Lack of genetic variation of an invasive clonal plant Eichhornia crassipes in China revealed by RAPD and ISSR markers

Random amplified polymorphic DNAs (RAPDs) and inter-simple sequence repeats (ISSRs) markers were used to analyze genetic structure of six populations of invasive plant Eichhornia crassipes that were sampled from its introduced regions in Southern China. Using 25 RAPD primers and 18 ISSR primers, 172 RAPD bands and 145 ISSR bands were produced respectively. But no polymorphic band was detected either within population or among populations by both markers, indicating the genetic diversity of E. crassipes in Southern China is extremely low, and all populations most likely consist of the same genotype. This study suggested that some other adaptability related factors, other than the genetic diversity, are responsible to the E. crassipes rapid expansion in China.     

Clonal diversity and structure of the invasive aquatic plant Eichhornia crassipes in China

The information on diversity and spatial distribution of clones of an invasive clonal plant is crucial for the understanding of its clonal structure and invasive history. In this paper, random amplified polymorphic DNA (RAPD) markers were used to explore the clonal diversity and clonal structure of Eichhornia crassipes (Mart.) Solms in natural populations, and their possible effects on the plant success as an invader are also discussed. Five populations covering the entire distribution area in China were studied, sampling 43 individuals per population at an interval of 1 m in a sampling plot. Twelve RAPD primers produced 69 reproducible bands, with 22 being polymorphic. Only five RAPD phenotypes (clones) were detected in these five populations, but each population consisted of at least three clones, contrary to the traditional expectations that E. crassipes populations should be monoclonal. The diversity of clones within populations is thought to be mainly resulted from multiple introductions by humans. The evenness of distribution of clones varied slightly and most clones were widespread, suggesting clonal growth is the predominant mode of regeneration in all the populations. A single clone dominated each population and this clone might be the first one introduced into China or the genotype with a higher phenotypic plasticity, which could survive and reproduce via clonal growth in various habitats. The clones in each population were highly intermixed, especially in river populations, suggesting this species has a guerilla clonal structure which can be facilitated by water current.     

Inferring the introduction history of the invasive apomictic grass Cortaderia jubata using microsatellite markers

Aim Reconstructing the introduction history of exotic species is critical to understanding ecological and evolutionary processes that underlie invasive spread and to designing strategies that prevent or manage invasions. The aims of this study were to infer the introduction history of the invasive apomictic bunchgrass Cortaderia jubata and to determine if molecular data support the postulated horticultural origin of invasive populations. Location Invaded areas in the USA (California, Maui) and New Zealand; native areas in Bolivia, Ecuador and Peru. Methods We used nuclear microsatellite markers to genotype 281 plants from invaded areas in California, Maui and New Zealand, and 77 herbarium specimens from native South America, and compared the genotypic and clonal variation of C. jubata from the invaded and native ranges. Clonal diversity was determined from genotypic diversity using two analytical methods. Results Invasive C. jubata from invaded regions in California, Maui and New Zealand consisted of the same single clone that probably originated from a single introduced genotype. In contrast, 14 clones were detected in herbarium specimens from the native areas of Bolivia, Ecuador and Peru. The invasive clone matched the most common clone identified in herbarium specimens from southern Ecuador where horticultural stock is presumed to have originated. Main conclusions The lack of clonal and genotypic diversity in invasive plants, but moderately high diversity detected in native plants, indicates a significant reduction in genetic variation associated with the introduction of C. jubata outside of its native range. Based on historical accounts of the horticultural introduction of C. jubata and the results of this study, a severe founder effect probably occurred during deliberate introduction of C. jubata into cultivation. Our results are consistent with the postulated horticultural origin of invasive C. jubata and point to southern Ecuador as the geographical source of invasive populations.     

The absence of genotypic diversity in a successful parthenogenetic invader

Invasiveness might depend on the ability of genetically diverse populations of exotic species to adapt to novel environments, which suggests a paradox since exotic species are expected to lose genetic diversity when introduced. The apparent need for genetic diversity is particularly important for exotic species that lack bi-parental reproduction and genetic recombination. We used genetic marker studies to determine the genotypic diversity of invasive US populations of the clonal New Zealand mudsnail (Potamopyrgus antipodarum). We report here on a three-pronged survey of allozyme, microsatellite DNA, and mitochondrial DNA genetic markers of invasive populations with a focus on the western US. Combining the three types of genetic markers, we discovered four distinct genotypes of P. antipodarum. These results show that only one genotype (US 1) occupied the vast majority of the western US range, and a second occurred in the Great Lakes in the eastern US (US 2). Two other genotypes occurred in the western US (US 1a and US 3), but were restricted to populations near the presumed source of invasion in the middle Snake River, ID. These results suggest that P. antipodarum spread across a broad geographic range in the western US from a single introduced source population, and that the populations are comprised of a single clonal lineage.     

The genetic consequences of a demographic bottleneck in an introduced biological control insect

Population bottlenecks may result in the loss of genetic diversity, with potentially negative consequences for species of interest in conservation biology, including rare species, invasive species and biological control agents. We examined mtDNA sequence data and four variable microsatellite loci (SSRs) in the melaleuca psyllid Boreioglycaspis melaleucae, which was introduced from Australia to Florida as a biological control agent of the invasive plant Melaleuca quinquenervia. We sampled psyllids in the native and introduced ranges as well as individuals stored from the original founding population. There was a clear loss of mtDNA haplotype diversity, as well as a loss of rare microsatellite alleles, in the introduced range. However, there was little genetic differentiation between the home and introduced ranges, and no evidence for a genetic bottleneck based on an analysis of heterozygosity with the microsatellite markers. Overall, the data showed that the demographic bottleneck had a limited effect on the genetics of populations in the new range.     

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.     

A few north Appalachian populations are the source of European black locust

The role of evolution in biological invasion studies is often overlooked. In order to evaluate the evolutionary mechanisms behind invasiveness, it is crucial to identify the source populations of the introduction. Studies in population genetics were carried out on Robinia pseudoacacia L., a North American tree which is now one of the worst invasive tree species in Europe. We realized large‐scale sampling in both the invasive and native ranges: 63 populations were sampled and 818 individuals were genotyped using 113 SNPs. We identified clonal genotypes in each population and analyzed between and within range population structure, and then, we compared genetic diversity between ranges, enlarging the number of SNPs to mitigate the ascertainment bias. First, we demonstrated that European black locust was introduced from just a limited number of populations located in the Appalachian Mountains, which is in agreement with the historical documents briefly reviewed in this study. Within America, population structure reflected the effects of long‐term processes, whereas in Europe it was largely impacted by human activities. Second, we showed that there is a genetic bottleneck between the ranges with a decrease in allelic richness and total number of alleles in Europe. Lastly, we found more clonality within European populations. Black locust became invasive in Europe despite being introduced from a reduced part of its native distribution. Our results suggest that human activity, such as breeding programs in Europe and the seed trade throughout the introduced range, had a major role in promoting invasion; therefore, the introduction of the missing American genetic cluster to Europe should be avoided.     

Initial genetic diversity enhances population establishment and alters genetic structuring of a newly established Daphnia metapopulation

When newly created habitats are initially colonized by genotypes with rapid population growth rates, later arriving colonists may be prevented from establishing. Although these priority effects have been documented in multiple systems, their duration may be influenced by the diversity of the founding population. We conducted a large‐scale field manipulation to investigate how initial clonal diversity influences temporal and landscape patterns of genetic structure in a developing metapopulation. Six genotypes of obligately asexual Daphnia pulex were stocked alone (no clonal diversity) or in combination (‘high’ clonal diversity) into newly created experimental woodland ponds. We also measured the population growth rate of all clones in the laboratory when raised on higher‐quality and lower‐quality resources. Our predictions were that in the 3 years following stocking, clonally diverse populations would be more likely to persist than nonclonally diverse populations and exhibit evidence for persistent founder effects. We expected that faster growing clones would be found in more pools and comprise a greater proportion of individuals genotyped from the landscape. Genetic composition, both locally and regionally, changed significantly following stocking. Six of 27 populations exhibited evidence for persistent founder effects, and populations stocked with ‘high’ clonal diversity were more likely to exhibit these effects than nonclonally diverse populations. Performance in the laboratory was not predictive of clonal persistence or overall dominance in the field. Hence, we conclude that although laboratory estimates of fitness did not fully explain metapopulation genetic structure, initial clonal diversity did enhance D. pulex population establishment and persistence in this system.     

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.     

Genetic uniformity and long‐distance clonal dispersal in the invasive androgenetic Corbicula clams

The clam genus Corbicula is an interesting model system to study the evolution of reproductive modes as it includes both sexual and asexual (androgenetic) lineages. While the sexual populations are restricted to the native Asian areas, the androgenetic lineages are widely distributed being also found in America and Europe where they form a major aquatic invasive pest. We investigated the genetic diversity of native and invasive Corbicula populations through a worldwide sampling. The use of mitochondrial and nuclear (microsatellite) markers revealed an extremely low diversity in the invasive populations with only four, undiversified, genetic lineages distributed across Europe and America. On the contrary, in the native populations, both sexual and androgenetic lineages exhibited much higher genetic diversity. Remarkably, the most abundant and widely distributed invasive forms, the so‐called form A and form R found in America and Europe respectively, are fixed for the same single COI (cytochrome c oxydase subunit I) haplotype and same multilocus genotype. This suggests that form R, observed in Europe since the 1980s, derived directly from form A found in America since the 1920s. In addition, this form shares alleles with some Japanese populations, indicating a Japanese origin for this invasive lineage. Finally, our study suggests that few androgenetic Corbicula individuals successfully invaded the non‐native range and then dispersed clonally. This is one striking case of genetic paradox raising the issue of invasive and evolutionary success of genetically undiversified 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.     

Weedy adaptation in Setaria spp. II. Genetic diversity and population genetic structure in S. glauca,S. geniculata,and S. faberii (Poaceae)

Setaria glauca (yellow foxtail), S. geniculata (knotroot foxtail), and S. faberii (giant foxtail) are important cosmopolitan weeds of temperate and tropical regions. Isozyme markers were used to investigate genetic diversity and population genetic structure in 94 accessions of yellow foxtail, 24 accessions of knotroot foxtail, and 51 accessions of giant foxtail, collected mainly from North America and Eurasia. Giant foxtail populations were nearly identical genetically, with only one population exhibiting isozyme polymorphism. Yellow and knotroot foxtail populations had low genetic diversity but marked population differentiation. Although the latter species are similar morphologically, they are readily distinguished electrophoretically, with Nei's genetic identity being 0.83. In both species, genetic divergence between accessions from Eurasia and North America was minimal. Populations from the native ranges had slightly greater genetic diversity than those from the respective introduced ranges. Yellow foxtail populations genetically clustered into Asian, European, and North American groups. Within North America, yellow foxtail populations from Iowa were genetically diverse whereas populations collected from other North American locations were nearly monomorphic for the same multilocus genotype. Knotroot foxtail populations in North America were genetically differentiated into northern and southern groups on either side of a line at ≈37° N latitude. No genetic patterning was evident in knotroot foxtail populations from Eurasia. In both yellow and knotroot foxtail, patterns of population genetic structure have been influenced by several factors, including genetic bottlenecks associated with founder events, genetic drift, and natural selection.     

The importance of setting the right genetic distance threshold for identification of clones using amplified fragment length polymorphism: a case study with five species in the tropical plant genus Piper

Amplified fragment length polymorphism (AFLP) has been widely used for clone identification, but numerous studies have shown that clonemates do not always present identical AFLP fingerprints. Pairwise AFLP distances that distinguish known clones from nonclones have been used to identify a threshold genetic dissimilarity distance below which samples are considered to represent a single clone. Most studies to date have reported threshold values between 2% and 4%. Here, I determine the consistency of the clonal threshold across five species in the tropical plant genus Piper, and evaluate the sensitivity of genetic diversity indices and estimates of frequency of clonal reproduction to the threshold value selected. I sampled multiple ramets per individual from widely distributed plants for each of the five Piper species to set a threshold at the point where the error rate of clonal assignments was lowest. I then sampled all individuals of each shade‐tolerant species in a 1‐ha plot, and of each light‐demanding species in 25 × 35‐m plot, to estimate the frequency of asexual recruitment in natural populations using a series of different thresholds including the threshold set with the preliminary sampling. Clonal threshold values for the different species ranged from 0% to 5% AFLP genetic dissimilarity distance. To determine the sensitivity of estimates of clonal reproduction, I calculated several clonal diversity indexes for the natural populations of each of the five species guided by the range in clonal threshold values observed across the five Piper species. I show that small changes in the value of the clonal threshold can lead to very different conclusions regarding the level of clonal reproduction in natural populations.     

Comparison of genetic diversity of the invasive weed Rubus alceifolius poir. (Rosaceae) in its native range and in areas of introduction, using amplified fragment length polymorphism (AFLP) markers

Theory predicts that colonization of new areas will be associated with population bottlenecks that reduce within-population genetic diversity and increase genetic differentiation among populations. This should be especially true for weedy plant species, which are often characterized by self-compatible breeding systems and vegetative propagation. To test this prediction, and to evaluate alternative scenarios for the history of introduction, the genetic diversity of Rubus alceifolius was studied with amplified fragment length polymorphism (AFLP) markers in its native range in southeast Asia and in several areas where this plant has been introduced and is now a serious weed (Indian Ocean islands, Australia). In its native range, R. alceifolius showed great genetic variability within populations and among geographically close populations (populations sampled ranging from northern Vietnam to Java). In Madagascar, genetic variability was somewhat lower than in its native range, but still considerable. Each population sampled in the other Indian Ocean islands (Mayotte, La Réunion, Mauritius) was characterized by a single different genotype of R. alceifolius for the markers studied, and closely related to individuals from Madagascar. Queensland populations also included only a single genotype, identical to that found in Mauritius. These results suggest that R. alceifolius was first introduced into Madagascar, perhaps on multiple occasions, and that Madagascan individuals were the immediate source of plants that colonized other areas of introduction. Successive nested founder events appear to have resulted in cumulative reduction in genetic diversity. Possible explanations for the monoclonality of R. alceifolius in many areas of introduction are discussed.     

Genetic analyses of the Asian longhorned beetle (Coleoptera,Cerambycidae, <Emphasis Type="Italic">Anoplophora glabripennis</Emphasis>), in North America,Europe and Asia

The Asian longhorned beetle, (Coleoptera, Cerambycidae, Anoplophora glabripennis (Motschulsky)), is endemic to China and Korea and an important invasive insect in North America and Europe. We analyzed mitochondrial DNA sequence data of invasive populations of A. glabripennis in North America and Europe, and microsatellite allele frequency data of beetles from North America. We show that populations in New York City and Long Island NY; New Jersey, Chicago, IL, and Toronto, Canada have limited genetic diversity compared to populations in China. In addition, the data suggest that separate introduction events were responsible for many of the populations in North America and for European populations in Austria, France, Germany and Italy. Populations on Long Island, NY are suspected to have been initiated by the transport of cut wood from New York City. A. glabripennis beetles found in Jersey City, NJ appear to be derived from an expansion of the New York City, NY population, whereas beetles found in Linden, NJ are an expansion from the Carteret, NJ population. Limited genetic diversity did not stop this invasive insect from establishing damaging populations in North America. Founders of introduced A. glabripennis populations in North America and Europe are likely derived from populations in China that are themselves invasive, rendering difficult the identification of source populations. Invasiveness in an insect’s natural range could be an important predictor of potential pest status of introduced populations.     

Genetic diversity and clonal vs. sexual reproduction in Fallopia spp. (Polygonaceae)

Although fundamental to the study of invasion mechanisms, the relationship between mode of reproduction and plant invasion is not well understood. Fallopia japonica (Japanese knotweed), a highly aggressive invasive plant in both Europe and North America, serves as a model species for examining this relationship. In Britain, F. japonica var. japonica is a single female clone reproducing solely through vegetative growth or obligate hybridization with other Fallopia spp. In the U.S., however, there is more evidence for sexual reproduction. Here, simple sequence repeat (SSR) markers were developed, and three Massachusetts populations were sampled at regular intervals. The amount of sexual and clonal reproduction in each population was determined based on within-population genetic diversity. Clonal growth was apparent, but the populations together contained 26 genotypes and had evidence of sexual reproduction. One genotype that was present in all populations matched the single aggressive British clone of F. japonica var. japonica. Also, a potentially diagnostic marker for the F. sachalinensis genome provided evidence of inter- and intraspecific sexual reproduction and introgression. These differences observed in U.S. populations compared to European populations have significant implications for management of Fallopia spp. in the U.S. and underscore the importance of regional studies of invasive species.     

Rare sexual reproduction events in the clonal reproduction system of introduced populations of the little fire ant

A unique reproductive system has previously been described in Wasmannia auropunctata, a widespread invasive ant species, where males are produced clonally, female queens are parthenogens, and female workers are produced sexually. However, these findings were mostly based on samples originating from only a limited part of the native range of the species in South America. We used microsatellite markers to uncover the reproductive modes displayed by a large number of nests collected in various invasive W. auropunctata populations introduced 40 years ago into New Caledonia, where the species now forms a single 450-km-long supercolony. Although the main reproduction system in New Caledonia remained clonality for both male and female reproductives, we found evidence of rare sexual reproduction events that led to the production of both new queen and male clonal lineages. All clonal lineages observed in New Caledonia potentially derived from sexual reproduction, recombination, and mutation events from a single female and a single male genotype. Hence, the male and female gene pools are not strictly separated in New Caledonia and the two sexes do not follow independent evolutionary trajectories. Our results also suggest genetic determination for both parthenogenesis and caste. We discuss the evolutionary implications of the emergence of sex in the clonal reproduction system of introduced populations of W. auropunctata.     

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

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

Sunshine versus gold: The effect of population age on genetic structure of an invasive mosquito

The genetic diversity and structure of invasive species are affected by the time since invasion, but it is not well understood how. We compare likely the oldest populations of Aedes aegypti in continental North America with some of the newest to illuminate the range of genetic diversity and structure that can be found within the invasive range of this important disease vector. Aedes aegypti populations in Florida have probably persisted since the 1600‐1700s, while populations in southern California derive from new invasions that occurred in the last 10 years. For this comparison, we genotyped 1,193 individuals from 28 sites at 12 highly variable microsatellites and a subset of these individuals at 23,961 single nucleotide polymorphisms (SNPs). This is the largest sample analyzed for genetic structure for either region, and it doubles the number of southern California populations previously analyzed. As predicted, the older populations (Florida) showed fewer indicators of recent founder effect and bottlenecks; in particular, these populations have dramatically higher genetic diversity and lower genetic structure. Geographic distance and driving distance were not good predictors of genetic distance in either region, especially southern California. Additionally, southern California had higher levels of genetic differentiation than any comparably sized documented region throughout the worldwide distribution of the species. Although population age and demographic history are likely driving these differences, differences in climate and transportation practices could also play a role.     

Horticulture,hybrid cultivars and exotic plant invasion: a case study of Wisteria (Fabaceae)

Exotic Wisteria species are highly favoured for their horticultural qualities and have been cultivated in North America since the early 1800s. This study determines the identity, genetic diversity and hybrid status of 25 Asian Wisteria cultivars using plastid, mitochondrial and nuclear DNA data. Fifteen (60%) hybrid cultivars were identified. All of the ‘Wisteria sinensis’ cultivars sampled are hybrids with W. floribunda. Although W. sinensis and W. floribunda are recognized invasive species in the southeastern USA, the relationships of horticultural cultivars to naturalized plants was previously unknown. Haplotype analysis of nuclear data identifies four haplotypes shared between cultivated stock and naturalized populations in the southeastern USA. In addition, US invasive haplotypes are present in New Zealand‐derived cultivars although, to date, naturalized Wisteria has not been documented in New Zealand. Finally, these data are used to make recommendations to horticulturalists of select species cultivars which may be less likely to invade US landscapes. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 593–601.