Isolation and characterization of microsatellite markers in the invasive shrub Mahonia aquifolium (Berberidaceae) and their applicability in related species

Microsatellite loci were isolated from a Mahonia aquifolium cultivar. We describe the variability of 10 loci in invasive European and native North American M. aquifolium and their transspecies amplification in native Mahonia repens and Mahonia pinnata from North America and one species of the related genus Berberis (Berberis vulgaris), native to Europe. The markers should be useful to reveal the genetic origin of invasive Mahonia populations and differences in the genetic make up between invasive and native populations.     

Invasive Mahonia plants outgrow their native relatives

Invasive populations often grow more vigorously than conspecific populations in the native range. This has frequently been attributed to evolutionary changes resulting either from founder effects, or from natural selection owing to enemy release. Another mechanism contributing to evolutionary change has largely been neglected in the past: Many invasive plant species do actually descend from cultivated plants and were therefore subject to breeding, including hybridization and artificial selection. In a common garden experiment, we compared invasive Central European populations of the ornamental shrub, Mahonia, with native populations of its putative parental species, Mahonia aquifolium and M. repens, from North America. We hypothesized that plants of invasive populations show increased growth and retained high levels of heritable variation in phenotypic traits. Indeed, invasive Mahonia plants grew larger in terms of stem length, number of leaves and above-ground biomass than either of the two native species, which did not differ significantly from each other. Since there are no hints on release of invasive Mahonia populations from natural enemies, it is likely that hybridization and subsequent selection by breeders have lead to an evolutionary increase of plant vigour in the introduced range. Further on, heritable variation was not consistently reduced in invasive populations compared with populations of the two native species. We suggest that interspecific hybridization among the Mahonia species has counteracted the harmful effects of genetic bottlenecks often associated with species introductions. Based on this case study, we conclude that, more attention has to be paid on the role of plant breeding when assessing the mechanisms behind successful plant invasions in future.     

Are plant populations in expanding ranges made up of escaped cultivars? The case of Ilex aquifolium in Denmark

Rapid range expansions are becoming more prevalent, especially as climate continues to change. The escape of ornamental plants within their native range represents a significant, but often overlooked component of this process. Few studies have focused on the role of ornamental plants in range expansions using molecular markers to identify the possibility of mixed native and cultivar populations. The purpose of the present study was to determine the genetic variation of a native woody plant with ornamental conspecifics at the edge of its distribution. We selected Ilex aquifolium L. (English holly) which grows naturally in Denmark but is spreading eastward in Scandinavia, possibly due to a combination of climate change and introduction of more frost-tolerant cultivars. We sampled 187 individuals from older and recently established populations, and cultivated I. aquifolium throughout Denmark, and compared them using nuclear SSR and AFLP. The overall results showed no structure or clustering of plants from the historical or the expanding range, or of wild or cultivated plants. The only clusters found were for a group of cultivated hybrid I. aquifolium. The samples represented four genetic groupings, indicating either hybridization between cultivated and wild I. aquifolium or replacement of the latter by cultivars. Thus, ornamental genotypes of I. aquifolium contribute to the northeastern range expansion of the species and eventual invasiveness within its native range.     

Potential selection in native grass populations by exotic invasion

Ecological impacts of invasive plant species are well documented, but the genetic response of native species to invasive dominance has been often overlooked. Invasive plants can drastically alter site conditions where they reach dominance, potentially exerting novel selective pressures on persistent native plant populations. Do native plant populations in old exotic invasions show evidence of selection when compared to conspecific populations in adjacent, noninvaded areas? We employ amplified fragment length polymorphism (AFLP) analysis to screen a large number of loci from two native grass species (Hesperostipa comata (Trin. & Rupr.) Barkworth and Sporobolus airoides Torr.) that occur in old infestations of the invasive forb Acroptilon repens. We then compare observed locus by locus F_ST values with distributions of F_ST estimated from simulation models under expectation of neutrality. We also compare the proportion of loci possibly linked to selection and those not linked to selection which exhibit parallel trends in divergence between two community types (invaded, noninvaded). Few loci (H. comata, 2.6%; S. airoides, 8.7%) in the two native grasses may be linked to genes under the influence of selection. Also, loci linked to selection showed a greater portion of parallel trends in divergence than neutral loci. Genetic similarities between community types were less than genetic similarity within community types suggesting differentiation in response to community alteration. These results indicate that a small portion of scored AFLP loci may be linked to genes undergoing selection tied to community dominance by an invasive species. We propose that native plants in communities dominated by exotic invasives may be undergoing natural selection.     

Seedling recruitment in the invasive clonal shrub, Mahonia aquifolium Pursh (Nutt.)

The clonal shrub, Mahonia aquifolium, is an aggressive invader of some forests in central Germany. We analysed the importance of seedling recruitment for the local dynamics of these invasive populations. In contrast to many other clonal plants, repeated seedling recruitment takes place in M. aquifolium, contributing to the colonization of available space within populations. Thus, sexual reproduction is not only important for dispersal to new sites but also for the local invasion␣process. In situations with grass competition (Calamagrostis epigejos) the number of successful recruits is reduced, and clonal growth is the predominant mode of reproduction. Since seedling recruitment results in an increasing number of genets as well as ramets, the investigated populations are still below their carrying capacities. Seedling densities are higher beneath the canopy of adults than outside adult patches, and increase with increasing cover of adult ramets. The spatially clumped pattern of seedling emergence results in density-dependent mortality especially in the most crowded seedling clusters. However, adult neighbourhood imposes a much higher mortality. Therefore, we expect that the importance of seedling recruitment for the dynamics of local M. aquifolium populations will decline as the invasion process continues. Received: 15 January 1996 / Accepted: 28 September 1996     

Tracing the introduction history of a potentially invasive ornamental shrub: variation in frost hardiness and climate change

Nursery catalogues have recently been used as a method for tracing the introduction history of invasive plants. Information on species, cultivars, plant size and price can help quantify historical changes in propagule pressure. Propagule pressure of invasive ornamentals has to be combined with climatic data to understand shifting distribution patterns of (potentially) invasive plants. In this study the spread of Ilex aquifolium L. cultivars was investigated at the edge of its distribution margin in Denmark. Danish nursery catalogues from 1841–2007 were studied to 1) reconstruct the introduction history of potentially invasive ornamental genotypes of I. aquifolium in Denmark; 2) explore potential factors that could explain naturalization of the species, i.e. nursery location, plant size, hardiness, price and year; 3) investigate whether or not marketing has been more common inside the historical range of the species in western Denmark; and 4) test the hypothesis that marketing of this frost‐sensitive species has increased parallel with higher temperatures in the past 100 years. Price, plant size, hardiness, location, and year were recorded for a total of 3213 entries. Through time, increasing prices and the availability of frost‐hardy cultivars were the most significant factors contributing to the sale of I. aquifolium in Denmark. The number of nurseries per area, and hence propagule pressure, was higher in eastern Denmark, outside the natural range of the species. The proportion of very frost‐hardy cultivars increased with milder winter temperatures. The main conclusion from this combined nursery catalogue and climate study are that nurseries in an indirect way play an important role in increasing the propagule pressure of potentially invasive ornamentals and thus can affect distribution of native species.     

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.     

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.     

Inferring the invasion history of coral berry Ardisia crenata from China to the USA using molecular markers

Genetic comparisons between native and invasive populations of a species can provide insights into its invasion history information, which is useful for guiding management and control strategies. The coral berry Ardisia crenata was introduced to Florida last century as a cultivated ornament plant, and has since spread widely throughout the southern regions of the USA. Previously, the genetic variation among 20 natural populations of A. crenata across its distribution center in southern China was quantified using seven microsatellite markers. Here we expand on that work by additionally sampling individuals from four other native populations in Taiwan and Japan, and from five invasive populations in the USA. We also examined the results from one chloroplast intergenic spacer region (trnF-trnL) in all 29 populations. Our aim is to identify the invasion source and subsequent history of the species?? spread throughout the southern USA. We observed lower genetic diversity in the invasive populations based on both microsatellite and chloroplast markers. Our data show that the invasive populations can be clustered with native populations in southeastern China, inferring this region as the geographic origin of A. crenata cultivars invading the USA. We further classified invasive individuals into invasive I and invasive II clusters. Nantou in Taiwan and Xihu in mainland China are the most closely related populations to those, which identify the former as potential sources for host-specific control agents. Our results, combined with the known introduction records, suggest that A. crenata was first multiply introduced into Florida and then secondarily colonized Louisiana and Texas from Florida.     

Clash of the titans: a multi-species invasion with high gene flow in the globally invasive titan acorn barnacle

The goal of this study was to investigate the phylogeny, invasion history and genetic structure of the global invader Megabalanus coccopoma. First, we created a Bayesian phylogeny using cytochrome oxidase I and 16S mitochondrial genes of samples we collected and sequences available on GenBank for all species in the genus Megabalanus. Second, we compared the genetic differences within and between native and invasive populations verified as M. coccopoma by constructing a haplotype network of the COI sequences and estimating gene diversity (h) and nucleotide diversity (π). Finally, we ran an analysis of molecular variance and calculated pairwise Φ _ST to evaluate the similarity among populations. We identified several lineages that correspond to putatively different species of Megabalanus and uncovered nomenclature discrepancies among GenBank samples and undocumented lineages from our own collections. However, we found that the majority of samples were indeed M. coccopoma. Among populations of M. coccopoma, levels of within-population genetic diversity were not significantly different (p _h  = 0.131, p _π  = 0.129) between native (h = 0.970, π = 0.00708) and non-native populations (h = 0.950, π = 0.00605) and analysis of molecular variance analyses revealed that 98.34 % of the genetic variation was partitioned within populations with a significant global Φ _ST  = 0.017. Our results revealed that invasions in at least the southeastern United States and Brazil are composed of multiple lineages; however, we found that most of the global invasion occurred from a single lineage, M. coccopoma, and that no significant genetic differentiation exists between native and non-native populations of this species.     

Can Invasive Species Enhance Competitive Ability and Restoration Potential in Native Grass Populations?

Native plant individuals often persist within communities dominated by exotics but the influence of this exposure on native populations is poorly understood. Selection for traits contributing to competitive ability may lead to native plant populations that are more tolerant of the presence of exotic invaders. In this way, long‐term coexistence with an exotic may confer competitive advantages to remnant (experienced) native populations and be potentially beneficial to restoration. In past studies we have documented genetic differentiation within native grass populations exposed to the exotic invader Russian knapweed (Acroptilon repens). Here, we examine populations of a cool‐season grass, needle‐and‐thread (Hesperostipa comata [Trin. & Rupr.]) and a warm season, alkali sacaton (Sporobolus airoides [Torr.]) collected from Russian knapweed‐invaded sites and adjacent noninvaded sites to assess their relative competitive ability against a novel exotic neighbor, Canada thistle (Cirsium arvense). Experienced S. airoides (from within A. repens invasions) appear to better tolerate (accumulate biomass, leaf nitrogen content, and to initiate new tillers) the presence of a novel competitor (C. arvense). Experienced and inexperienced H. comata genets differ in their response to the presence of C. arvense. Relative neighbor effects of native grasses on C. arvense were generally greater from experienced grasses. The ability to compete with novel neighbors may be driven by general competitive traits rather than species‐specific coevolutionary trajectories. Irrespective of competitive mechanisms, the conservation of native species populations within weed invasions may provide an important restoration tool by retaining unique components of native gene pools selected by competitive interactions with exotics.     

The impact of the flower mite Aceria acroptiloni on the invasive plant Russian knapweed,Rhaponticum repens,in its native range

Rhaponticum repens (L.) Hidalgo is a clonal Asteraceae plant native to Asia and highly invasive in North America. We conducted open-field experiments in Iran to assess the impact of the biological control candidate, Aceria acroptiloni Shevchenko & Kovalev (Acari, Eriophyidae), on the target weed. Using three different experimental approaches, we found that mite attack reduced the biomass of R. repens shoots by 40–75 %. Except for the initial year of artificial infestation by A. acroptiloni of R. repens shoots, the number of seed heads was reduced by 60–80 % and the number of seeds by 95–98 %. Morphological investigations of the mite complex attacking R. repens at the experimental field site revealed that A. acroptiloni was by far the dominant mite species. We conclude that the mite A. acroptiloni is a promising biological control candidate inflicting significant impact on the above-ground biomass and reproductive output of the invasive plant R. repens.     

Altering native community assembly history influences the performance of an annual invader

Understanding the determinants of early invasion resistance is a major challenge for designing plant communities that efficiently repel invaders. Recent evidence highlighted the significant role of priority effects in early community assembly as they affect species composition, structure and functional properties, but the consequences of native community assembly history on the success of subsequent invasions has not been elucidated yet. In a greenhouse experiment, we investigated how (1) the identity of the first native colonizing species (one of two grasses: Dactylis glomerata and Lolium perenne, or two legumes: Onobrychis viciifolia and Trifolium repens), each introduced four weeks before the rest of the native community, and (2) timing of species establishment (synchronous vs. sequential sowing), influenced early establishment success of Ambrosia artemisiifolia, an annual noxious weed in Europe. First colonizer identity and establishment timing both affected early biomass production and composition of the community, and had implications for A. artemisiifolia early invasion success. Invasion success decreased when all native individuals were sown simultaneously, quickly generating a high biomass production, while it increased when the productive N-fixing legume T. repens was sown first. These findings support that native species assembly history matters to invasion resistance in the early growth stages, thus opening the way to more effective invasive species management strategies in restoration.     

Impacts of Carpobrotus edulis (L.) N.E.Br. on the Germination,Establishment and Survival of Native Plants: A Clue for Assessing Its Competitive Strength

Does Carpobrotus edulis have an impact on native plants? How do C. edulis’ soil residual effects affect the maintenance of native populations? What is the extent of interspecific competition in its invasion process? In order to answer those questions, we established pure and mixed cultures of native species and C. edulis on soil collected from invaded and native areas of Mediterranean coastal dunes in the Iberian Peninsula. We examined the impact of the invader on the germination, growth and survival of seeds and adult plants of two native plant species (Malcolmia littorea (L.) R.Br, and Scabiosa atropurpurea L.) growing with ramets or seeds of C. edulis. Residual effects of C. edulis on soils affected the germination process and early growth of native plants in different ways, depending on plant species and density. Interspecific competition significantly reduced the germination and early growth of native plants but this result was soil, density, timing and plant species dependent. Also, at any density of adult individuals of C. edulis, established native adult plants were not competitive. Moreover, ramets of C. edulis had a lethal effect on native plants, which died in a short period of time. Even the presence of C. edulis seedlings prevents the recruitment of native species. In conclusion, C. edulis have strong negative impacts on the germination, growth and survival of the native species M. littorea and S. atropurpurea. These impacts were highly depended on the development stages of native and invasive plants. Our findings are crucial for new strategies of biodiversity conservation in coastal habitats.     

The Population Genetics of Cultivation: Domestication of a Traditional Chinese Medicine,Scrophularia ningpoensis Hemsl. (Scrophulariaceae)

Background

Domestic cultivation of medicinal plants is an important strategy for protecting these species from over harvesting. Some species of medicinal plants have been brought into cultivation for more than hundreds years. Concerns about severe loss of genetic diversity and sustainable cultivation can potentially limit future use of these valuable plants. Genetic studies with comprehensive sampling of multiple medicinal species by molecular markers will allow for assessment and management of these species. Here we examine the population genetic consequences of cultivation and domestication in Scrophularia ningpoensis Hemsl. We used chloroplast DNA and genomic AFLP markers to clarify not only the effects of domestication on genetic diversity, but also determine the geographic origins of cultivars and their genetic divergence from native populations. These results will allow both better management of cultivated populations, but also provide insights for crop improvement.

Results

Twenty-one cpDNA haplotypes of S. ningpoensis were identified. Wild populations contain all haplotypes, whereas only three haplotypes were found in cultivated populations with wild populations having twice the haplotype diversity of cultivated populations. Genetic differentiation between cultivated populations and wild populations was significant. Genomic AFLP markers revealed similar genetic diversity patterns. Furthermore, Structure analysis grouped all wild populations into two gene pools; two of which shared the same gene pool with cultivated S. ningpoensis. The result of Neighbor-Joining analysis was consistent with the structure analysis. In principal coordinate analysis, three cultivated populations from Zhejiang Province grouped together and were separated from other cultivated populations.

Conclusions

These results suggest that cultivated S. ningpoensis has experienced dramatic loss of genetic diversity under anthropogenic influence. We postulate that strong artificial selection for medicinal quality has resulted in genetic differentiation between cultivated and wild populations. Furthermore, it appears that wild populations in Jiangxi-Hunan area were involved in the origin of cultivated S. ningpoensis.     

Genetic diversity of endangered Manglietia patungensis assessed by inter simple sequence repeat and sequence-related amplified polymorphism markers

Manglietia patungensis Hu is an endangered plant native to China. Knowledge of its genetic diversity and structure would aid its conservation. This study assessed nine natural populations of M. patungensis using two methods: inter simple sequence repeat (ISSR) and sequence-related amplified polymorphism (SRAP) markers. Using 10 ISSR primer pairs, 334 bands were generated, and 10 SRAP primer pairs generated 276 bands. The percent of polymorphic bands (91.32% and 93.48%), Nei's genetic diversity (0.3448 and 0.3323), and Shannon's information index (0.5075 and 0.4935) revealed a high level of genetic diversity at the species level. Total heterozygosity was 0.3439 by ISSR and 0.3281 by SRAP. The mean heterozygosity was 0.2323 by ISSR and 0.2521 by SRAP. The coefficient of genetic differentiation among natural populations was 0.3245 by ISSR and 0.2316 by SRAP. These data indicated higher levels of genetic diversity of M. patungensis within, rather than among, populations. Estimates of gene flow among natural populations were 1.0411 and 1.0589, which implied a certain amount of gene exchange among populations. A Mantel test revealed no significant correlation between genetic and geographic distance. ISSR and SRAP markers are both effective for genetic diversity research in M. Patungensis. Based on these results, conservation of M. patungensis should be performed both in situ and ex situ.     

Genetic structure,diversity, and hybridization in populations of the rare arctic relict <Emphasis Type="Italic">Euphrasia hudsoniana</Emphasis> (Orobanchaceae) and its invasive congener <Emphasis Type="Italic">Euphrasia stricta</Emphasis>

Arctic relict populations, which persist in disjunct locations far south of a species’ normal range, are at the frontline of climate change and may be especially susceptible to the negative impacts of climate warming. Further, these relict populations may face increasing contact with, or become outcompeted by, invasive species if the invasive taxa are spreading along with the warming climate. Relict populations are simultaneously of particular conservation importance due to their unique genetic make-up and potential for adaptations to warmer temperatures compared to populations at the core of the species range. In this study, we used genotyping-by-sequencing to study the population genetics of Euphrasia hudsoniana, a polyploid arctic disjunct of conservation concern, at the southern edge of its range along the northwestern shore of Lake Superior. In addition, we examined evidence for hybridization with its invasive congener, E. stricta. Overall, we found clear differentiation between the native and invasive species indicated by nearly all analyses. There was limited evidence for gene flow from the invasive into the native species, but patterns were consistent with more extensive gene flow in the opposite direction. Differentiation among native populations was low, yet two of the five populations fell into a separate, distinct group based on STRUCTURE analyses. Continued genetic monitoring of these populations will help elucidate whether hybridization with invasives is a burgeoning threat for this arctic relict.     

Risk Assessment of Transgenic Virus-resistant White Clover: Non-target Plant Community Characterisation and Implications for Field Trial Design

One of the most difficult elements of the ecological risk assessment of transgenic plants is investigation of their potential impact on biodiversity in complex non-target communities. This problem is particularly acute for pasture plants, since many have a track record of invasiveness. In this paper we develop an understanding of some aspects of the ecology of Trifolium repens L. (white clover) in montane vegetation communities in southeastern Australia as part of a larger project investigating potential ecological risks associated with commercial release of newly-developed transgenic virus-resistant T. repens cultivars. We use a combination of floristic surveys and biomass sampling to determine the habitat affinity of T. repens, the structure and composition of associated communities, and the scale at which different abiotic and biotic factors correlate with T. repens abundance. We also compare the abundance of native and exotic species within specific morpho-functional groups and use this to determine the relative significance of T. repens as a community constituent, and to identify native species that would be most at risk by expanding populations of T. repens. We found that T. repens comprises a relatively small component of the total community cover and biomass, but is one of the most abundant herbaceous species in mesic Poa – dominated grasslands and Poa-Eucalyptus woodlands in the study area, and that T. repens abundance is correlated at the within-community scale primarily with soil moisture and fertility. At smaller scales T. repens is limited by the hierarchical dominance of native graminoids and we conclude that competition for inter-tussock space in mesic communities is the most likely mechanism by which expanding populations of T. repens would influence populations of associated native species. These results have significant implications for the manner in which future analysis and risk quantification stages of the risk assessment of virus-resistant T.␣repens and other transgenic pasture plants in complex plant communities are performed.     

Population genetics of introduced bullfrogs, Rana (Lithobates) catesbeianus, in the Willamette Valley, Oregon, USA

The American bullfrog, Rana (Lithobates) catesbeianus, is endemic to eastern North America, but has been introduced to approximately 40 countries on four continents and is considered one of the hundred worst invasive alien species in the world. Here, we investigated the genetics of invasive bullfrogs in the Willamette Valley, Oregon, USA, where bullfrogs are widespread and abundant to determine: (1) the minimum number of bullfrog introductions; (2) the native source population(s); and (3) whether genetic variation is reduced compared to source populations. To answer these questions, we analyzed partial sequences of the mitochondrial cytochrome b gene for 251 bullfrogs from the Willamette Valley and the native range. We found that bullfrogs from the Mississippi River basin and Great Lakes region were introduced at least once to the Willamette Valley. Genetic variation measured as haplotype diversity (h) and nucleotide diversity (?? _n ) was not significantly different between Willamette Valley and source populations. Our results were in contrast to a recent genetic analysis of invasive bullfrog populations in Europe, which found that genetic variation in European bullfrog populations was much lower than in source populations. European bullfrogs also originated from different source populations than Willamette Valley bullfrogs. The difference in genetic composition between Willamette Valley and European bullfrogs is likely due to differences in their invasion histories and may have implications for the potential of bullfrogs in these different regions to adapt and expand.