within-population diversity in their native range and few introductions of few individuals to the invasive range. We test these predictions with Geranium carolinianum, a winter annual herb native to North America and invasive in China. We measure the extent of founder effects using allozymes and microsatellites, and ask whether this is consistent with its colonization history and patterns of diversity in the native range. In the native range, genetic diversity is higher and structure is lower than expected based on life history traits. In China, our results provide evidence for multiple introductions near Nanjing, Jiangsu province, with subsequent range expansion to the west and south. Patterns of genetic diversity across China reveal weak founder effects that are driven largely by low-diversity populations at the expansion front, away from the introduction location. This suggests that reduced diversity in China has resulted from successive founder events during range expansion, and that the loss of genetic diversity in the Nanjing area was mitigated by multiple introductions from diverse source populations. This has implications for the future of G. carolinianum in China, as continued gene flow among populations should eventually increase genetic diversity within the more recently founded populations.     

Genetic and epigenetic changes during the invasion of a cosmopolitan species (Phragmites australis)

While many introduced invasive species can increase genetic diversity through multiple introductions and/or hybridization to colonize successfully in new environments, others with low genetic diversity have to persist by alternative mechanisms such as epigenetic variation. Given that Phragmites australis is a cosmopolitan reed growing in a wide range of habitats and its invasion history, especially in North America, has been relatively well studied, it provides an ideal system for studying the role and relationship of genetic and epigenetic variation in biological invasions. We used amplified fragment length polymorphism (AFLP) and methylation‐sensitive (MS) AFLP methods to evaluate genetic and epigenetic diversity and structure in groups of the common reed across its range in the world. Evidence from analysis of molecular variance (AMOVA) based on AFLP and MS‐AFLP data supported the previous conclusion that the invasive introduced populations of P. australis in North America were from European and Mediterranean regions. In the Gulf Coast region, the introduced group harbored a high level of genetic variation relative to originating group from its native location, and it showed epigenetic diversity equal to that of the native group, if not higher, while the introduced group held lower genetic diversity than the native. In the Great Lakes region, the native group displayed very low genetic and epigenetic variation, and the introduced one showed slightly lower genetic and epigenetic diversity than the original one. Unexpectedly, AMOVA and principal component analysis did not demonstrate any epigenetic convergence between native and introduced groups before genetic convergence. Our results suggested that intertwined changes in genetic and epigenetic variation were involved in the invasion success in North America. Although our study did not provide strong evidence proving the importance of epigenetic variation prior to genetic, it implied the similar role of stable epigenetic diversity to genetic diversity in the adaptation of P. australis to local environment.     

Reconstructing the invasion history of Heracleum persicum (Apiaceae) into Europe

Sparse, incomplete and inappropriate historical records of invasive species often hamper invasive species management interventions. Population genetic analyses of invaders might provide a suitable context for the identification of their source populations and possible introduction routes. Here, we describe the population genetics of Heracleum persicum Desf. ex Fisch and trace its route of introduction into Europe. Microsatellite markers revealed a significantly higher genetic diversity of H. persicum in its native range, and the loss of diversity in the introduced range may be attributed to a recent genetic bottleneck. Bayesian cluster analysis on regional levels identified three and two genetic clusters in the native and the introduced ranges, respectively. A global structure analysis revealed two worldwide distinct genetic groups: one primarily in Iran and Denmark, the other primarily in Norway. There were also varying degrees of admixture in England, Sweden, Finland and Latvia. Approximate Bayesian computation indicated two independent introductions of H. persicum from Iran to Europe: the first one in Denmark and the second one in England. Finland was subsequently colonized by English populations. In contrast to the contemporary hypothesis of English origin of Norwegian populations, we found Finland to be a more likely source for Norwegian populations, a scenario supported by higher estimated histor‐ical migration from Finland to Norway. Genetic diversity per se is not a primary determinant of invasiveness in H. persicum. Our results indicate that, due to either pre‐adaptations or rapid local adaptations, introduced populations may have acqu‐ired invasiveness after subsequent introductions, once a suitable environment was encountered.     

Range expansion by invasion: genetic characterization of invasion of the greenside darter (Etheostoma blennioides) at the northern edge of its distribution

Species introductions in freshwater ecosystems are often complex processes, yet an understanding of the nature of the introduction can inform management and conservation actions. The greenside darter (Etheostoma blennioides), until recently a species of special concern, expanded its Canadian range and is now common and widespread in the Grand River watershed (GRW). This is despite there being no evidence of greenside darter in the GRW prior to 1990. The goal of this study was to genetically characterize the GRW greenside darter introduction. Greenside darter were sampled in the GRW, the three known native watersheds in Canada, and one site from Ohio. We measured genetic diversity and population structure, and tested for population bottlenecks using eight microsatellite loci. Genotype assignment was used to identify possible introduction sources. Populations in the GRW showed similar genetic diversity to native watershed populations with no evidence for recent or historical population bottlenecks. Genotype assignment showed that one of the Canadian watersheds and the Ohio site were not potential sources of the GRW greenside darter, whereas the Thames River watershed was the most likely source. Substantial population genetic structure exists among the sample sites in the GRW. Clearly, the current widespread and abundant distribution of the greenside darter in the GRW is not the result of recent expansion of an existing native population, but rather multiple introductions into at least three sites in the GRW, followed by rapid population growth. Although the GRW E. blennioides is introduced, it harbours considerable genetic diversity and represents an important northern range extension for this species.     

A population genetic signature of human releases in an invasive ladybeetle

Biological invasions have been accelerated by a variety of human activities. Propagule pressure, the number of introduced individuals and independent introductions, is probably to be influenced by these human activities and may be an important factor for successful range expansion in new environments. We tested whether the current distribution of the predatory ladybeetle Coccinella septempunctata in the introduced range (USA) is the result of multiple historical human introductions or natural range expansion from the first established populations in the USA. To test this hypothesis, we compared historical records of propagule size, propagule number, specific introduction locations and the date of each introduction, with estimates of genetic variation in mitochondrial DNA (cytochrome oxidase I). Our results indicated that genetic diversity in the introduced range was positively correlated with historical records of propagule size and number and negatively correlated with distance to nearest introduction point, suggesting that multiple human releases were successful. Higher genetic diversity in populations found near introduction points suggest that initial founder effects were limited, but lower genetic diversity found farther from introduction points is probably the result of serial founder effects during secondary range expansion. These results suggest that the current distribution of C. septempunctata in the introduced range is the result of a combination of human releases and short‐range expansion from multiple established populations in the introduced range.     

Founder events predict changes in genetic diversity during human‐mediated range expansions

Intentional or accidental introduction of species to new locations is predicted to result in loss of genetic variation and increase the likelihood of inbreeding, thus reducing population viability and evolutionary potential. However, multiple introductions and large founder numbers can prevent loss of genetic diversity and may therefore facilitate establishment success and range expansion. Based on a meta‐analysis of 119 introductions of 85 species of plants and animals, we here show a quantitative effect of founding history on genetic diversity in introduced populations. Both introduction of large number of individuals and multiple introduction events significantly contribute to maintaining or even increasing genetic diversity in introduced populations. The most consistent loss of genetic diversity is seen in insects and mammals, whereas introduced plant populations tend to have higher genetic variation than native populations. However, loss or gain of genetic diversity does not explain variation in the extent to which plant or animal populations become invasive outside of their native range. These results provide strong support for predictions from population genetics theory with respect to patterns of genetic diversity in introduced populations, but suggest that invasiveness is not limited by genetic bottlenecks.     

Are bottlenecks associated with colonization? Genetic diversity and diapause variation of native and introduced Rhagoletis completa populations

The success of invasive species appears to be a paradox: despite experiencing strong population bottlenecks, invasive species are able to successfully establish in new environments. We studied how the walnut husk fly, Rhagoletis completa, was able to successfully colonize California from the Midwestern United States, by examining genetic diversity and diapause variation of native and introduced fly populations. Climate plays an important role in the successful establishment of introduced insects, because insect diapause is highly dependent upon external climatic conditions. We examined if: (1) fly populations show signs of a population bottleneck, (2) native and introduced flies differ in diapause length when exposed to California and Midwestern climatic conditions, and (3) population genetic diversity is related to variation in diapause length. We assessed if fly diapause conformed more to a model of establishment by local adaptation or to a model of a highly plastic “general-purpose genotype”. Our results indicate that only two populations close to the original introduced location showed signs of a population bottleneck, and native and introduced populations did not differ in genetic diversity. Genetic diversity increased in the northern introduced populations, suggesting that multiple introductions have occurred. Flies emerged about 2 weeks earlier under the Midwestern treatment than the California treatment, and introduced flies emerged about a week earlier than native flies. All flies emerged when walnuts are typically available in California. Although variance in diapause length differed between populations, it did not vary between populations or regions. Furthermore, genetic diversity was not associated with diapause variation. Therefore, multiple introductions and a “general-purpose genotype” appear to have facilitated the fly’s invasion into California.     

Molecular markers reconstruct the invasion history of variable leaf watermilfoil (<Emphasis Type="Italic">Myriophyllum heterophyllum</Emphasis>) and distinguish it from closely related species

Genetic variation is increasingly recognized as an important factor influencing the establishment and spread of introduced species, and depends on both the introduction history and partitioning of genetic variation within and among potential source populations. We examine patterns of genetic variation in native and introduced populations of variable leaf watermilfoil, Myriophyllum heterophyllum, using chloroplast (trnL-F) and ribosomal (ITS) DNA sequences, as well as amplified fragment length polymorphisms (AFLPs). We identify a strong phylogeographic break distinguishing populations located on the Atlantic Coastal Plain (ACP) versus other (“Continental”) portions of the native range. Within these distinct biogeographic regions, we also find genetic variation to be strongly partitioned among populations as analysis of molecular variance (AMOVA) partitioned 91 and 75% of cpDNA and ITS diversity among populations, respectively. We demonstrate that the introduced ranges of variable leaf watermilfoil (northeastern and western US) result from multiple independent introductions from a variety of source populations, including lineages from both the ACP and Continental portions of the native range. In addition, we used our molecular markers to demonstrate that variable leaf watermilfoil is genetically distinct from three closely-related species that it is morphologically similar to. In particular, we demonstrate that M. heterophyllum is clearly distinct from a morphologically similar native species in the western US, M. hippuroides—whose distinctiveness from M. heterophyllum has been questioned—and therefore confirm the introduction of M. heterophyllum in the western US. Furthermore, we provide the first evidence for hybridization between these two species. Finally, our molecular markers identify previously unrecognized genetic variation in these four species, and therefore demonstrate the need for further taxonomic investigation.     

Reconciling molecular signatures across markers: mitochondrial DNA confirms founder effect in invasive North American house finches (<Emphasis Type="Italic">Carpodacus mexicanus</Emphasis>)

Well-characterized species introductions provide opportunities to compare the genetic signatures of known founder effects across classes of molecular markers. The release of small numbers of house finches (Carpodacus mexicanus) into the eastern United States in the 1940s led to substantial interest in the effects of this introduction on genetic diversity in this now abundant species, an issue that has been highlighted by a recent Mycoplasma disease epidemic that most intensively affects the introduced and potentially genetically depauperate house finch populations. Previous studies comparing genetic diversity levels in native and introduced house finch populations produced seemingly disparate results: comparisons based on amplified fragment length polymorphism, RFLP mtDNA, and allozyme markers found essentially equivalent levels of diversity in eastern and western populations, whereas microsatellite markers showed clear reductions in diversity in the introduced populations. Here we employ sequence variation at the ND2 mtDNA locus to further compare levels of diversity between the four native and five introduced house finch populations that were previously examined in the microsatellite study. We found substantially lower ND2 haplotype richness and diversity across all introduced populations of house finches. The majority of sequence variation (78%) was detected within subpopulations, with the remainder (22%) explained by the historical status of each population (native or introduced). Our results are consistent with previous microsatellite evidence for a founder effect during the introduction of eastern house finches, and suggest that the mtDNA founder effect was particularly severe, likely owing to a male-biased sex ratio at the time of introduction coupled with the lower effective population size of clonally inherited markers. We discuss how the inconsistencies between past studies of house finch diversity can inform the usefulness of distinct marker sets for detecting molecular signatures of founder events.     

Population genetics of introduced and native populations of the green mussel, Perna viridis: determining patterns of introduction

Genetic variation can be used to determine routes of introduction of non-native species and whether introduced populations lost variation during establishment. The present study sought to determine whether multiple, geographically isolated non-native populations of the green mussel, Perna viridis, were the product of a stepping stone expansion of a single introduction or from multiple independent introductions from the native range. Measurements of genetic variation were compared among five introduced populations and three populations from within the native range. We sequenced 650 bp of the mitochondrial gene cytochrome oxidase I from 280 samples from five introduced populations and another 190 samples from three native populations. Haplotype frequencies of all introduced populations were not significantly different from each other, but virtually all populations differed from samples taken from the native range. Measurements of genetic variation tended to suggest that introduced populations had less variation than most native populations and there was no evidence for admixture in any of the introduced populations. The genetic data and Monte Carlo simulations both provide compelling evidence of a stepping-stone pattern of introduction of P. viridis from the native range to Trinidad, and from Trinidad to other locations in the Caribbean and United States. The lack of genetic variation in introduced populations suggests that the initial introduction was relatively small and the lack of admixture suggests a single original source population.     

Long live the alien: is high genetic diversity a pivotal aspect of crested porcupine (Hystrix cristata) long‐lasting and successful invasion?

Studying the evolutionary dynamics of an alien species surviving and continuing to expand after several generations can provide fundamental information on the relevant features of clearly successful invasions. Here, we tackle this task by investigating the dynamics of the genetic diversity in invasive crested porcupine (Hystrix cristata) populations, introduced to Italy about 1500 years ago, which are still growing in size, distribution range and ecological niche. Using genome‐wide RAD markers, we describe the structure of the genetic diversity and the demographic dynamics of the H. cristata invasive populations and compare their genetic diversity with that of native African populations of both H. cristata and its sister species, H. africaeaustralis. First, we demonstrate that genetic diversity is lower in both the invasive Italian and the North Africa source range relative to other native populations from sub‐Saharan and South Africa. Second, we find evidence of multiple introduction events in the invasive range followed by very limited gene flow. Through coalescence‐based demographic reconstructions, we also show that the bottleneck at introduction was mild and did not affect the introduced genetic diversity. Finally, we reveal that the current spatial expansion at the northern boundary of the range is following a leading‐edge model characterized by a general reduction of genetic diversity towards the edge of the expanding range. We conclude that the level of genome‐wide diversity of H. cristata invasive populations is less important in explaining its successful invasion than species‐specific life‐history traits or the phylogeographic history in the native source range.     

Founding events in species invasions: genetic variation, adaptive evolution, and the role of multiple introductions

Invasive species are predicted to suffer from reductions in genetic diversity during founding events, reducing adaptive potential. Integrating evidence from two literature reviews and two case studies, we address the following questions: How much genetic diversity is lost in invasions? Do multiple introductions ameliorate this loss? Is there evidence for loss of diversity in quantitative traits? Do invaders that have experienced strong bottlenecks show adaptive evolution? How do multiple introductions influence adaptation on a landscape scale? We reviewed studies of 80 species of animals, plants, and fungi that quantified nuclear molecular diversity within introduced and source populations. Overall, there were significant losses of both allelic richness and heterozygosity in introduced populations, and large gains in diversity were rare. Evidence for multiple introductions was associated with increased diversity, and allelic variation appeared to increase over long timescales (~100 years), suggesting a role for gene flow in augmenting diversity over the long‐term. We then reviewed the literature on quantitative trait diversity and found that broad‐sense variation rarely declines in introductions, but direct comparisons of additive variance were lacking. Our studies of Hypericum canariense invasions illustrate how populations with diminished diversity may still evolve rapidly. Given the prevalence of genetic bottlenecks in successful invading populations and the potential for adaptive evolution in quantitative traits, we suggest that the disadvantages associated with founding events may have been overstated. However, our work on the successful invader Verbascum thapsus illustrates how multiple introductions may take time to commingle, instead persisting as a ‘mosaic of maladaptation’ where traits are not distributed in a pattern consistent with adaptation. We conclude that management limiting gene flow among introduced populations may reduce adaptive potential but is unlikely to prevent expansion or the evolution of novel invasive behaviour.     

The worldwide expansion of the Argentine ant

Aim The aim of this study was to determine the number of successful establishments of the invasive Argentine ant outside native range and to see whether introduced supercolonies have resulted from single or multiple introductions. We also compared the genetic diversity of native versus introduced supercolonies to assess the size of the propagules (i.e. the number of founding individuals) at the origin of the introduced supercolonies. Location Global. Methods We used mitochondrial DNA (mtDNA) markers and microsatellite loci to study 39 supercolonies of the Argentine ant Linepithema humile covering both the native (n = 25) and introduced range (n = 14). Results Data from three mitochondrial genes and 13 nuclear microsatellites suggest that the introduced supercolonies studied originated from at least seven founding events out of the native area in Argentina (primary introductions). The distribution of mtDNA haplotypes also suggests that supercolonies in the introduced range each derive from a single source supercolony and that one of these source supercolonies has been particularly successful, being the basis of many introduced populations spread across the world. Comparison of the genetic diversity of supercolonies based on the five most diverse loci also revealed that native and introduced supercolonies have greatly overlapping ranges of diversity, although the genetic diversity is on average less in introduced than in native supercolonies. Main conclusions Both primary introductions (from the native range) and secondary introductions (from sites with established invasive supercolonies) were important in the global expansion of the Argentine ant. In combination with the similar social organization of colonies in the native and introduced range, this indicates that invasiveness did not evolve recently as a unique and historically contingent event (e.g. reduction of genetic diversity) in this species. Rather, native L. humile supercolonies have characteristics that make them pre‐adapted to invade new – and in particular disturbed – habitats when given the opportunity. These results have important implications with regard to possible strategies to be used to control invasive ants.     

Loss of genetic diversity and increased embryonic mortality in non‐native lizard populations

Many populations are small and isolated with limited genetic variation and high risk of mating with close relatives. Inbreeding depression is suspected to contribute to extinction of wild populations, but the historical and demographic factors that contribute to reduced population viability are often difficult to tease apart. Replicated introduction events in non‐native species can offer insights into this problem because they allow us to study how genetic variation and inbreeding depression are affected by demographic events (e.g. bottlenecks), genetic admixture and the extent and duration of isolation. Using detailed knowledge about the introduction history of 21 non‐native populations of the wall lizard Podarcis muralis in England, we show greater loss of genetic diversity (estimated from microsatellite loci) in older populations and in populations from native regions of high diversity. Loss of genetic diversity was accompanied by higher embryonic mortality in non‐native populations, suggesting that introduced populations are sufficiently inbred to jeopardize long‐term viability. However, there was no statistical correlation between population‐level genetic diversity and average embryonic mortality. Similarly, at the individual level, there was no correlation between female heterozygosity and clutch size, infertility or hatching success, or between embryo heterozygosity and mortality. We discuss these results in the context of human‐mediated introductions and how the history of introductions can play a fundamental role in influencing individual and population fitness in non‐native species.     

Evidence for multiple introductions of Centaurea stoebe micranthos (spotted knapweed,Asteraceae) to North America

Invasive species’ success may depend strongly on the genetic resources they maintain through the invasion process. We ask how many introductions have occurred in the North American weed Centaurea stoebe micranthos (Asteraceae), and explore whether genetic diversity and population structure have changed as a result of introduction. We surveyed individuals from 15 European native range sites and 11 North American introduced range sites at six polymorphic microsatellite loci. No significant difference existed in the total number of alleles or in the number of private alleles found in each range. Shannon–Weaver diversity of phenotype frequencies was also not significantly different between the ranges, while expected heterozygosity was significantly higher in the invasive range. Population structure was similar between the native range and the invasive range, and isolation by distance was not significant in either range. Traditional assignment methods did not allocate any North American individuals to the sampled European populations, while Bayesian assignment methods grouped individuals into nine genetic clusters, with three of them shared between North America and Europe. Invasive individuals tended to have genetically admixed profiles, while natives tended to assign more strongly to a single cluster. Many North American individuals share assignment with Romania and Bulgaria, suggesting two separate invasions that have undergone gene flow in North America. Samples from three other invasive range sites were genetically distinct, possibly representing three other unique introductions. Multiple introductions and the maintenance of high genetic diversity through the introduction process may be partially responsible for the invasive success of C. stoebe micranthos.