INTRASPECIFIC CHLOROPLAST DNA VARIATION AND BIOGEOGRAPHY OF NORTH AMERICAN LIRIODENDRON L. (MAGNOLIACEAE)

Restriction site variation in chloroplast DNA (cpDNA) was surveyed to analyze population dynamics in Liriodendron tulipifera L., a woody angiosperm found in eastern North America. Two cpDNA haplotypes, differing by the presence or absence of five restriction site changes (nucleotide sequence divergence estimated as approximately 0.15%) are geographically structured; 61 widespread populations possess the “northern” haplotype and three isolated populations of central Florida possess the “southern” haplotype. This geographic break in cpDNA distribution corresponds to patterns of geographic distribution revealed by a previous survey of allozyme variation, with the exception that analyses of allozyme data further divided the populations containing the northern cpDNA haplotype into two groups, a widespread upland group and a coastal intermediate group. Analyses of these two independent data sets together support the hypothesis that L. tulipifera survived the glacial advances of the Pleistocene in two distinct refugia, possibly as different taxa, and the intermediate coastal group was putatively formed from recent hybridizations between these entities.     

PATTERNS OF HYBRIDIZATION IN THE PIRIQUETA CAROLINIANA COMPLEX IN CENTRAL FLORIDA: EVIDENCE FOR AN EXPANDING HYBRID ZONE

We have identified a broad zone of hybridization between two morphologically and ecologically distinct herbaceous perennial taxa (morphotypes) within the Piriqueta caroliniana complex, which extends more than 300 km across the central Florida peninsula. Phylogeographic analyses indicate that the caroliniana morphotype has been present in north and central Florida since the early Pleistocene and that the viridis morphotype has immigrated into southern Florida much more recently. We examine the distribution of diagnostic morphological characters and nuclear genetic markers to assess the extent and patterns of introgression in this system. The results are consistent with the hypothesis that this hybrid zone has expanded north in recent history as viridis alleles have introgressed into regions that were previously occupied by populations of caroliniana. Genetic markers diagnostic for caroliniana have consistently high frequencies across the hybrid zone, whereas markers for viridis are extremely variable among populations with frequency reversals in adjacent populations. The latter pattern is probably the result of the combined stochastic effects of dispersal and drift on viridis alleles as they introgressed northward. Additional evidence for the recent expansion of this hybrid zone comes from patterns of variation for morphological and genetic markers. As expected for an expanding hybrid zone, within-population morphological variation was greatest toward the advancing front of introgression and levels of genetic variation for neutral diagnostic markers were greatest in the region of initial contact and lower in areas of recent expansion. The observed patterns of variation suggest that at least some hybrid genotypes have high fitnesses, which has led to the expansion of the hybrid zone via the displacement of parental genotypes in central Florida.     

HISTORICAL BIOGEOGRAPHY AND CONTEMPORARY PATTERNS OF MITOCHONDRIAL DNA VARIATION IN WHITE-TAILED DEER FROM THE SOUTHEASTERN UNITED STATES

Mitochondrial DNA (mtDNA) was used to characterize patterns of geographic variation among white-tailed deer (Odocoileus virginianus) populations in the southeastern United States. Fifteen restriction enzymes were employed to survey and map 99 restriction sites in 142 deer from 18 localities in five southeastern states. Phylogenetic analysis revealed three primary groups of haplotypes: (1) southern Florida and the Florida Keys, (2) the remainder of peninsular Florida northward to South Carolina, and (3) the Florida panhandle westward to Mississippi. Geographical heterogeneity in haplotype frequencies suggests that stochastic lineage sorting or isolation by distance are not important determinates of mtDNA differentiation among deer populations. The pattern of mtDNA variation in white-tailed deer is concordant spatially with those observed in unrelated taxa suggesting the common influence of historical biogeographic events. The data (1) support previous hypotheses that relate contemporary patterns of intraspecific phylogeography in northern Florida to the physiogeographic history of the region; and (2) suggest that genetic differentiation in southern Florida may be attributable to episodes of Pleistocene dispersal. Despite potentially high vagility and human intervention, ecological and demographic characteristics of deer have effectively preserved the historical pattern of intraspecific mtDNA differentiation.     

Effects of natural habitat fragmentation on an endemic scrub lizard (Sceloporus woodi): an historical perspective based on a mitochondrial DNA gene genealogy

The Florida scrub lizard, Sceloporus woodi, is endemic to scrub habitat patches along the central portion of the Florida peninsula and xeric coastal regions. Scrub ecosystems are the patchily distributed remnants of previously widespread habitats formed during the Pleiocene and early Pleistocene. Scrub lizards appear to have limited dispersal capabilities due to high habitat specificity and low mobility. To assess the population structure and phylogeography of S. woodi, 135 samples were collected from 16 patches on five major ridges in Florida, USA. Analysis of 273 bp of mitochondrial DNA (mtDNA) cytochrome b reveals a very strong geographic distribution of genetic diversity. Haplotype frequencies are significantly different in 63 of 66 comparisons between patches. With one exception, samples from the five major ridges are characterized by fixed differences in haplotype distribution and deep evolutionary separations (3-10%). Fixed genetic differences were also observed between northern and southern segments of several ridges. Analysis of molecular variance (AMOVA) shows an estimated 10.4% total genetic variation within patches, 17.5% among patches (within ridges), and 72.1% among ridges. This strong population structure among patches within ridges indicates that the distribution of S. woodi is tightly linked to sandy scrub habitat and that the discontinuous distribution of scrub habitats significantly inhibits dispersal and gene flow. Phylogeographic analyses indicate a pattern of dispersal down the Florida peninsula during the late Pliocene-early Pleistocene, followed by habitat fragmentation and variant isolation events. Therefore, the deep genetic structuring among scrub lizard populations on separate ridges is attributed to ancient isolation events induced by a shift from dry (xeric) to wet (mesic) conditions on the Florida peninsula. These findings indicate that some scrub lizard populations have persisted in isolation for time frames in excess of 1 Myr, providing a case history on the genetic consequences of habitat fragmentation.     

Patterns of introgression across an expanding hybrid zone: analysing historical patterns of gene flow using nonequilibrium approaches

Previous studies suggest that the hybrid zone between two taxa in the Piriqueta caroliniana complex in central Florida (south-eastern North America) has recently expanded with hybrids replacing parental genotypes across a broad region of the Florida peninsula. Here I use patterns of genetic disequilibria and levels of differentiation among populations to infer historical patterns of introgression across this broad hybrid zone. There were strong positive associations among taxon-specific alleles at the southern extent of hybridization, but disequilibria values were close to zero across the central and northern portions of the hybrid zone. Levels of among-population differentiation in the central portion of the hybrid zone were relatively low, and increased towards the northern extent of hybridization. The high levels of disequilibria are coincident with the sharp clines at the southern end of the hybrid zone, suggesting that there is a tension zone in this region that is maintained by selection against hybrids and dispersal from parental regions. The levels of disequilibria within populations and patterns of differentiation among populations are consistent with historical introgression and northward expansion of this hybrid zone, which may have slowed or ceased in recent generations.     

SPECIES-WIDE POPULATION STRUCTURE IN A SOUTHEASTERN U.S. FRESHWATER FISH,HETERANDRIA FORMOSA: GENE FLOW AND BIOGEOGRAPHY

The phylogeography of the freshwater fish fauna of the southeastern United States has almost achieved paradigm status in evolutionary biology (Avise 1992), and the major geographic features responsible for shaping species distributions are well-characterized. Nevertheless, variation among species in distributions of allele or haplotype frequencies suggests that species-specific processes (e.g., migration) may also play a role in establishing those distributions. There has also been relatively little investigation into how population structure may differ among subregions in the Southeast, for example, on the Florida peninsula versus the U.S. mainland to the northwest and/or northeast. The geology of the peninsula is such that both physical and biotic fluctuations may have been (and still be) particularly important in establishing the population structure of freshwater taxa. This possibility leads to two interesting questions in population genetics. (1) Does gene flow in freshwater species of the region better approximate a one- or two-dimensional pattern? (2) Are populations on the peninsula farther from migration-genetic drift equilibrium than their counterparts on the mainland? These questions were addressed by examining the population strucuture of a livebearing fish, Heterandria formosa; several features of the biology of the species make it particularly likely that recent gene flow has been important in its evolution. I surveyed electrophoretic variation in 34 populations distributed throughout the species range. The phylogeographic patterns observed are in general concordance with those found in other species, although with some differences. A two-dimensional hypothesis of gene flow on the Florida peninsula better explains the data than does a one-dimensional one. There is no evidence that populations on the peninsula are farther from migration-drift equilibrium than those to the northwest. Populations in the northeast have lower genetic diversity than those to the south and west and show no isolation by distance; those results are consistent with a recent range expansion into the northeast, although smaller historical effective population sizes could also explain the pattern.     

A biogeographical profile of the sand cockroach Arenivaga floridensis and its bearing on origin hypotheses for Florida scrub biota

Florida scrub is a xeric ecosystem associated with the peninsula's sand ridges, whose intermittent Pliocene–Pleistocene isolation is considered key to scrub endemism. One scrub origin hypothesis posits endemics were sourced by the Pliocene dispersal of arid‐adapted taxa from southwestern North America; a second invokes Pleistocene migration within eastern North America. Only one study to date has explicitly tested these competing hypotheses, supporting an eastern origin for certain scrub angiosperms. For further perspective, we conducted a genetic analysis of an endemic arthropod, the Florida sand cockroach (Arenivaga floridensis), with two aims: (1) to reconstruct the peninsular colonization and residence history of A. floridensis and (2) determine whether its biogeographic profile favors either origin hypothesis. We sequenced the cox2 mitochondrial gene for 237 specimens (65 populations) as well as additional loci (cox1, nuclear H3) for a subset of Florida roaches and congeners. Using Network and Bayesian inference methods, we identified three major lineages whose genetic differentiation and phylogeographical structure correspond with late Pliocene peninsula insularization, indicating Arenivaga was present and broadly distributed in Florida at that time. Stem and crown divergence estimates (6.36 Ma; 2.78 Ma) between A. floridensis and western sister taxa span a period of extensive dispersal by western biota along an arid Gulf Coast corridor. These phylogeographical and phylogenetic results yield a biogeographic profile consistent with the western origin hypothesis. Moreover, age estimates for the roach's peninsular residence complement those of several other endemics, favoring a Pliocene (or earlier) inception of the scrub ecosystem. We argue that eastern versus western hypotheses are not mutually exclusive; rather, a composite history of colonization involving disparate biotas better explains the diverse endemism of Florida scrub.     

Intervarietal and intravarietal genetic structure in Douglas‐fir: nuclear SSRs bring novel insights into past population demographic processes,phylogeography, and intervarietal hybridization

Douglas‐fir (Pseudotsuga menziesii) is one of numerous wide‐range forest tree species represented by subspecies/varieties, which hybridize in contact zones. This study examined the genetic structure of this North American conifer and its two hybridizing varieties, coastal and Rocky Mountain, at intervarietal and intravarietal level. The genetic structure was subsequently associated with the Pleistocene refugial history, postglacial migration and intervarietal hybridization/introgression. Thirty‐eight populations from the USA and Canada were genotyped for 13 nuclear SSRs and analyzed with simulations and traditional population genetic structuring methods. Eight genetic clusters were identified. The coastal clusters embodied five refugial populations originating from five distinct refugia. Four coastal refugial populations, three from California and one from western Canada, diverged during the Pleistocene (56.9–40.1 ka). The three Rocky Mountain clusters reflected distinct refugial populations of three glacial refugia. For Canada, ice covered during the Last Glacial Maximum, we present the following three findings. (1) One refugial population of each variety was revealed in the north of the distribution range. Additional research including paleodata is required to support and determine whether both northern populations originated from cryptic refugia situated south or north of the ice‐covered area. (2) An interplay between intravarietal gene flow of different refugial populations and intervarietal gene flow by hybridization and introgression was identified. (3) The Canadian hybrid zone displayed predominantly introgressants of the Rocky Mountain into the coastal variety. This study provides new insights into the complex Quaternary dynamics of this conifer essential for understanding its evolution (outside and inside the native range), adaptation to future climates and for forest management.     

Genetic insights into the biogeography of the southeastern North American endemic, Ceratiola ericoides (Empetraceae)

The southeastern United States harbors an unusually large number of endemic plant taxa, which may reflect the refugial nature of the region during Pleistocene glacial maxima. Understanding the genetic diversity and structure of extant plant taxa can provide insights into the biogeographical processes that shaped them genetically. Here, we investigate the levels and partitioning of allozyme diversity in the southeastern North American endemic, Ceratiola ericoides, which displayed greater genetic variation and structure than other endemics. Central Florida populations represent a center of genetic diversity, whereas South Carolina and Georgia Fall Line sandhill populations have a subset of the Central Florida genetic diversity and may be relicts of a once continuous distribution. This much broader, continuous distribution throughout the southeastern United States occurred during glacial maxima when the scrub habitat, dominated by C. ericoides, expanded considerably owing to drier climatic conditions. Georgia Coastal Plain populations appear to have been independently founded more recently by propagules from Central Florida and the Fall Line sandhills because they have an even more limited subset of genetic diversity and greater genetic heterogeneity among populations. Since their establishment, coastal plain populations appear to have had little, if any, gene exchange among each other or with the relatively proximate Fall Line sandhill populations. These data underscore the importance of understanding the genetic composition and historical biogeography of species before intelligent management or restoration decisions can be made regarding their preservation.     

Gene flow versus selection pressure and ancestral differentiation in the composition of species: Analysis of populational variation ofJuniperus ashei buch. Using terpenoid data

Summary Twenty-four populations ofJuniperus ashei were sampled throughout the range of this taxon and the terpenoids of the foliage were analyzed by gas/liquid chromatography. Population differentiation was investigated by analysis of variance and numerical taxonomy. Three south Texas and one Mexican population clustered together with the rest of the taxon appearing fairly uniform. No evidence was found of hybridization or introgression with other taxa. Disjunct populations in Oklahoma and the Ozarks, which have been genetically isolated from the central population for thousands of years, showed no signs of differentiation nor genetic drift. The present pattern of distribution probably dates from the Pleistocene. The south Texas and Mexican populations appear to be the primitive elements of the species. Populational differences have apparently been maintained in adjacent populations in spite of seemingly large gene flow and conversely, chemical uniformity is being maintained in many disjunct populations where there is little or no gene flow.     

Multiple introgression events and range shifts in Schizocodon (Diapensiaceae) during the Pleistocene

Range shifts during the Pleistocene shaped the unique phylogeographical structures of numerous species. Accompanying species migration, sister taxa may have experienced multiple introgression events. Here, we report the signature of introgression events in multiple areas in Schizocodon, herbs endemic to Japan, using amplified fragment length polymorphism (AFLP) fingerprinting and plastid DNA haplotyping in 48 populations. Although the present distributions of S. soldanelloides and S. ilicifolius are mainly allopatric, the species share plastid DNA haplotypes in each region (north‐eastern, north‐central, south‐central and south‐western Japan); in contrast, the specific groups were highly supported by AFLP analyses. These results support the occurrence of multiple introgression events in Schizocodon. Notably, the disjunct plastid haplotypes found only in S. ilicifolius var. intercedens suggest complete plastid DNA replacement at local areas from S. soldanelloides into S. ilicifolius var. ilicifolius. Furthermore, we found that S. soldanelloides experienced range contraction and expansion during glacial and interglacial cycles based on mismatch distribution analysis and ecological niche modelling. Based on several pieces of evidence, our study supports the idea that historical range shifts associated with Pleistocene climatic oscillations favoured multiple and regional introgression events in Schizocodon. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 46–63.     

Perpendicular axes of differentiation generated by mitochondrial introgression

Differential introgression of mitochondrial vs. nuclear DNA generates discordant patterns of geographic variation and can promote population divergence and speciation. We examined a potential case of mitochondrial introgression leading to two perpendicular axes of differentiation. The Eastern Yellow Robin Eopsaltria australis, a widespread Australian bird, shows a deep mitochondrial split that is perpendicular to north–south nuclear DNA and plumage colour differentiation. We propose a scenario to explain this pattern: (i) first, both nuclear and mitochondrial genomes differentiated in concert during north–south population divergence; (ii) later, their histories disconnected after two mitochondrial introgression events resulting in a deep mitochondrial split perpendicular to the nuclear DNA structure. We explored this scenario by coalescent modelling of ten mitochondrial genes and 400 nuclear DNA loci. Initial mitochondrial and nuclear genome divergences were estimated to have occurred in the early Pleistocene, consistent with the proposed scenario. Subsequent climatic transitions may have driven later mitochondrial introgression. We consider neutral introgression unlikely and instead propose that the evidence is more consistent with adaptive mitochondrial introgression and selection against incompatible mitochondrial‐nuclear combinations. This likely generated an axis of coastal‐inland mitochondrial differentiation in the face of nuclear gene flow, perpendicular to the initial north–south axis of differentiation (reflected in genomewide nuclear DNA and colour variation).     

High levels of population subdivision in a morphologically conserved Mediterranean toad (Alytes cisternasii) result from recent,multiple refugia: evidence from mtDNA,microsatellites and nuclear genealogies

Pleistocene glaciations often resulted in differentiation of taxa in southern European peninsulas, producing the high levels of endemism characteristic of these regions (e.g. the Iberian Peninsula). Despite their small ranges, endemic species often exhibit high levels of intraspecific differentiation as a result of a complex evolutionary history dominated by successive cycles of fragmentation, expansion and subsequent admixture of populations. Most evidence so far has come from the study of species with an Atlantic distribution in northwestern Iberia, and taxa restricted to Mediterranean‐type habitats remain poorly studied. The Iberian Midwife toad (Alytes cisternasii) is a morphologically conserved species endemic to southwestern and central Iberia and a typical inhabitant of Mediterranean habitats. Applying highly variable genetic markers from both mitochondrial and nuclear genomes to samples collected across the species’ range, we found evidence of high population subdivision within A. cisternasii. Mitochondrial haplotypes and microsatellites show geographically concordant patterns of genetic diversity, suggesting population fragmentation into several refugia during Pleistocene glaciations followed by subsequent events of geographical and demographic expansions with secondary contact. In addition, the absence of variation at the nuclear β‐fibint7 and Ppp3caint4 gene fragments suggests that populations of A. cisternasii have been recurrently affected by episodes of extinction and recolonization, and that documented patterns of population subdivision are the outcome of recent and multiple refugia. We discuss the evolutionary history of the species with particular interest in the increasing relevance of Mediterranean refugia for the survival of genetically differentiated populations during the Pleistocene glaciations as revealed by studies in co‐distributed taxa.     

Genetic variation in the green anole lizard (Anolis carolinensis) reveals island refugia and a fragmented Florida during the quaternary

The green anole lizard (Anolis carolinensis) is a model organism for behavior and genomics that is native to the southeastern United States. It is currently thought that the ancestors of modern green anoles dispersed to peninsular Florida from Cuba. However, the climatic changes and geological features responsible for the early diversification of A. carolinensis in North America have remained largely unexplored. This is because previous studies (1) differ in their estimates of the divergence times of populations, (2) are based on a single genetic locus or (3) did not test specific hypotheses regarding the geologic and topographic history of Florida. Here we provide a multi-locus study of green anole genetic diversity and find that the Florida peninsula contains a larger number of genetically distinct populations that are more diverse than those on the continental mainland. As a test of the island refugia hypothesis in Pleistocene Florida, we use a coalescent approach to estimate the divergence times of modern green anole lineages. We find that all demographic events occurred during or after the Upper Pliocene and suggest that green anole diversification was driven by population divergence on interglacial island refugia in Florida during the Lower Pleistocene, while the region was often separated from continental North America. When Florida reconnected to the mainland, two separate dispersal events led to the expansion of green anole populations across the Atlantic Seaboard and Gulf Coastal Plain.     

Genetic structure of island populations of <Emphasis Type="Italic">Prunus lannesiana</Emphasis> var. <Emphasis Type="Italic">speciosa</Emphasis> revealed by chloroplast DNA,AFLP and nuclear SSR loci analyses

The wild flowering cherry Prunus lannesiana var. speciosa is highly geographically restricted, being confined to the Izu Islands and neighboring peninsulas in Japan. In an attempt to elucidate how populations of this species have established we investigated the genetic diversity and differentiation in seven populations (sampling 408 individuals in total), using three kinds of genetic markers: chloroplast DNA (cpDNA), amplified fragment length polymorphisms (AFLPs), and 11 nuclear SSR polymorphic loci. Eight haplotypes were identified based on the cpDNA sequence variations, 64 polymorphic fragments were scored for the AFLP markers, and a total of 154 alleles were detected at the 11 nuclear SSR loci. Analysis of molecular variance showed that among-population variation accounted for 16.55, 15.04 and 7.45% of the total detected variation at the cpDNA, AFLPs, and SSR loci, respectively. Thus, variation within populations accounted for most of the genetic variance for all types of markers, although the genetic differentiation among populations was also highly significant. For cpDNA variation, no clear structure was found among the populations, except that of the most distant island, although an “isolation by distance” pattern was found for each marker. Both neighbor-joining trees and structure analysis indicate that the genetic relationships between populations reflect geological variations between the peninsula and the islands and among the islands. Furthermore, hybridization with related species may have affected the genetic structure, and some genetic introgression is likely to have occurred.     

Stand structure and dynamics of sand pine differ between the Florida panhandle and peninsula

Size and age structures of stand populations of numerous tree species exhibit uneven or reverse J-distributions that can persist after non-catastrophic disturbance, especially windstorms. Among disjunct populations of conspecific trees, alternative distributions are also possible and may be attributed to more localized variation in disturbance. Regional differences in structure and demography among disjunct populations of sand pine (Pinus clausa (Chapm. ex Engelm.) Vasey ex Sarg.) in the Florida panhandle and peninsula may result from variation in hurricane regimes associated with each of these populations. We measured size, age, and growth rates of trees from panhandle and peninsula populations and then compiled size and age class distributions. We also characterized hurricanes in both regions over the past century. Size and age structures of panhandle populations were unevenly distributed and exhibited continuous recruitment; peninsula populations were evenly sized and aged and exhibited only periodic recruitment. Since hurricane regimes were similar between regions, historical fire regimes may have been responsible for regional differences in structure of sand pine populations. We hypothesize that fires were locally nonexistent in coastal panhandle populations, while periodic high intensity fires occurred in peninsula populations over the past century. Such differences in local fire regimes could have resulted in the absence of hurricane effects in the peninsula. Increased intensity of hurricanes in the panhandle and current fire suppression patterns in the peninsula may shift characteristics of sand pine stands in both regions.     

Phylogeography of the Temminck’s Stint (Calidris temminckii): historical vicariance but little present genetic structure in a regionally endangered Palearctic wader

Aim We study the population differentiation and phylogeography of the Temminck’s Stint (Calidris temminckii). Specifically, we seek signs of past and present population size changes and dispersal events and evaluate management and conservation unit status of the populations. We also study the possibility of introgression as the origin of two mitochondrial DNA (mtDNA) lineages found and estimate the divergence time of the lineages. Location Northern Eurasia. Methods We analysed 583 bp of mtDNA control region domains I and II and 11 microsatellite loci from 13 localities throughout the breeding range. In addition, we used mitochondrial cytochrome c oxidase subunit I (COI), a barcoding gene, to search for signs of introgression. Results More population differentiation was found from microsatellites than from mtDNA, although differentiation was weak in both markers. Signs of past population growth were observed, in addition to more recent decline in some areas. Both control region and COI sequences revealed two maternal lineages coexisting in Fennoscandia and in north‐west Siberia. No signs of introgression were detected. Lineage divergence time was estimated to have occurred during the glacial periods of Pleistocene. Main conclusions Slight differences in mtDNA and microsatellite differentiation and diversity may reflect different features – such as the mutation rate and effective population size – of the markers used, or female‐biased dispersal pattern and high male site‐fidelity of the species. The coexistence of the two mitochondrial lineages is most likely a consequence of post‐glacial mixing of two refugial Pleistocene populations. Based on genetic information alone, global conservation concerns are not imminent. However, fast decline of a marginal Bothnian Bay population and the smallness and remoteness of a Central Yakutian population warrant conservation actions.     

Population genomic variation reveals roles of history,adaptation and ploidy in switchgrass

Geographic patterns of genetic variation are shaped by multiple evolutionary processes, including genetic drift, migration and natural selection. Switchgrass (Panicum virgatum L.) has strong genetic and adaptive differentiation despite life history characteristics that promote high levels of gene flow and can homogenize intraspecific differences, such as wind‐pollination and self‐incompatibility. To better understand how historical and contemporary factors shape variation in switchgrass, we use genotyping‐by‐sequencing to characterize switchgrass from across its range at 98 042 SNPs. Population structuring reflects biogeographic and ploidy differences within and between switchgrass ecotypes and indicates that biogeographic history, ploidy incompatibilities and differential adaptation each have important roles in shaping ecotypic differentiation in switchgrass. At one extreme, we determine that two Panicum taxa are not separate species but are actually conspecific, ecologically divergent types of switchgrass adapted to the extreme conditions of coastal sand dune habitats. Conversely, we identify natural hybrids among lowland and upland ecotypes and visualize their genome‐wide patterns of admixture. Furthermore, we determine that genetic differentiation between primarily tetraploid and octoploid lineages is not caused solely by ploidy differences. Rather, genetic diversity in primarily octoploid lineages is consistent with a history of admixture. This suggests that polyploidy in switchgrass is promoted by admixture of diverged lineages, which may be important for maintaining genetic differentiation between switchgrass ecotypes where they are sympatric. These results provide new insights into the mechanisms shaping variation in widespread species and provide a foundation for dissecting the genetic basis of adaptation in switchgrass.     

Salicaceae detoxification abilities in Florida tiger swallowtail butterflies (Papilio glaucus maynardi Gauthier): Novel ability or Pleistocene holdover?

Abstract Florida populations of the eastern tiger swallowtail butterfly, Papilio glaucus L., have unique morphological features and ecological adaptations that have contributed to their subspecies status (P. g. maynardi Gauthier). We describe geographically unique abilities for detoxification of Carolina willow, Salix caroliniana Michx. (Salicaceae), for several Florida populations of P. g. maynardi. Of all the approximately 570 worldwide species of the Papilionidae, such Salicaceae detoxification abilities exist only in the allopatric North American western and northernmost species (P. rutulus Lucas, P. eurymedon Lucas and P. canadensis Rothschild & Jordan). Females of P. glaucus collected from populations in southeastern USA were examined for oviposition preference in 5‐choice assays, and displayed a low preference for Salicaceae (<5%), but larvae from Florida populations exhibited a high survival (>60%) on these plants. Detoxification abilities have previously shown to be autosomally inherited, and can be transferred via natural or hand‐paired interspecific hybrid introgression. However, these Florida populations are at least 700–1 500 km from the nearest hybrids or the hybrid species, P. appalachiensis Pavulaan & Wright, which possess these detoxification abilities. In any case, the Z ( = X)‐linked oviposition preferences for Salicaceae are lacking in these Florida populations, illustrating genetic independence of oviposition preference determination and larval survival/performance abilities. The origins of detoxification abilities are unlikely to be due to recent climate‐driven introgression, and may represent ancestral trait carry‐overs from interglacial refugium populations of the Pleistocene epoch.     

The impact of Pleistocene glaciation across the range of a widespread European coastal species

There is a growing consensus that much of the contemporary phylogeography of northern hemisphere coastal taxa reflects the impact of Pleistocene glaciation, when glaciers covered much of the coastline at higher latitudes and sea levels dropped by as much as 150 m. The genetic signature of postglacial recolonization has been detected in many marine species, but the effects of coastal glaciation are not ubiquitous, leading to suggestions that species may intrinsically differ in their ability to respond to the environmental change associated with glacial cycles. Such variation may indeed have a biological basis, but apparent differences in population structure among taxa may also stem from our heavy reliance on individual mitochondrial loci, which are strongly influenced by stochasticity during coalescence. We investigated the contemporary population genetics of Syngnathus typhle, one of the most widespread European coastal fish species, using a multilocus data set to investigate the influence of Pleistocene glaciation and reduced sea levels on its phylogeography. A strong signal of postglacial recolonization was detected at both the northern and eastern ends of the species’ distribution, while southern populations appear to have been relatively unaffected by the last glacial cycle. Patterns of population variation and differentiation at nuclear and mitochondrial loci differ significantly, but simulations indicate that these differences can be explained by the stochastic nature of the coalescent process. These results demonstrate the strength of a multilocus approach to phylogeography and suggest that an overdependence on mitochondrial loci may provide a misleading picture of population‐level processes.