The genetic structure of populations of the vulnerable aquatic macrophyte <Emphasis Type="Italic">Ranunculus nipponicus</Emphasis> (Ranunculaceae)

Ranunculus nipponicus (Makino) Nakai is a vulnerable aquatic macrophyte in the Kinki district, which is the southernmost distribution of this species in Japan. The genetic diversity and structure within and among eleven extant populations were assessed using the inter-simple sequence repeats (ISSR) polymerase chain reaction in association with combinations of propagation pattern (clonal and/or seeds) and genotypic geographical structure. In total, 53 bands were amplified, of which 18 (34%) were polymorphic. Analysis of the ISSR bands identified 46 genotypes among 81 individuals from one stream population and 72 distinct genotypes among 147 individuals in the Kinki district. An unweighted pair group method with arithmetic mean (UPGMA) dendrogram showed some unity among upstream and downstream subpopulations within one stream and eleven populations. The Shannon index of genetic diversity was 0.109 for one stream population and 0.313 for total genetic diversity, suggesting relatively high genetic diversity. Analysis of molecular variance (AMOVA) revealed that 84.1% of the total genetic diversity occurred among populations and the remaining diversity (15.9%) occurred within populations. Significant genetic differentiation occurred among populations in the Kinki district. These results suggest that conservation of each population is important for maintaining genetic diversity of R. nipponicus in this district. An erratum to this article can be found at     

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.     

Genomics reveals the history of a complex plant invasion and improves the management of a biological invasion from the South African–Australian biotic exchange

Many plants exchanged in the global redistribution of species in the last 200 years, particularly between South Africa and Australia, have become threatening invasive species in their introduced range. Refining our understanding of the genetic diversity and population structure of native and alien populations, introduction pathways, propagule pressure, naturalization, and initial spread, can transform the effectiveness of management and prevention of further introductions. We used 20,221 single nucleotide polymorphisms to reconstruct the invasion of a coastal shrub, Chrysanthemoides monilifera ssp. rotundata (bitou bush) from South Africa, into eastern Australia (EAU), and Western Australia (WAU). We determined genetic diversity and population structure across the native and introduced ranges and compared hypothesized invasion scenarios using Bayesian modeling. We detected considerable genetic structure in the native range, as well as differentiation between populations in the native and introduced range. Phylogenetic analysis showed the introduced samples to be most closely related to the southern‐most native populations, although Bayesian analysis inferred introduction from a ghost population. We detected strong genetic bottlenecks during the founding of both the EAU and WAU populations. It is likely that the WAU population was introduced from EAU, possibly involving an unsampled ghost population. The number of private alleles and polymorphic SNPs successively decreased from South Africa to EAU to WAU, although heterozygosity remained high. That bitou bush remains an invasion threat in EAU, despite reduced genetic diversity, provides a cautionary biosecurity message regarding the risk of introduction of potentially invasive species via shipping routes.     

Domestication and the distribution of genetic variation in wild and cultivated populations of the Mesoamerican fruit tree Spondias purpurea L. (Anacardiaceae)

Domestication occurs as humans select and cultivate wild plants in agricultural habitats. The amount and structure of variation in contemporary cultivated populations has been shaped, in part, by how genetic material was transferred from one cultivated generation to the next. In some cultivated tree species, domestication involved a shift from sexually reproducing wild populations to vegetatively propagated cultivated populations; however, little is known about how domestication has impacted variation in these species. We employed AFLP data to explore the amount, structure, and distribution of variation in clonally propagated domesticated populations and sexually reproducing wild populations of the Neotropical fruit tree, Spondias purpurea (Anacardiaceae). Cultivated populations from three different agricultural habitats were included: living fences, backyards, and orchards. AFLP data were analysed using measures of genetic diversity (% polymorphic loci, Shannon's diversity index, Nei's gene diversity, panmictic heterozygosity), population structure (F(ST) analogues), and principal components analyses. Levels of genetic variation in cultivated S. purpurea populations are significantly less than variation found in wild populations, although the amount of diversity varies in different agricultural habitats. Cultivated populations have a greater proportion of their genetic variability distributed among populations than wild populations. The genetic structure of backyard populations resembles that of wild populations, but living fence and orchard populations have 1/3 more variability distributed among populations, most likely a reflection of relative levels of vegetative reproduction. Finally, these results suggest that S. purpurea was domesticated in two distinct regions within Mesoamerica.     

Genetic diversity pattern of Stipa purpurea populations in the hinterland of Qinghai–Tibet Plateau

Stipa purpurea is among constructive endemic species in alpine meadow and steppe on the Qinghai–Tibet Plateau. To reveal the genetic diversity of this species and its relationship with geographic distribution pattern, we sampled eight populations across a northward transect with an increasing aridity gradient in the hinterland of Qinghai–Tibet Plateau. Their genetic diversity was quantified using eight intersimple sequence repeat (ISSR) primers. We found that S. purpurea had relatively low genetic diversity ( H _e = 0.135) but significant genetic differentiation among populations ( G _st = 0.391), indicating relatively more genetic diversity retained within populations. A Mantel test revealed a significant relationship between genetic and geographic distance in the S. purpurea populations. The genetic diversity tended to decrease with increasing latitude and longitude, while no significant relationship was found between genetic diversity and altitude, suggesting the possible influences of humidity and temperature on genetic diversity of alpine plant. We propose conservation measures for this plant on the plateau.     

The use of random amplified polymorphic DNA (RAPD) for hybrid detection in Scirpus from the river Schelde (Belgium)

Along the unique freshwater tidal zone of the river Schelde (Belgium), plants of Scirpus species occur primarily in small and fragmented populations. The majority of these are native Scirpus lacustris, S. pungens, S. triqueter, S. tabernaemontani and intermediate morphological forms. The distribution area of S. triqueter is even restricted to this tidal habitat. However, several cultivated S. tabernaemontani strains have recently been introduced. The latter species is often used to stabilize riverbanks. To determine the existing genetic diversity among these species, stems from plants from 44 different locations were subjected to random amplified polymorphic DNA (RAPD) analysis, using 22 decanucleotide primers. Data analysis of the amplified DNA fragments enabled us to unambiguously differentiate among these Scirpus taxa. Hybridization between S. triqueter and S. tabernaemontani was documented, and the studied hybrids were always genetically more similar to S. triqueter than to S. tabernaemontani . Among the introduced clones, at least two different origins could be distinguished. Several of the introduced S. tabernaemontani clones were clearly different from individuals of the same species that were native to this region. As there were different hybrid genotypes owing to different hybrid events or to introgression, this complex should be safeguarded from genetic pollution. New S. tabernaemontani genotypes with markers not previously observed in that hybrid complex of the unique tidal freshwater habitat should be avoided in replantation projects.     

Uniformity of the nuclear and chloroplast genomes of Spartina maritima (Poaceae), a salt-marsh species in decline along the Western European Coast

Spartina maritima is a salt-marsh species from European and African Atlantic coasts. In the northern range of the species (including north-west France), a rapid decline of the populations has been observed during the 20th century. In this paper, the molecular diversity of 10 populations of S. maritima from France has been investigated using nuclear and chloroplast DNA markers: inter-simple sequence polymorphism (ISSR), randomly amplified polymorphic DNA (RAPD), inter-retrotransposon amplified polymorphism (IRAP), and PCR-RFLP of a 5 kb long portion of chloroplast DNA. The results reveal an extremely low level of genetic variation in this species: only one nuclear marker (out of 98) was polymorphic, with the presence of two genotypes randomly distributed among the populations. The lack of genetic diversity is interpreted in light of the almost exclusive vegetative propagation of the species in its northern range, the colonization history of the populations, and the origin of S. maritima (2n = 60) in the hexaploid lineage of the genus and in the context of the management of S. maritima populations.     

Do exotic generalist predators alter host plant preference of a native willow beetle?

1 Selection can favour herbivores that choose host plants benefitting their offspring either by enhancing growth rates or by increasing larval defences against native predators. For exotic predator species that feed on herbivores, their success with invading new habitats may depend upon overcoming defences used by native prey. Whether exotic predators can alter herbivore host choice has remained unexamined. Therefore, we compared the efficacy of larval defence by Chrysomela knabi (a native beetle species) that had fed on two native willow hosts: Salix sericea (a phenolic glycoside (PG)-rich species) and Salix eriocephala (a PG-poor species), when attacked by exotic generalist predators. In addition, the preference and performance of C. knabi on S. sericea and S. eriocephala was examined.
2  Chrysomela knabi preferred and performed better on S. sericea. In a common garden, adult C. knabi were nine-fold more common and oviposited five-fold more frequently on S. sericea than on S. eriocephala . In the laboratory, adult feeding preference on leaf discs and survival rates of larvae were both greater on S. sericea , and time to pupation was shorter.
3  Chrysomela knabi larvae produced significantly more salicylaldehyde when fed S. sericea leaves than when fed S. eriocephala leaves. Additionally, those larvae with greater salicylaldehyde had reduced predation by two exotic generalist predators, Harmonia axyridis larvae and juvenile Tenodera aridifolia sinensis .
4 The results obtained in the present study suggest that selection favoured the preference of C. knabi for PG-rich willow plants because larvae grew and survived better and that selection by common exotic generalist predators would reinforce this preference.     

Evidence for multiple sources of invasion and intraspecific hybridization in Brachypodium sylvaticum (Hudson) Beauv. in North America

We compared the levels and distribution of genetic diversity in Eurasian and North American populations of Brachypodium sylvaticum (Huds.) Beauv. (false brome), a newly invasive perennial bunchgrass in western North America. Our goals were to identify source regions for invasive populations, determine the number of independent invasion events, and assess the possibility that postinvasion bottlenecks and hybridization have affected patterns of genetic diversity in the invaded range. We tested the hypothesis that this Eurasian grass was accidentally introduced into two areas in Oregon and one site in California by examining nuclear microsatellites and chloroplast haplotype variation in 23 introduced and 25 native populations. In the invaded range, there was significantly lower allelic richness (R(S)), observed heterozygosity (H(O)) and within-population gene diversity (H(S)), although a formal test failed to detect a significant genetic bottleneck. Most of the genetic variation existed among populations in the native range but within populations in the invaded range. All of the allelic variation in the invaded range could be explained based on alleles found in western European populations. The distribution of identified genetic clusters in the North American populations and the unique alleles associated with them is consistent with two historical introductions in Oregon and a separate introduction to California. Further analyses of population structure indicate that intraspecific hybridization among genotypes from geographically distinct regions of western Europe occurred following colonization in Oregon. The California populations, however, are more likely to be derived from one or perhaps several genetically similar regions in the native range. The emergence and spread of novel recombinant genotypes may be facilitating the rapid spread of this invasive species in Oregon.     

Fitness and genetic architecture of parent and hybrid willows in common gardens

Models of hybrid zone dynamics incorporate different patterns of hybrid fitness relative to parental species fitness. An important but understudied source of variation underlying these fitness differences is the environment. We investigated the performance of two willow species and their F1, F2, and backcross hybrids using a common-garden experiment with six replicated gardens that differed in soil moisture. Aboveground biomass, catkin production, seed production per catkin, and seed germination rate were significantly different among genetic classes. For aboveground biomass and catkin production, hybrids generally had intermediate or inferior performance compared to parent species. Salix eriocephala had the highest performance for all performance measures, but in two gardens F, plants had superior or equal performance for aboveground biomass and female catkin production. Salix eriocephala and backcrosses to S. eriocephala had the highest numbers of filled seeds per catkin and the highest estimates of total fitness in all gardens. Measures of filled seeds per catkin and germination rate tend to support the model of endogenous hybrid unfitness, and these two measures had major effects on estimates of total seed production per catkin. We also estimated how the two willow species differ genetically in these fitness measures using line cross analysis. We found a complex genetic architecture underlying the fitness differences between species that involved additive, dominance, and epistatic genetic effects for all fitness measures. The environment was important in the expression of these genetic differences, because the type of epistasis differed among the gardens for above-ground biomass and for female catkin production. These findings suggest that fine-scale environmental variation can have a significant impact on hybrid fitness in hybrid zones where parents and hybrids are widely interspersed.     

Genetic uniformity characterizes the invasive spread of water hyacinth (Eichhornia crassipes), a clonal aquatic plant

Aquatic plant invasions are often associated with long‐distance dispersal of vegetative propagules and prolific clonal reproduction. These reproductive features combined with genetic bottlenecks have the potential to severely limit genetic diversity in invasive populations. To investigate this question we conducted a global scale population genetic survey using amplified fragment length polymorphism markers of the world’s most successful aquatic plant invader –Eichhornia crassipes (water hyacinth). We sampled 1140 ramets from 54 populations from the native (South America) and introduced range (Asia, Africa, Europe, North America, Central America and the Caribbean). Although we detected 49 clones, introduced populations exhibited very low genetic diversity and little differentiation compared with those from the native range, and ～80% of introduced populations were composed of a single clone. A widespread clone (‘W’) detected in two Peruvian populations accounted for 70.9% of the individuals sampled and dominated in 74.5% of the introduced populations. However, samples from Bangladesh and Indonesia were composed of different genotypes, implicating multiple introductions to the introduced range. Nine of 47 introduced populations contained clonal diversity suggesting that sexual recruitment occurs in some invasive sites where environmental conditions favour seedling establishment. The global patterns of genetic diversity in E. crassipes likely result from severe genetic bottlenecks during colonization and prolific clonal propagation. The prevalence of the ‘W’ genotype throughout the invasive range may be explained by stochastic sampling, or possibly because of pre‐adaptation of the ‘W’ genotype to tolerate low temperatures.     

Broad-scale latitudinal patterns of genetic diversity among native European and introduced house sparrow (Passer domesticus) populations

Introduced species offer unique opportunities to study evolution in new environments, and some provide opportunities for understanding the mechanisms underlying macroecological patterns. We sought to determine how introduction history impacted genetic diversity and differentiation of the house sparrow (Passer domesticus), one of the most broadly distributed bird species. We screened eight microsatellite loci in 316 individuals from 16 locations in the native and introduced ranges. Significant population structure occurred between native than introduced house sparrows. Introduced house sparrows were distinguished into one North American group and a highly differentiated Kenyan group. Genetic differentiation estimates identified a high magnitude of differentiation between Kenya and all other populations, but demonstrated that European and North American samples were differentiated too. Our results support previous claims that introduced North American populations likely had few source populations, and indicate house sparrows established populations after introduction. Genetic diversity also differed among native, introduced North American, and Kenyan populations with Kenyan birds being least diverse. In some cases, house sparrow populations appeared to maintain or recover genetic diversity relatively rapidly after range expansion (<50 years; Mexico and Panama), but in others (Kenya) the effect of introduction persisted over the same period. In both native and introduced populations, genetic diversity exhibited large-scale geographic patterns, increasing towards the equator. Such patterns of genetic diversity are concordant with two previously described models of genetic diversity, the latitudinal model and the species diversity model.     

Reed Canary Grass (Phalaris arundinacea) as a Biological Model in the Study of Plant Invasions

Invasive species pose a serious threat to native plant communities and are an important contributor to loss of biodiversity. In the case of Phalaris arundinacea, L. (Poaceae), reed canary grass, a cool-season, long-lived perennial plant native to Eurasia and North America, nonnative agronomically important genotypes were introduced to North America for numerous uses such as forage and soil stabilization. Following repeated introductions, reed canary grass became an aggressive invader that takes over natural wet prairies, stream-banks and wetlands. Reed canary grass can outcompete native plant species, resulting in monospecific stands with concomitant loss of plant and insect diversity and ultimately to alteration in ecosystem function. Abiotic factors such as disturbance, changes in hydrological regime, and particularly nutrient runoff to wetlands can enhance reed canary grass establishment and vegetative spread. In addition, the species' capacity for early season growth, rapid vegetative spread, high stem elongation potential, wide physiological tolerance, and high architectural plasticity make the species highly aggressive under a wide range of ecological conditions. The change in life-history and environmental conditions responsible for the enhanced aggressiveness observed between native and invasive genotypes are not yet understood. Hence, reed canary grass provides an ideal study system to test a number of ecological and genetic hypotheses to explain why some plant species become extremely aggressive when transported into a new geographical area. To date, genetic studies have found that invasive populations have high genetic diversity and that genotypes differ in their phenotypic plasticity and response to ecological conditions, which may contribute to their invasion success. Finally comparative studies currently underway on European native and American invasive genotypes of reed canary grass should shed light on the mechanisms responsible for reed canary grass's aggressiveness and should provide an experimental protocol to test ecological and genetic hypotheses about what makes a species invasive.     

Genetic variation in wild and cultivated populations of the haploid-diploid red alga Gracilaria chilensis: how farming practices favor asexual reproduction and heterozygosity.

The extent of changes in genetic diversity and life-history traits associated with farming was investigated in the haploid-diploid red alga, Gracilaria chilensis, cultivated in Chile. This alga belongs to one of the most frequently cultivated seaweed genera around the world. Fifteen farmed populations, 11 wild populations, and two subspontaneous populations were sampled along the Chilean coast. The frequency of reproductive versus vegetative individuals and of haploid versus diploid individuals was checked in each population. In addition, the distribution of genetic variation in wild and cultivated populations was analyzed using six microsatellite markers. Our results first demonstrated that farmed populations are maintained almost exclusively by vegetative propagation. Moreover, the predominance of diploid individuals in farms showed that farming practices had significantly modified life-history traits as compared to wild populations. Second, the expected reduction in genetic diversity due to a cultivation bottleneck and subsequent clonal propagation was detected in farms. Finally, our study suggested that cultural practices in the southern part of the country contributed to the spread of selected genotypes at a local scale. Altogether, these results document for the first time that involuntary selection could operate during the first step of domestication in a marine plant.     

GENETIC VARIATION IN WILD AND CULTIVATED POPULATIONS OF THE HAPLOID– DIPLOID RED ALGA GRACILARIA CHILENSIS: HOW FARMING PRACTICES FAVOR ASEXUAL REPRODUCTION AND HETEROZYGOSITY

The extent of changes in genetic diversity and life-history traits associated with farming was investigated in the haploid–diploid red alga, Gracilaria chilensis , cultivated in Chile. This alga belongs to one of the most frequently cultivated seaweed genera around the world. Fifteen farmed populations, 11 wild populations, and two subspontaneous populations were sampled along the Chilean coast. The frequency of reproductive versus vegetative individuals and of haploid versus diploid individuals was checked in each population. In addition, the distribution of genetic variation in wild and cultivated populations was analyzed using six microsatellite markers. Our results first demonstrated that farmed populations are maintained almost exclusively by vegetative propagation. Moreover, the predominance of diploid individuals in farms showed that farming practices had significantly modified life-history traits as compared to wild populations. Second, the expected reduction in genetic diversity due to a cultivation bottleneck and subsequent clonal propagation was detected in farms. Finally, our study suggested that cultural practices in the southern part of the country contributed to the spread of selected genotypes at a local scale. Altogether, these results document for the first time that involuntary selection could operate during the first step of domestication in a marine plant.     

Allozymic diversity of Allium tricoccum (Ait.) Solander var. burdickii Hanes in isolated populations of Nova Scotia (Canada)

 Allium tricoccumvar. burdickii is a rare species in Nova Scotia, growing in isolated populations. Genetic variation was investigated in three populations using cellulose acetate gel electrophoresis. The results showed that all 13 enzyme loci were polymorphic. An excess of heterozygotes was found in all populations. A total of 29 genotypes were detected in the populations. Very few genotypes were common to two populations and none to the three populations. Genetic diversity among populations was compared with geographic distance and previous work. The comparison of the genetic distance matrix to the geographic distance matrix indicated no relationship. F-statistics suggested that the populations are predominantly maintained through vegetative propagation. Occasional sexual reproduction and gene flow might occur in order to maintain high level of variation among populations. Environmental conditions could also influence population genetic structure as they occur in highly different habitats. Received November 13, 2000 Accepted February 26, 2001     

High genetic diversity is not essential for successful introduction

Some introduced populations thrive and evolve despite the presumed loss of diversity at introduction. We aimed to quantify the amount of genetic diversity retained at introduction in species that have shown evidence of adaptation to their introduced environments. Samples were taken from native and introduced ranges of Arctotheca populifolia and Petrorhagia nanteuilii. Using microsatellite data, we identified the source for each introduction, estimated genetic diversity in native and introduced populations, and calculated the amount of diversity retained in introduced populations. These values were compared to those from a literature review of diversity in native, confamilial populations and to estimates of genetic diversity retained at introduction. Gene diversity in the native range of both species was significantly lower than for confamilials. We found that, on average, introduced populations showing evidence of adaptation to their new environments retained 81% of the genetic diversity from the native range. Introduced populations of P. nanteuilii had higher genetic diversity than found in the native source populations, whereas introduced populations of A. populifolia retained only 14% of its native diversity in one introduction and 1% in another. Our literature review has shown that most introductions demonstrating adaptive ability have lost diversity upon introduction. The two species studied here had exceptionally low native range genetic diversity. Further, the two introductions of A. populifolia represent the largest percentage loss of genetic diversity in a species showing evidence of substantial morphological change in the introduced range. While high genetic diversity may increase the likelihood of invasion success, the species examined here adapted to their new environments with very little neutral genetic diversity. This finding suggests that even introductions founded by small numbers of individuals have the potential to become invasive.     

High Levels of Genetic Diversity in Salix viminalis of the Czech Republic as Revealed by Microsatellite Markers

Willows (Salix spp.) grown as short rotation coppice are recognised as an important bioenergy crop, and breeding programmes are underway in several countries, including the Czech Republic. The basket willow Salix viminalis is one of the few willow species that is widespread in the Czech Republic and thus a potential source of diversity, but the most extensive germplasm collection available shows evidence of redundancy. To investigate levels of variation in natural populations of this species for use in crop improvement programmes, a set of 38 microsatellite markers was used to assess genetic diversity and population structure among 84 S. viminalis individuals collected from seven Czech rivers (the Odra, Be?va, Morava, Dyje, Jihlava, Sázava and Vltava), covering a wide geographic distribution. The markers detected 6.95 alleles per locus on average with 92?% of the sampled individuals having a unique multilocus genotype giving a high clonal richness measure among all samples (R?=?0.952). Three sets of putative clones (with identical genotypes as determined by the markers used here) were also identified. Significant levels of genetic diversity were revealed within all sampling sites. With the exception of sites on the Odra and Morava, pairwise F _ST (0.02?C0.1) values indicated moderate differentiation between sites. Principal coordinates analysis revealed some separation of the Dyje individuals from all others. This was in agreement with the population structure results derived from Bayesian analyses using STRUCTURE software. These results provide the first evidence that potentially useful levels of genotypic variation are present within natural S. viminalis populations in the Czech Republic.     

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.     

Morphological and molecular evidence for hybridization and introgression in a willow (Salix) hybrid zone

Hybrid zones provide biologists with the opportunity to examine genetic and ecological interactions between differentiated populations. Accurate identification of hybrid genealogies is considered a necessary prerequisite to understanding observed patterns of hybridization-related phenomena. We analysed molecular and morphological data from individuals in a hybrid zone between two species of willows (Salix sericea Marshall and S. eriocephala Michaux) and report the use of randomly amplified polymorphic DNA (RAPD), chloroplast DNA (cpDNA), and ribosomal DNA (rDNA) markers, as well as vegetative morphology and foliar chemistry data to identify individuals in terms of hybrid genealogy and to infer the direction and extent of backcrossing and introgression within the hybrid zone. A novel version of a maximum likelihood estimate approach (developed for this study) was used to calculate hybrid index scores from RAPD marker data; this method produced results similar to those obtained using traditional arithmetic methods. Distribution of rDNA, cpDNA, and chemistry data were examined within the graphical context of RAPD-based hybrid index score histograms and principal component analyses (PCA) on RAPD and morphology data. Seven of the 21 plants classified as S. eriocephala in the field were possible introgressants. Another plant presented an unequivocal example of backcrossed S. sericea chemistry and RAPD markers. Inter- and intraspecific chloroplast diversity found within the hybrid zone suggests both historic introgression (perhaps in a glacial refugium), and contemporary hybridization. Patterns of inheritance and expression within the hybrid zone suggest that morphological characters are often not expressed in a simple additive fashion, and problems associated with both morphological and molecular data are considered.