Phylogeographic insights into the invasion history and secondary spread of the signal crayfish in Japan

Successful invasion by nonindigenous species is often attributed to high propagule pressure, yet some foreign species become widespread despite showing reduced genetic variation due to founder effects. The signal crayfish (Pacifastacus leniusculus) is one such example, where rapid spread across Japan in recent decades is believed to be the result of only three founding populations. To infer the history and explore the success of this remarkable crayfish invasion, we combined detailed phylogeographical and morphological analyses conducted in both the introduced and native ranges. We sequenced 16S mitochondrial DNA of signal crayfish from across the introduced range in Japan (537 samples, 20 sites) and the native range in western North America (700 samples, 50 sites). Because chela size is often related to aggressive behavior in crayfish, and hence, their invasion success, we also measured chela size of a subset of specimens in both introduced and native ranges. Genetic diversity of introduced signal crayfish populations was as high as that of the dominant phylogeographic group in the native range, suggesting high propagule pressure during invasion. More recently established crayfish populations in Japan that originated through secondary spread from one of the founding populations exhibit reduced genetic diversity relative to older populations, probably as a result of founder effects. However, these newer populations also show larger chela size, consistent with expectations of rapid adaptations or phenotypic responses during the invasion process. Introduced signal crayfish populations in Japan originate from multiple source populations from a wide geographic range in the native range of western North America. A combination of high genetic diversity, especially for older populations in the invasive range, and rapid adaptation to colonization, manifested as larger chela in recent invasions, likely contribute to invasion success of signal crayfish in Japan.     

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.     

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.     

The invasion history of the exotic freshwater zooplankter Daphnia lumholtzi (Cladocera, Crustacea) in North America: a genetic analysis

Daphnia lumholtzi is a planktonic crustacean native to subtropical regions in Africa, Asia and Australia. Since its invasion to the southern USA in ~1990 it has spread across North America as far north as the Laurentian Great Lakes. We assessed invasion history using microsatellite makers and to explore the influence of mean annual temperature on the genetic structure along a latitudinal gradient in North America. Genotypic data were obtained from 9 microsatellite markers for 178 individuals from 13 populations (eight populations introduced to North America and five populations in the native range). Pairwise F_st values as well as Bayesian clustering showed a strong subdivision between native and introduced populations. Bayesian clustering identified multiple genetic clusters in recently invaded locations, suggestive of multiple invasions from various sources, including Asia and Africa. Using variation partitioning, we determined the amount of variation for genetic clusters of populations in the invaded range due to mean annual air temperature and the year of first detection. The results point to a primary introduction into the southern range of North America, with a subsequent northward expansion, and multiple introductions possibly from both the native range and by secondary spread from previously-invaded locations. Separate analysis of genetic clusters within the invaded range suggests additional effects of temperature conditions on geographic genetic structure, possibly as a consequence of D. lumholtzi’s tropical origin.     

Living in two worlds: Evolutionary mechanisms act differently in the native and introduced ranges of an invasive plant

Identifying the factors that influence spatial genetic structure among populations can provide insights into the evolution of invasive plants. In this study, we used the common reed (Phragmites australis), a grass native in Europe and invading North America, to examine the relative importance of geographic, environmental (represented by climate here), and human effects on population genetic structure and its changes during invasion. We collected samples of P. australis from both the invaded North American and native European ranges and used molecular markers to investigate the population genetic structure within and between ranges. We used path analysis to identify the contributions of each of the three factors—geographic, environmental, and human‐related—to the formation of spatial genetic patterns. Genetic differentiation was observed between the introduced and native populations, and their genetic structure in the native and introduced ranges was different. There were strong effects of geography and environment on the genetic structure of populations in the native range, but the human‐related factors manifested through colonization of anthropogenic habitats in the introduced range counteracted the effects of environment. The between‐range genetic differences among populations were mainly explained by the heterogeneous environment between the ranges, with the coefficient 2.6 times higher for the environment than that explained by the geographic distance. Human activities were the primary contributor to the genetic structure of the introduced populations. The significant environmental divergence between ranges and the strong contribution of human activities to the genetic structure in the introduced range suggest that invasive populations of P. australis have evolved to adapt to a different climate and to human‐made habitats in North America.     

Genetic reconstruction of a bullfrog invasion to elucidate vectors of introduction and secondary spread

Reconstructing historical colonization pathways of an invasive species is critical for uncovering factors that determine invasion success and for designing management strategies. The American bullfrog (Lithobates catesbeianus) is endemic to eastern North America, but now has a global distribution and is considered to be one of the worst invaders in the world. In Montana, several introduced populations have been reported, but little is known of their sources and vectors of introduction and secondary spread. We evaluated the genetic composition of introduced populations at local (Yellowstone River floodplain) and regional (Montana and Wyoming) scales in contrast to native range populations. Our objectives were to (1) estimate the number of introductions, (2) identify probable native sources, (3) evaluate genetic variation relative to sources, and (4) characterize properties of local‐ and regional‐scale spread. We sequenced 937 bp of the mitochondrial cytochrome b locus in 395 tadpoles collected along 100 km of the Yellowstone River, from three additional sites in MT and a proximate site in WY. Pairwise Φ_ST revealed high divergence among nonnative populations, suggesting at least four independent introductions into MT from diverse sources. Three cyt b haplotypes were identical to native haplotypes distributed across the Midwest and Great Lakes regions, and AMOVA confirmed the western native region as a likely source. While haplotype (H_d = 0.69) and nucleotide diversity (π = 0.005) were low in introduced bullfrogs, the levels of diversity did not differ significantly from source populations. In the Yellowstone, two identified haplotypes implied few introduction vectors and a significant relationship between genetic and river distance was found. Evidence for multiple invasions and lack of subsequent regional spread emphasizes the importance of enforcing legislation prohibiting bullfrog importation and the need for continuing public education to prevent transport of bullfrogs in MT. More broadly, this study demonstrates how genetic approaches can reveal key properties of a biological invasion to inform management strategies.     

Genetic admixture accelerates invasion via provisioning rapid adaptive evolution

Genetic admixture, the intraspecific hybridization among divergent introduced sources, can immediately facilitate colonization via hybrid vigor and profoundly enhance invasion via contributing novel genetic variation to adaption. As hybrid vigor is short‐lived, provisioning adaptation is anticipated to be the dominant and long‐term profit of genetic admixture, but the evidence for this is rare. We employed the 30 years' geographic‐scale invasion of the salt marsh grass, Spartina alterniflora, as an evolutionary experiment and evaluated the consequences of genetic admixture by combining the reciprocal transplant experiment with quantitative and population genetic surveys. Consistent with the documentation, we found that the invasive populations in China had multiple origins from the southern Atlantic coast and the Gulf of Mexico in the US. Interbreeding among these multiple sources generated a “hybrid swarm” that spread throughout the coast of China. In the northern and mid‐latitude China, natural selection greatly enhanced fecundity, plant height and shoot regeneration compared to the native populations. Furthermore, genetic admixture appeared to have broken the negative correlation between plant height and shoot regeneration, which was genetically‐based in the native range, and have facilitated the evolution of super competitive genotypes in the invasive range. In contrast to the evolved northern and mid‐latitude populations, the southern invasive populations showed slight increase of plant height and shoot regeneration compared to the native populations, possibly reflecting the heterotic effect of the intraspecific hybridization. Therefore, our study suggests a critical role of genetic admixture in accelerating the geographic invasion via provisioning rapid adaptive evolution.     

Tracing the invasion history of mealy plum aphid, Hyalopterus pruni (Hemiptera: Aphididae), in North America: a population genetics approach

Biological invasions are typically the outcome of complex patterns of introduction, establishment, and spread, and genetic methods are excellent tools to resolve such histories for non-native organisms. The mealy plum aphid, Hyalopterus pruni, is an invasive pest of dried plum in California. We examined nine microsatellite loci and DNA sequences from three mitochondrial genes (1,148 bp) in populations throughout the native and invaded ranges of H. pruni to assess key invasion parameters, including geographic origins of invasive populations, number of introductions, and levels of genetic diversity and gene flow. Our results provide evidence for multiple invasions of H. pruni into North America, suggesting that aphids in California may have been introduced from Spain, and aphids in the eastern United States and Vancouver, Canada were likely introduced from central or northern Europe. H. pruni populations in California were characterized by low genetic diversity relative to native populations, while the two other North American populations were less genetically impoverished. Gene flow among introduced populations was low, but does appear to occur with some regularity. These findings provide a framework for more detailed studies of H. pruni, but also represent a model for how population genetics approaches can be used to study invasion biology and aid the development of optimized management methods for agricultural pests. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Multiple introductions,admixture and bridgehead invasion characterize the introduction history of Ambrosia artemisiifolia in Europe and Australia

Admixture between differentiated populations is considered to be a powerful mechanism stimulating the invasive success of some introduced species. It is generally facilitated through multiple introductions; however, the importance of admixture prior to introduction has rarely been considered. We assess the likelihood that the invasive Ambrosia artemisiifolia populations of Europe and Australia developed through multiple introductions or were sourced from a historical admixture zone within native North America. To do this, we combine large genomic and sampling data sets analysed with approximate Bayesian computation and random forest scenario evaluation to compare single and multiple invasion scenarios with pre‐ and postintroduction admixture simultaneously. We show the historical admixture zone within native North America originated before global invasion of this weed and could act as a potential source of introduced populations. We provide evidence supporting the hypothesis that the invasive populations established through multiple introductions from the native range into Europe and subsequent bridgehead invasion into Australia. We discuss the evolutionary mechanisms that could promote invasiveness and evolutionary potential of alien species from bridgehead invasions and admixed source populations.     

Self‐fertilization,long‐distance flash invasion and biogeography shape the population structure of Pseudosuccinea columella at the worldwide scale

Population genetic studies are efficient for inferring the invasion history based on a comparison of native and invasive populations, especially when conducted at species scale. An expected outcome in invasive populations is variability loss, and this is especially true in self‐fertilizing species. We here focus on the self‐fertilizing Pseudosuccinea columella, an invasive hermaphroditic freshwater snail that has greatly expanded its geographic distribution and that acts as intermediate host of Fasciola hepatica, the causative agent of human and veterinary fasciolosis. We evaluated the distribution of genetic diversity at the largest geographic scale analysed to date in this species by surveying 80 populations collected during 16 years from 14 countries, using eight nuclear microsatellites and two mitochondrial genes. As expected, populations from North America, the putative origin area, were strongly structured by selfing and history and harboured much more genetic variability than invasive populations. We found high selfing rates (when it was possible to infer it), none‐to‐low genetic variability and strong population structure in most invasive populations. Strikingly, we found a unique genotype/haplotype in populations from eight invaded regions sampled all over the world. Moreover, snail populations resistant to infection by the parasite are genetically distinct from susceptible populations. Our results are compatible with repeated introductions in South America and flash worldwide invasion by this unique genotype/haplotype. Our study illustrates the population genetic consequences of biological invasion in a highly selfing species at very large geographic scale. We discuss how such a large‐scale flash invasion may affect the spread of fasciolosis.     

Geographic structure, genetic diversity and source tracking of Spartina alterniflora

Aim To examine the distribution and structure of genetic variation among native Spartina alterniflora and to characterize the evolutionary mechanisms underlying the success of non‐native S. alterniflora. Location Intertidal marshes along the Atlantic, Gulf and Pacific coasts of North America. Methods amova , parsimony analysis, haplotype networks of chloroplast DNA (cpDNA) sequences, neighbour‐joining analysis, Bayesian analysis of population structure, and individual assignment testing were used. Results Low levels of gene flow and geographic patterns of genetic variation were found among native S. alterniflora from the Atlantic and Gulf coasts of North America. The distribution of cpDNA haplotypes indicates that Atlantic coast S. alterniflora are subdivided into ‘northern’ and ‘southern’ groups. Variation observed at microsatellite loci further suggests that mid‐Atlantic S. alterniflora are differentiated from S. alterniflora found in southern Atlantic and New England coastal marshes. Comparisons between native populations on the Atlantic and Gulf coasts and non‐native Pacific coast populations substantiate prior studies demonstrating reciprocal interspecific hybridization in San Francisco Bay. Our results corroborate historical evidence that S. alterniflora was introduced into Willapa Bay from multiple source populations. However, we found that some Willapa Bay S. alterniflora are genetically divergent from putative sources, probably as a result of admixture following secondary contact among previously allopatric native populations. We further recovered evidence in support of models suggesting that S. alterniflora has secondarily spread within Washington State, from Willapa Bay to Grays Harbor. Main conclusions Underlying genetic structure has often been cited as a factor contributing to ecological variation of native S. alterniflora. Patterns of genetic structure within native S. alterniflora may be the result of environmental differences among biogeographical provinces, of migration barriers, or of responses to historical conditions. Interactions among these factors, rather than one single factor, may best explain the distribution of genetic variation among native S. alterniflora. Comprehensive genetic comparisons of native and introduced populations can illustrate how biological invasions may result from dramatically different underlying factors – some of which might otherwise go unrecognized. Demonstrating that invasions can result from several independent or interacting mechanisms is important for improving risk assessment and future forecasting. Further research on S. alterniflora not only may clarify what forces structure native populations, but also may improve the management of non‐native populations by enabling post‐introduction genetic changes and the rapid evolution of life‐history traits to be more successfully exploited.     

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.     

Deciphering the worldwide invasion of the Asian long‐horned beetle: A recurrent invasion process from the native area together with a bridgehead effect

Retracing introduction routes is crucial for understanding the evolutionary processes involved in an invasion, as well as for highlighting the invasion history of a species at the global scale. The Asian long‐horned beetle (ALB) Anoplophora glabripennis is a xylophagous pest native to Asia and invasive in North America and Europe. It is responsible for severe losses of urban trees, in both its native and invaded ranges. Based on historical and genetic data, several hypotheses have been formulated concerning its invasion history, including the possibility of multiple introductions from the native zone and secondary dispersal within the invaded areas, but none have been formally tested. In this study, we characterized the genetic structure of ALB in both its native and invaded ranges using microsatellites. In order to test different invasion scenarios, we used an approximate Bayesian “random forest” algorithm together with traditional population genetics approaches. The strong population differentiation observed in the native area was not geographically structured, suggesting complex migration events that were probably human‐mediated. Both native and invasive populations had low genetic diversity, but this characteristic did not prevent the success of the ALB invasions. Our results highlight the complexity of invasion pathways for insect pests. Specifically, our findings indicate that invasive species might be repeatedly introduced from their native range, and they emphasize the importance of multiple, human‐mediated introductions in successful invasions. Finally, our results demonstrate that invasive species can spread across continents following a bridgehead path, in which an invasive population may have acted as a source for another invasion.     

Biological invasions in soil: DNA barcoding as a monitoring tool in a multiple taxa survey targeting European earthworms and springtails in North America

Biological invasions are increasingly recognized as a potent force altering native ecosystems worldwide. Many of the best documented cases involve the massive invasions of North America by plant and animal taxa native to Europe. In this study, we use DNA barcoding to survey the occurrence and genetic structure of two major groups of soil invertebrates in both their native and introduced ranges: Collembola and earthworms. Populations of ten species of earthworms and five species of Collembola were barcoded from both continents. Most of these species exhibited a similar genetic structure of large and stable populations in North America and Europe, a result supporting a scenario of multiple invasions. This was expected for earthworm species involved in human economic activities, but not foreseen for Collembola species de facto unintentionally introduced. This study also establishes that invasive species surveys employing DNA barcoding gain additional resolution over those based on morphology as they allow evaluation of cryptic lineages exhibiting different invasion histories.     

Recent admixture generates heterozygosity–fitness correlations during the range expansion of an invading species

Admixture, the mixing of historically isolated gene pools, can have immediate consequences for the genetic architecture of fitness traits. Admixture may be especially important for newly colonized populations, such as during range expansion and species invasions, by generating heterozygosity that can boost fitness through heterosis. Despite widespread evidence for admixture during species invasions, few studies have examined the demographic history leading to admixture, how admixture affects the heterozygosity and fitness of invasive genotypes, and whether such fitness effects are maintained through time. We address these questions using the invasive plant Silene vulgaris, which shows evidence of admixture in both its native Europe and in North America where it has invaded. Using multilocus genotype data in conjunction with approximate Bayesian computation analysis of demographic history, we showed that admixture during the invasion of North America was independent from and much younger than admixture in the native range of Europe. We tested for fitness consequences of admixture in each range and detected a significant positive heterozygosity–fitness correlation (HFC) in North America; in contrast, no HFC was present in Europe. The lack of HFC in Europe may reflect the longer time since admixture in the native range, dissipating associations between heterozygosity at markers and fitness loci. Our results support a key short‐term role for admixture during the early stages of invasion by generating HFCs that carry populations past the threat of extinction from inbreeding and demographic stochasticity.     

Dispersal Pathways and Genetic Differentiation among Worldwide Populations of the Invasive Weed Centaurea solstitialis L. (Asteraceae)

The natural history of introduced species is often unclear due to a lack of historical records. Even when historical information is readily available, important factors of the invasions such as genetic bottlenecks, hybridization, historical relationships among populations and adaptive changes are left unknown. In this study, we developed a set of nuclear, simple sequence repeat markers and used these to characterize the genetic diversity and population structure among native (Eurasian) and non-native (North and South American) populations of Centaurea solstitialis L., (yellow starthistle). We used these data to test hypotheses about the invasion pathways of the species that were based on historical and geographical records, and we make inferences about historical relationships among populations and demographic processes following invasion. We confirm that the center of diversity and the native range of the species is likely the eastern Mediterranean region in the vicinity of Turkey. From this region, the species likely proceeded to colonize other parts of Europe and Asia via a slow, stepwise range expansion. Spanish populations were the primary source of seed to invade South America via human-mediated events, as was evident from historical records, but populations from the eastern Mediterranean region were also important. North American populations were largely derived from South America, but had secondary contributors. We suggest that the introduction history of non-native populations from disparate parts of the native range have allowed not just one, but multiple opportunities first in South America then again in North America for the creation of novel genotypes via intraspecific hybridization. We propose that multiple intraspecific hybridization events may have created especially potent conditions for the selection of a noxious invader, and may explain differences in genetic patterns among North and South America populations, inferred differences in demographic processes, as well as morphological differences previously reported from common garden experiments.     

When invasion increases population genetic structure: a study with <Emphasis Type="Italic">Centaurea diffusa</Emphasis>

Biological invasions offer excellent systems to study the evolutionary processes involved in introductions of species to new ranges. Molecular markers can reveal invasion histories and the effects of introductions on amounts and structuring of genetic variation. We used five polymorphic microsatellite loci to elucidate genetic diversity and population structure between native range and introduced range populations of a prominent North American rangeland weed, Centaurea diffusa (Asteraceae). We found that the total number of alleles and the number of private alleles was slightly higher in the native Eurasian range, and that allelic richness did not differ between the ranges, indicating overall levels of diversity were similar in Eurasia and North America. It therefore seems unlikely that this invasion has been affected by genetic bottlenecks or founder effects. Indeed, results of assignment tests suggest that multiple introductions have contributed to North America’s C. diffusa invasion. Additionally, assignment tests show that both Eurasian and North American sites had a strong pattern of mixed genetic ancestry. This mixed assignment corresponded to a lack of geographic population structure among Eurasian samples. The lack of population structure in the native range conflicts with general expectations and findings to date for invasion genetics, and cautions that even species’ native ranges may show signs of recent ecological upheaval. Despite the mixed assignments, North American samples showed strong population structure, suggesting that the invasion has been characterized by long-range dispersal of genetically distinct propagules across the introduced range.     

REDUCED DROUGHT TOLERANCE DURING DOMESTICATION AND THE EVOLUTION OF WEEDINESS RESULTS FROM TOLERANCE–GROWTH TRADE‐OFFS

The increased reproductive potential, size, shoot allocation, and growth rate of weedy plants may result from reduced resource allocation to other aspects of plant growth and defense. To investigate whether changes in resource allocation occurred during domestication or the evolution of weediness, we compared the mycorrhizal responsiveness, growth, and drought tolerance of nine native ruderal, nine agriculturally weedy (four U.S. weedy and five Australian weedy), and 14 domesticated populations (eight ancient landraces and six improved cultivars) of the common sunflower (Helianthus annuus). Domesticated sunflower cultivars were less drought tolerant, but had higher plant growth and fecundity and coarser roots than wild populations. There were no changes in level of drought tolerance between improved cultivars and ancient landrace plants, but there was an increase in allocation to flowers with recent selection. Weedy populations were intermediate between domesticated cultivars and native ruderal populations for plant growth rate, root architecture, and drought tolerance. Weedy populations benefited most from mycorrhizal inoculation by having fewer wilted leaves and wetter soil. Overall, we found that trade‐offs between drought tolerance and several aspects of plant growth, including growth rate, allocation to flowering, and root architecture, govern evolution during sunflower domestication and the invasion of disturbed habitat.     

Historical range expansion determines the phylogenetic diversity introduced during contemporary species invasion

For a species rapidly expanding its geographic range, such as during biological invasion, most alleles in the introduced range will have their evolutionary origins in the native range. Yet, the way in which historical processes occurring over evolutionary time in the native range contribute to the diversity sampled during contemporary invasion is largely unknown. We used chloroplast DNA (cpDNA) gene genealogies and coalescent methods to study two congeneric plants, Silene latifolia and S. vulgaris. We examined how phylogenetic diversity was shaped by demographic growth and historical range expansions in the native European range, and how this history affected the diversity sampled during their recent invasion of North America. Genealogies from both species depart from neutrality, likely as a result of demographic expansion in the ancestral range, the timing of which corresponds to shortly after each species originated. However, the species differ in the spatial distribution of cpDNA lineages across the native range. Silene latifolia shows a highly significant phylogeographic structure that most likely reflects different avenues of the post-glacial expansion into northern Europe from Mediterranean refugia. By contrast, cpDNA lineages in S. vulgaris have been widely scattered across Europe during, or since, the most recent post-glacial expansion. These different evolutionary histories resulted in dramatic differences in how phylogenetic diversity was sampled during invasion of North America. In S. latifolia, relatively few, discrete invasion events from a structured native range resulted in a rather severe genetic bottleneck, but also opportunities for admixture among previously isolated lineages. In S. vulgaris, lack of genetic structure was accompanied by more representative sampling of phylogenetic diversity during invasion, and reduced potential for admixture. Our results provide clear insights into how historical processes may feed forward to influence the phylogenetic diversity of species invading new geographic ranges.     

ORIGINS OF GENOTYPIC VARIATION IN NORTH AMERICAN DANDELIONS INFERRED FROM RIBOSOMAL DNA AND CHLOROPLAST DNA RESTRICTION ENZYME ANALYSIS

Restriction enzyme analysis of ribosomal DNA (rDNA) and chloroplast DNA (cpDNA) is used to assess the relative contribution of hybridization and mutation as sources of genotypic variation in weedy asexual dandelions, with focus on the dandelion flora of North America. Of 318 North American dandelions surveyed, 145 rDNA-cpDNA clones are detected. The combined rDNA-cpDNA genotypes show that most of the polymorphic rDNA and cpDNA restriction sites or lengths in these plants are also present in weedy asexual dandelions collected from natural populations in Europe and in asexual and diploid taxa (microspecies) chosen to represent diverse Eurasian members of the genus. However, of 222 combined rDNA-cpDNA genotypes found in 427 asexual plants surveyed, only 9 genotypes are found in both North American and Eurasian dandelions. Two rDNA and three cpDNA characters are unique to individual plants in North America and are consistent with mutational origins of genotypic variation in asexual lineages. But the array of genotypic diversity, characterized by different combinations of the rDNA and cpDNA characters, show that multiple hybridization events are a more important source of genotypic variation than mutation in the asexual polyploids. The rDNA and cpDNA data also indicate polyphyletic origin of several asexual Taraxacum taxa.