Phylogenetic and biogeographic diversification of Rhus (Anacardiaceae) in the Northern Hemisphere

Sequences of internal transcribed spacers (ITS) of nuclear ribosomal DNA, the chloroplast ndhF gene, and chloroplast trnL-F regions (trnL intron, and trnL [UAA] 3' exon-trnF [GAA] intergenic spacer) were used for phylogenetic analyses of Rhus, a genus disjunctly distributed in Asia, Europe, Hawaii, North America, and Northern Central America. Both ITS and cpDNA data sets support the monophyly of Rhus. The monophyly of subgenus Rhus was suggested by the combined cpDNA and ITS data, and largely supported in the cpDNA data except that Rhus microphylla of subgenus Lobadium was nested within it. The monophyly of subgenus Lobadium was strongly supported in the ITS data, whereas the cpDNA data revealed two main clades within the subgenus, which formed a trichotomy with the clade of subgenus Rhus plus R. microphylla. The ITS and cpDNA trees differ in the positions of Rhus michauxii, R. microphylla, and Rhus rubifolia, and hybridization may have caused this discordance. Fossil evidence indicates that Rhus dates back to the early Eocene. The penalized likelihood method was used to estimate divergence times, with fossils of Rhus subgenus Lobadium, Pistacia and Toxicodendron used for age constraints. Rhus diverged from its closest relative at 49.1+/-2.1 million years ago (Ma), the split of subgenus Lobadium and subgenus Rhus was at 38.1+/-3.0 Ma. Rhus most likely migrated from North America into Asia via the Bering Land Bridge during the Late Eocene (33.8+/-3.1 Ma). Rhus coriaria from southern Europe and western Asia diverged from its relatives in eastern Asia at 24.4+/-3.2 Ma. The Hawaiian Rhus sandwicensis diverged from the Asian Rhus chinensis at 13.5+/-3.0 Ma. Subgenus Lobadium was inferred to be of North American origin. Taxa of subgenus Lobadium then migrated southward to Central America. Furthermore, we herein make the following three nomenclatural combinations: (1) Searsia leptodictya (Diels) T. S. Yi, A. J. Miller and J. Wen, comb. nov., (2) Searsia pyroides (A. Rich.) T. S. Yi, A. J. Miller and J. Wen, comb. nov., and (3) Searsia undulata (Jacq.) T. S. Yi, A. J. Miller and J. Wen, because our analyses support the segregation of Searsia from Rhus.     

Genetic,morphological, and niche variation in the widely hybridizing Rhus integrifolia-Rhus ovata species complex

Hybridization and introgression are common processes among numerous plant species that present both challenges and opportunities for studies of species delimitation, phylogenetics, taxonomy and adaptation. Rhus integrifolia and R. ovata are two ecologically important shrubs native to the southwestern USA and Mexico, and are known to hybridize frequently, but the morphological, genetic and ecological implications of hybridization in these species are poorly studied on a broad geographic scale. Analyses were conducted using leaf morphology, genetic variation of plastid and nuclear loci, and species distribution models for both species and their putative hybrid introgressants across 19 localities in California and Arizona, USA. These analyses revealed evidence for morphological and genetic distinction among localities comprising putative parental species, but a high degree of morpho-genetic intermediacy among localities with putative hybrids. Comparison of morphological and genetic population structure among localities revealed evidence for putative local adaptation or widespread phenotypic plasticity. Multiple regression models identified a weak but statistically significant negative association between leaf area and precipitation. Finally, species distribution modeling inferred northward range shifts over time, with both species predicted to occupy more coastal regions in the future, possibly increasing the frequency of hybridization among them. These findings underscore the importance of integrative assessment of multiple data sources in the study of hybridizing species and highlight the R. integrifolia-ovata complex as a powerful model for investigating the adaptive implications of hybridization.     

MORPHOLOGICAL AND GENETIC VARIABILITY IN PLANTAGO CORDATA (PLANTAGINACEAE), A THREATENED AQUATIC PLANT

Morphological and genetic variation were studied in Plantago cordata Lam., an imperiled aquatic plant. The existence of distinct seasonal morphs was confirmed by numerical morphological analyses of specimens from 53 populations. Plants from two North Carolina populations possessed leaves that were significantly shorter, narrower, and fewer-veined than populations in other portions of the species range. Genetic variation within and among ten populations in seven states/provinces (Illinois, Missouri, New York, North Carolina, Ohio, Ontario, and Wisconsin) was examined electrophoretically. The species is an apparent allopolyploid with fixed heterozygosity observed at 67% of the loci surveyed. Electrophoretic variation was mostly partitioned among populations. The genetic identity among populations was high except for North Carolina populations which differed at 27% of the loci surveyed.     

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.     

Hybridization between the escaped Rosa rugosa (Rosaceae) and native R. blanda in eastern North America

Rosa rugosa, a vigorous ornamental shrub introduced from Asia in the 19th century, is now naturalized in coastal northeastern North America, where it occasionally grows in sympatry with the native R. blanda. To document hybridization between these species, evaluate its extent across the area of sympatry, and examine the use of morphology as a field monitoring tool, we sampled 179 individuals of parental species and putative hybrids in 13 pure and 11 mixed populations. We developed allele-specific primers to assay single nucleotide polymorphisms (SNPs) markers from one chloroplast region and four low-copy nuclear introns. Our results revealed frequent bidirectional hybridization and infrequent introgression in sympatric populations of these species. The recurrent presence of F(1) hybrids in mixed populations indicated the weakness of early-acting reproductive barriers. Morphological data were concordant with molecular data and provided additional evidence for the presence of a few backcrosses. Morphological analyses yielded diagnostic characters for identifying hybrids and monitoring the hybrid zone. Such hybridization could ultimately lead to the genetic assimilation of R. blanda in mixed populations and to the formation of invasive hybrid genotypes, a phenomenon that is of economic and ecological concern because of the increasing number of exotic species worldwide.     

Complex Genetic Effects on Early Vegetative Development Shape Resource Allocation Differences Between Arabidopsis lyrata Populations

Costs of reproduction due to resource allocation trade-offs have long been recognized as key forces in life history evolution, but little is known about their functional or genetic basis. Arabidopsis lyrata, a perennial relative of the annual model plant A. thaliana with a wide climatic distribution, has populations that are strongly diverged in resource allocation. In this study, we evaluated the genetic and functional basis for variation in resource allocation in a reciprocal transplant experiment, using four A. lyrata populations and F₂ progeny from a cross between North Carolina (NC) and Norway parents, which had the most divergent resource allocation patterns. Local alleles at quantitative trait loci (QTL) at a North Carolina field site increased reproductive output while reducing vegetative growth. These QTL had little overlap with flowering date QTL. Structural equation models incorporating QTL genotypes and traits indicated that resource allocation differences result primarily from QTL effects on early vegetative growth patterns, with cascading effects on later vegetative and reproductive development. At a Norway field site, North Carolina alleles at some of the same QTL regions reduced survival and reproductive output components, but these effects were not associated with resource allocation trade-offs in the Norway environment. Our results indicate that resource allocation in perennial plants may involve important adaptive mechanisms largely independent of flowering time. Moreover, the contributions of resource allocation QTL to local adaptation appear to result from their effects on developmental timing and its interaction with environmental constraints, and not from simple models of reproductive costs.     

POPULATION VARIATION AND HYBRIDIZATION IN SEA-ROCKETS (CAKILE,CRUCIFERAE): SEED GLUCOSINOLATE CHARACTERS

In a study of intra- and interpopulation variation in seed glucosinolates, remarkable uniformity was found among individuals within the two chemically distinct subspecies of East Coast Cakile edentula. Glucosinolates were used as taxonomic markers in a study of hybridization between ssp. edentula and ssp. harperi where they form mixed populations on the Outer Banks of North Carolina. Thirteen plants with hybrid chemical profiles were detected in a sample of 89 individuals from four populations; the zone of hybridization is narrow, and has persisted for at least eight years. Artificial hybrids of the two subspecies are additive for kinds and intermediate for proportions of seed glucosinolates, and families of F₂'s show a range of recombinant chemical profiles. Natural hybridization was confirmed by recovering hybrid and parental types in 47 progeny of three presumptive wild hybrids. Glucosinolate profile characteristics segregated independently of the segregation patterns for fruit length and seed weight. Selection against particular glucosinolate phenotypes is one possible mechanism maintaining this narrow hybrid zone on the Outer Banks.     

GENETIC VARIATION IN PLANT-INSECT ASSOCIATIONS: SURVIVAL OF LEPTINOTARSA DECEMLINEATA POPULATIONS ON SOLANUM CAROLINENSE

Populations of the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae), from the east coast of the United States differ in their ability to survive on a wild host, Solanum carolinense (Solanaceae), but not on their most important cultivated host, Solanum tuberosum. On the wild host, the North Carolina population survived best, while populations from Virginia, New Jersey, and Connecticut exhibited uniformly low survival. Formal genetic studies of populations from Connecticut and North Carolina demonstrated heritable variation in the ability to survive on S. carolinense both between and within populations; the North Carolina population had the higher heritability for this trait. Overall, there was no genetic variation between populations or within the North Carolina population for survival on S. tuberosum, but such variation existed within the Connecticut population. Hybrids and backcrosses between these two lines all survived at intermediate levels, although survivorship did not appear to be inherited additively. Differences in survival were greater than differences in adult weight at emergence and development time of the survivors. Leptinotarsa decemlineata was first reported from North Carolina less than 100 years ago. The rapid expansion of L. decemlineata's host range in North Carolina is attributed to the poor synchrony between the insect and S. tuberosum compared to more northerly locations. In contrast to the prediction of a strong negative correlation in fitness on different host species, the ability of L. decemlineata to survive on S. carolinense was not correlated with that on S. tuberosum. Adult weight and female development time were significantly positively correlated across hosts. Our results are in accord with most previous studies in which strong negative correlations in fitness of specialized phytophagous insects feeding on different hosts were expected, sought, but not found.     

Unusual patterns of hybridization involving a narrow endemic Rhododendron species (Ericaceae) in Yunnan, China

? Premise of the study: One potential threat to rare species is genetic swamping caused by hybridization, but few studies have quantified this threat. Rhododendron cyanocarpum is a narrow endemic species that occurs sympatrically with potentially interfertile congeners throughout its range within Yunnan, China. We searched the entire distribution of R. cyanocarpum for hybrids and examined the patterns of hybridization to assess potential threat from hybridization. ? Methods: In a comprehensive field survey, we detected only one instance of hybridization involving R. cyanocarpum, with R. delavayi, at Huadianba near Dali. Material of both species and putative hybrids was examined using morphology, chloroplast DNA, nuclear ribosomal DNA, and Bayesian analysis of AFLP profiles. ? Key results: Of 10 putative hybrids, two were F(1)(')s and at least seven were F(2)(')s. Four backcrosses to R. delavayi were detected among material with R. delavayi-like morphology within the hybrid zone. Backcrosses to R. cyanocarpum were not detected. Therefore F(2)(')s outnumbered all other classes within the hybrid zone, a situation not previously confirmed for plants and extremely rare generally. Hybridization was asymmetrical, with R. delavayi as the maternal parent in all but one of the hybrids detected. ? Conclusions: Although natural hybridization is common in Rhododendron, it is rare in R. cyanocarpum and is apparently not accompanied by backcrossing toward R. cyanocarpum. Hence, there is no immediate risk of genetic swamping, unless habitat disturbance increases and changes the patterns of hybridization. Our study is the first to report a plant hybrid zone dominated by F(2) hybrids. This pattern might contribute to species barrier maintenance.     

RECENT VERSUS RELIC: DISCERNING THE GENETIC SIGNATURE OF FUCUS VESICULOSUS (HETEROKONTOPHYTA; PHAEOPHYCEAE) IN THE NORTHWESTERN ATLANTIC1

Fucus vesiculosus L. is one of the most widespread macrophytes in the northwestern Atlantic, ranging from North Carolina (USA) to Greenland (DK). We investigated genetic diversity, population differentiation, patterns of isolation by distance, and putative glacial refugial populations across seven locations from North Carolina (USA) to Cape Breton Island, Nova Scotia (Canada), with microsatellite analyses. Distinct northern versus southern (Delaware–North Carolina) populations were revealed by microsatellite data. Five of six microsatellite loci were fixed in populations in North Carolina, suggesting a recent founder event or a bottleneck, and the same homozygous genotype was found in herbarium materials collected on the North Carolina coast from more than 60 years ago. An additional set of individuals from the northern limit in Greenland was included in our analysis of mitochondrial intergenic spacer (mt IGS) haplotypes in the northwestern Atlantic. Remarkably, 184 of 188 F. vesiculosus specimens from North Carolina to Greenland shared the same haplotype. Recent colonization of the North American shore from Europe is hypothesized based upon the ubiquity of this common haplotype, which was earlier reported from Europe.     

Insertion-deletion variation at the yellow-achaete-scute region in two natural populations of Drosophila melanogaster

We have surveyed the region of the X chromosome of Drosophila melanogaster which encodes the yellow, achaete and scute genes for restriction map variation. Two natural populations, one from North Carolina, U.S.A. and the other from southern Spain were screened for variation at about 70 restriction sites and for variation due to DNA insertion or deletion events in 120 kilobases of DNA. Mean heterozygosity per nucleotide was estimated to be 0.0024 and 15 large insertions were found in the 49 chromosomes screened. Extensive disequilibrium between polymorphic sites were found across much of the region in the North Carolina population. The frequency of large insertions, which usually correspond to transposable genetic elements, is significantly lower than has been observed in autosomal regions of the genome. This is predicted for X-linked loci by certain models of transposable element evolution, where copy number is restricted by virtue of the recessive deleterious effects of the insertions. Our results appear to support such models. The deficiency of insertions may in this case be enhanced by hitch-hiking effects arising from the high level of disequilibrium.     

Hybridization and asymmetric introgression between <Emphasis Type="Italic">Rhododendron eriocarpum</Emphasis> and <Emphasis Type="Italic">R. indicum</Emphasis> on Yakushima Island,southwest Japan

We examined natural hybridization between two morphologically and ecologically divergent species on Yakushima Island-the light-purple flowered Rhododendron eriocarpum native to seaside habitats and the red flowered R. indicum native to riverside habitats. By investigation of morphological traits and amplified fragment length polymorphisms (AFLP) we found that hybrid individuals morphologically and genetically intermediate between the two species grow mainly in the seaside areas of the island. The degree of morphological and genetic variation was different among the seaside hybrid populations. Although most pollinator species were exclusive to one of the two flower color groups, the halictine bees of Lasioglossum were observed in both the color types. The crucial interspecific incompatibility after pollination has not been previously described. Geographic distance between the populations is likely to be an important primary factor in bringing about natural hybridization and determining the degree of introgression between R. eriocarpum and R. indicum. The fact that hybrids occur mostly in the seaside area on Yakushima Island indicates that asymmetrical introgression occurs from R. indicum to R. eriocarpum. Alternatively, strong habitat-mediated selection from recurrent floods may prevent the hybrids from colonizing riverside habitats.     

Captive breeding and the reintroduction of Mexican and red wolves

Mexican and red wolves were both faced with extinction in the wild until captive populations were established more than two decades ago. These captive populations have been successfully managed genetically to minimize mean kinship and retain genetic variation. Descendants of these animals were subsequently used to start reintroduced populations, which now number about 40-50 Mexican wolves in Arizona and New Mexico and about 100 red wolves in North Carolina. The original captive Mexican wolf population was descended from three founders. Merging this lineage with two other captive lineages, each with two founders, has been successfully carried out in the captive population and is in progress in the reintroduced population. This effort has resulted in increased fitness of cross-lineage wolves, or genetic rescue, in both the captive and reintroduced populations. A number of coyote-red wolf hybrid litters were observed in the late 1990s in the reintroduced red wolf population. Intensive identification and management efforts appear to have resulted in the elimination of this threat. However, population reintroductions of both Mexican and red wolves appear to have reached numbers well below the generally recommended number for recovery and there is no current effort to re-establish other populations.     

A Comparison of Hybridization between Mottled Ducks (<Emphasis Type="Italic">Anas fulvigula</Emphasis>)and Mallards (<Emphasis Type="Italic">A. platyrhynchos</Emphasis>) in Florida and South Carolina using Microsatellite DNA Analysis

Interspecific hybridization has been implicated in population declines for some waterfowl species within the mallard complex, and hybridization with mallards (Anas platyrhynchos) is currently considered the largest threat to mottled ducks (A. fulvigula), one North American member of that complex. We assessed genetic variation among 225 mottled ducks and mallards using five microsatellite loci, and detected significant overall differences between these species within two geographic areas. We characterized hybridization in Florida, where mottled ducks are endemic and mallards are beginning to appear on the breeding grounds, and in South Carolina, where mottled ducks were introduced outside their native range. We used Bayesian genetic mixture analysis in an attempt to distinguish between these closely related species. In Florida, we detected two distinct genetic groups, and 10.9% of our samples from Florida mottled ducks were inferred to have been hybrids. In contrast only 3.4% of Florida mallards were inferred to have been hybrids, suggesting asymmetric hybridization. Populations from different geographic areas within Florida exhibited hybridization rates ranging from 0% to 24%. These data indicate a genetic component would be appropriate in actively managing interspecific hybridization in Florida mottled ducks. In contrast, South Carolina mottled ducks and mallards cannot be differentiated.     

Origin of gynodioecy in Limnanthes: evidence from ecogeographic patterns of variation

Summary Evidence for hybrid origin of gene-cytoplasmic gynodioecy in Limnanthes douglasii is presented in terms of the parapatric distributions of putative parental taxa and the increased levels of genetic variation in gynodioecious populations. Attempts to produce gynodioecy through artificial hybridization between different accessions apparently failed due to the limited number of parental combinations used in making hybrids. Further studies are proposed on the hybrid origin model and on selective forces determining the fate of gynodioecy with its contributions to higher levels of hybridity and genetic variation.     

Nuclear-mitochondrial discordance and gene flow in a recent radiation of toads

Natural hybridization among recently diverged species has traditionally been viewed as a homogenizing force, but recent research has revealed a possible role for interspecific gene flow in facilitating species radiations. Natural hybridization can actually contribute to radiations by introducing novel genes or reshuffling existing genetic variation among diverging species. Species that have been affected by natural hybridization often demonstrate patterns of discordance between phylogenies generated using nuclear and mitochondrial markers. We used Amplified Fragment Length Polymorphism (AFLP) data in conjunction with mitochondrial DNA in order to examine patterns of gene flow and nuclear-mitochondrial discordance in the Anaxyrus americanus group, a recent radiation of North American toads. We found high levels of gene flow between putative species, particularly in species pairs sharing similar male advertisement calls that occur in close geographic proximity, suggesting that prezygotic reproductive isolating mechanisms and isolation by distance are the primary determinants of gene flow and genetic differentiation among these species. Additionally, phylogenies generated using AFLP and mitochondrial data were markedly discordant, likely due to recent and/or ongoing natural hybridization events between sympatric populations. Our results indicate that the putative species in the A. americanus group have experienced high levels of gene flow, and suggest that their North American radiation could have been facilitated by the introduction of beneficial genetic variation from admixture between divergent populations coming into secondary contact after glacial retreats.     

Hybridization and introgression in New World red mangroves, Rhizophora (Rhizophoraceae)

? Premise of the study: Hybridization is common in both animals and plants and can lead to a diverse array of outcomes ranging from the generation of new ecotypes or species to the breakdown of morphological differences. Here, we explore the extent of hybridization in the three currently recognized New World Rhizophora species-R. mangle, R. racemosa, and the putative hybrid species R. harrisonii. ? Methods: We assayed variation across the three recognized Rhizophora species using two noncoding chloroplast (cpDNA), two flanking microsatellite regions (FMRs), and six microsatellite loci. ? Key results: Gene genealogies of cpDNA and FMRs showed a strong phylogeographic break across the Central American Isthmus, but little relationship to recognized species boundaries. Instead, individuals collected in the same ocean basin and classified as R. mangle and R. racemosa by morphological characteristics were more closely related to each other than with similar looking individuals collected in the other ocean basin. Nonetheless, there were low, yet significant differences at microsatellite loci among co-occurring populations of R. mangle and R. racemosa in both ocean basins, suggesting that two taxonomic groups coexist. However, we found no genetic evidence that R. harrisonii was a hybrid species. Rather, R. harrisonii appears to represent a morphotype produced by ongoing hybridization and backcrossing between R. mangle and R. racemosa. ? Conclusions: Our data support ancient and persistent introgressive hybridization among new world Rhizophora and argue for a full revision of the systematic relationships of the group based on much finer morphological, ecological, and genetic analyses.     

Persistence of an endangered native duck,feral mallards,and multiple hybrid swarms across the main Hawaiian Islands

Interspecific hybridization is recognized as an important process in the evolutionary dynamics of both speciation and the reversal of speciation. However, our understanding of the spatial and temporal patterns of hybridization that erode versus promote species boundaries is incomplete. The endangered, endemic koloa maoli (or Hawaiian duck, Anas wyvilliana) is thought to be threatened with genetic extinction through ongoing hybridization with an introduced congener, the feral mallard (A. platyrhynchos). We investigated spatial and temporal variation in hybrid prevalence in populations throughout the main Hawaiian Islands, using genomic data to characterize population structure of koloa, quantify the extent of hybridization, and compare hybrid proportions over time. To accomplish this, we genotyped 3,308 double‐digest restriction‐site‐associated DNA (ddRAD) loci in 425 putative koloa, mallards, and hybrids from populations across the main Hawaiian Islands. We found that despite a population decline in the last century, koloa genetic diversity is high. There were few hybrids on the island of Kaua?i, home to the largest population of koloa. By contrast, we report that sampled populations outside of Kaua?i can now be characterized as hybrid swarms, in that all individuals sampled were of mixed koloa × mallard ancestry. Further, there is some evidence that these swarms are stable over time. These findings demonstrate spatial variation in the extent and consequences of interspecific hybridization, and highlight how islands or island‐like systems with small population sizes may be especially prone to genetic extinction when met with a congener that is not reproductively isolated.     

Genetic and phenotypic differentiation between invasive and native Rhododendron (Ericaceae) taxa and the role of hybridization

Hybridization has been repeatedly put forward to explain the invasiveness of Rhododendron ponticum L. in the British Isles. The present study investigates the pattern of ecotypic differentiation and hybridization among native North American R. catawbiense and R. maximum, native R. ponticum from Georgia and Spain, and invasive R. ponticum from Ireland and aims to assess the contribution of hybridization for Rhododendron invasion in the British Isles. Six populations per taxon were analyzed with AFLP markers for genetic dissimilarity, subjected to germination and growth experiments, and tested for frost hardiness. We assessed variation in morphological and ecological characteristics to identify traits displaying evidence of hybridization, thus, promoting invasiveness. Molecular marker analyses revealed a clear distinction between North American R. catawbiense and R. maximum on the one hand, and all R. ponticum populations on the other hand, displaying a complete intermixture of native Spanish and invasive Irish populations. Multivariate analyses of traits revealed leaf length-width ratio, relative growth rates (RGRs) in leaf length, root biomass, and shoot-root ratio to significantly discriminate between the different taxa and unequivocally assigned invasive Irish R. ponticum to the Spanish phenotypes. While the Irish R. ponticum had similar growth traits as conspecific native R. ponticum provenances, germination and biomass allocation were more similar to North American R. catawbiense and R. maximum. Hybridization did not contribute to explaining invasiveness of R. ponticum in Ireland. The similarity in germination and biomass allocation of invasive Irish R. ponticum and North American species has evolved independently and can more probably be attributed to an independent shift within the Ponticum cluster in Ireland.     

Patterns of hybridization and population genetic structure in the terrestrial orchids Liparis kumokiri and Liparis makinoana (Orchidaceae) in sympatric populations

We investigated the potential for gene flow and genetic assimilation via hybridization between common and rare species of the terrestrial orchid genus Liparis, focusing specifically on sympatric and allopatric populations of the common Liparis kumokiri and the rare Liparis makinoana. We utilized analyses of genetic diversity, morphology, and the spatial distributions of individuals and genotypes to quantify the dynamics of interspecific gene flow at within- and among-population scales. High levels of allozyme genetic diversity (HE) were found in populations of the rare L. makinoana (0.317), whereas the common L. kumokiri (N = 1744 from 14 populations) revealed a complete lack of variation. This contrast may reflect different breeding systems and associated rates of genetic drift (L. makinoana is self-incompatible, whereas L. kumokiri is self-compatible). At the two known sympatric sites, individuals were found that recombined parental phenotypes, possessing floral characteristics of L. kumokiri and vegetative characteristics of L. makinoana. These putative hybrids were the only individuals found segregating alleles diagnostic of both parental species. Analysis of these individuals indicated that hybrid genotypes were skewed towards L. kumokiri and later generation recombinants of L. kumokiri at both sympatric sites. Furthermore, Ripley's bivariate L(r) statistics revealed that at one site these hybrids are strongly spatially clustered with L. kumokiri. Nonetheless, the relatively low frequency of hybrids, absence of ongoing hybridization (no F1s or first generation backcrossess), and strong genetic differentiation between morphologically 'pure' parental populations at sympatric sites (FST = 0.708-0.816) indicates that hybridization was not an important bridge for gene flow. The results from these two species suggest that natural hybridization has not played an important role in the diversification of Liparis, but instead support the view that genetic drift and limited gene flow are primarily responsible for speciation in Liparis. Based on genetic data and current status of the species, implications of the research for conservation are considered to provide guidelines for appropriate conservation and management strategies.