Testing hypotheses of speciation in the Plethodon jordani species complex with allozymes and mitochondrial DNA sequences

Salamander populations of the Plethodon jordani species complex form a challenging system for applying the general lineage concept of species to diagnose population-level lineages. The present study reports and analyses mitochondrial-DNA haplotypes (∼1200 nucleotide bases from the genes encoding ND2, tRNA^Trp, and tRNA^Ala from 438 salamanders) from 100 populations representing six species of the P. jordani complex ( Plethodon amplus , Plethodon cheoah , Plethodon jordani , Plethodon meridianus , Plethodon metcalfi , and Plethodon montanus ) with comparative analyses of previously published allozymic data to reconstruct the evolutionary history of this group and to diagnose species lineages. Analyses of mitochondrial haplotypic data include nested-cladistic analysis of phylogeography, analysis of molecular variance, hierarchical analysis of nucleotide-diversity measures, and likelihood-based estimates of recent temporal changes in population size. New analyses of allozymic data include multidimensional scaling and principal component analyses, and both data sets are analysed and compared for congruent genetic structure using Mantel correlation tests. These analyses in combination identify the six named species as distinct evolutionary lineages despite sporadic genetic exchanges among them and some discordance between mitochondrial DNA and allozymic markers. Sexual isolation is not complete for any pair of these six species, but they replace each other geographically and appear to block the geographical spreading of their neighbours. The P. jordani complex is a strong study system for investigating the genetic and ecological processes responsible for vicariant speciation.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 25–51.     

Geographic variability in mitochondrial introgression among hybridizing populations of Golden-winged (<Emphasis Type="Italic">Vermivora chrysoptera</Emphasis>) and Blue-winged (<Emphasis Type="Italic">V.opinus</Emphasis>) Warblers

The rapidly declining Golden-winged Warbler (Vermivora chrysoptera) is of conservation concern owing in part to hybridization with the closely related Blue-winged Warbler (V. pinus). These species hybridize extensively in eastern North America and over the past century the Blue-winged Warbler has displaced the Golden-winged Warbler from substantial regions of its historic breeding range. A previous study suggested that these genetic interactions result in rapid and asymmetric introgression of Blue-winged Warbler mitochondrial DNA (mtDNA) into Golden-winged phenotype populations within the zones of contact, but more recent and extensive surveys have documented a more complex pattern of genetic interchange between these taxa. We surveyed mtDNA/phenotype associations in 104 individuals of known phenotype drawn from two locations with different histories of contact and found substantial variation between sites in the extent of introgression. Where both species have co-existed for more than a century, we found evidence of bi-directional introgression and the long-term persistence of Golden-winged mtDNA haplotypes. At the leading edge of the northward expansion of Blue-winged Warblers, we found predominantly Golden-winged Warbler mtDNA haplotypes in both Golden-winged and hybrid-phenotype individuals. Across both sites, genetic swamping does not appear to be occurring via the early immigration of Blue-winged Warbler females into populations dominated by Golden-winged Warbler phenotypes. Instead, the differing patterns of mitochondrial introgression may be driven by the relative local population sizes of the parental species coupled with subtle between-species differences in mate choice and habitat preferences.     

Introgression of mitochondrial DNA among lineages in a hybridogenetic ant

We report a remarkable pattern of incongruence between nuclear and mitochondrial variations in a social insect, the desert ant Cataglyphis hispanica. This species reproduces by social hybridogenesis. In all populations, two distinct genetic lineages coexist; non-reproductive workers develop from hybrid crosses between the lineages, whereas reproductive offspring (males and new queens) are typically produced asexually by parthenogenesis. Genetic analyses based on nuclear markers revealed that the two lineages remain highly differentiated despite constant hybridization for worker production. Here, we show that, in contrast with nuclear DNA, mitochondrial DNA (mtDNA) does not recover the two lineages as monophyletic. Rather, mitochondrial haplotypes cluster according to their geographical origin. We argue that this cytonuclear incongruence stems from introgression of mtDNA among lineages, and review the mechanisms likely to explain this pattern under social hybridogenesis.     

THE EXTENT OF INTROGRESSION OUTSIDE THE CONTACT ZONE BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (TELEOSTEI: CYPRINIDAE)

The cyprinid fishes, Notropis cornutus and N. chrysocephalus, hybridize in a long, narrow zone in the midwestern United States. To quantify the extent of introgression of genetic markers outside of this zone, samples were collected along transects starting near the region of contact (as defined by morphological characters), followed by samples progressively more distant. Diagnostic allozymic and mitochondrial DNA (mtDNA) restriction site markers were used to estimate the extent of introgression outside of the zone, while polymorphic allozyme and mtDNA markers were used to evaluate the potential for gene flow among populations within transects. Analysis of populations from the northern transect provided evidence for differentiation of populations for some of the markers; however, on average, enough gene flow has occurred to overcome substantial differentiation. Introgressed mtDNA and allozyme haplotypes were rare and found only in the population closest to the contact zone. The rarity of introgressed alleles in the more northern populations is consistent with the recent origin of these populations after the Wisconsin glaciation (less than 12,000 years bp) and/or selection maintaining the northern boundary of the contact zone. Analysis of populations from the southern transect revealed evidence for population subdivision but no evidence for introgression at the diagnostic allozyme loci; however, nearly all individuals from this transect possessed introgressed mtDNA haplotypes, with samples furthest from the contact zone exhibiting the highest frequencies of introgression. Patterns of variation for one of the polymorphic allozyme markers (Est-A) and introgressed mtDNAs were highly correlated, suggesting that allozymic heterogeneity at this locus is also the result of introgression. The most likely explanation for these data is that these introgressed haplotypes are indicators of a more southern position of the contact zone during the Pleistocene, with the contact zone shifting northward with the recession of the glacial front. Such movement implicates selection in the maintenance of distributional limits of these species, and hence, the width and position of the contact zone.     

ASYMMETRIC INTROGRESSION OF MITOCHONDRIAL DNA AMONG EUROPEAN POPULATIONS OF BLUE MUSSELS (MYTILUS SPP.)

Abstract.—Mytilus edulis and M. galloprovincialis are two blue mussel species that coexist in western Europe. Previously, we reported that M. galloprovincialis populations contain female and male haplotypes that are fixed in M. edulis populations as well as unique haplotypes. This study assesses whether paraphyly for these species is due to introgression or incomplete lineage extinction. The lineage extinction hypothesis predicts that the shared mtDNA haplotypes in M. galloprovincialis will be significantly diverged from those in M. edulis and form distinct sequence clades. In contrast, the introgression hypothesis proposes that M. edulis haplotypes have only recently been introduced into M. galloprovincialis through hybridization with relatively little divergence accumulating between the shared RFLP haplotypes. We examined 80 mtl6S gene sequences for both the maternal and paternal mtDNA lineages from mussels sampled from various European populations and found strong support for the introgression hypothesis. In addition, we found that M. edulis mtDNA haplotypes appear to be introgressing into mussel populations in the Baltic Sea, which have predominantly M. trossulus nuclear genotypes, indicating that introgressive hybridization is prevalent among European mussel populations.     

Rapid genetic assimilation of native wall lizard populations (Podarcis muralis) through extensive hybridization with introduced lineages

The Common Wall Lizard (Podarcis muralis) has established more than 150 non-native populations in Central Europe, stemming from eight geographically distinct evolutionary lineages. While the majority of these introduced populations are found outside the native range, some of these populations also exist at the northern range margin in southwestern Germany. To (i) infer the level of hybridization in contact zones of alien and native lineages; and (ii) compare the genetic diversity among purebred introduced, native and hybrid populations, we used a combination of maternally inherited markers (mtDNA: cytb) and Mendelian markers (microsatellites). Our results suggest a rapid genetic assimilation of native populations by strong introgression from introduced lineages. Discordant patterns of mtDNA and nDNA variation within hybrid populations may be explained by directed mate choice of females towards males of alien lineages. In contrast to previous studies, we found a nonlinear relationship between genetic diversity and admixture level. The genetic diversity of hybrid populations was substantially higher than in introduced and native populations belonging to a single lineage, but rapidly reaching a plateau of high genetic diversity at an admixture level of two. However, even introduced populations with low founder sizes and from one source population retained moderate levels of genetic diversity and no evidence for a genetic bottleneck was found. The extent of introgression and the dominance of alien haplotypes in mixed populations indicate that introductions of non-native lineages represent a serious threat to the genetic integrity of native populations due to the rapid creation of hybrid swarms.     

Contrasting patterns of mitochondrial DNA and microsatellite introgressive hybridization between lineages of lake whitefish (Coregonus clupeaformis); relevance for speciation

We performed a combined analysis of mitochondrial DNA (mtDNA) and microsatellite loci among lake whitefish (Coregonus clupeaformis) populations in order to assess the levels of congruence between both types of markers in defining patterns of genetic structuring, introgressive hybridization and inferring population origins in the hybrid zone of the St. John River basin. A second objective was to test the hypothesis that secondary contact between glacial lineages always resulted in the occurrence of sympatric dwarf and normal whitefish ecotypes. Fish were sampled from 35 populations and polymorphism was screened at mtDNA and six microsatellite loci for a total of 688 and 763 whitefish, respectively. Four lakes harbouring a single whitefish population of normal ecotype admixed with mtDNA haplotypes of different lineages were found. This confirmed that secondary contact between whitefish evolutionary lineages did not always result in the persistence of reproductively isolated ecotypes. Microsatellites further supported the definition of distinct glacial lineages by identifying lineage-specific allelic size groups. They also further supported the hypothesis that ecotypes originated from either a single founding lineage (sympatric divergence) or following secondary contacts between lineages (allopatric divergence), depending on the lake. In general, however, the pattern of population differentiation and introgressive hybridization observed at microsatellites was in sharp contrast with that depicted by mtDNA variation. Both factorial correspondence analysis and analysis of admixture proportion revealed a much more pronounced pattern of introgressive hybridization than depicted by mtDNA analyses. Variable levels of introgression indicated that environmental differences may be as important as the historical contingency of secondary contact in explaining the persistence of sympatric ecotypes and the differential pattern of introgressive hybridization among lakes. Whitefish populations from the St. John River basin hybrid zone represent a rare illustration of a continuum of both morphological and genetic differentiation within a given taxon, spanning from complete introgression to possibly complete reproductive isolation, depending on lakes. Thus, each lake may be viewed as a different temporal snapshot taken throughout the gradual process of speciation.     

REPRODUCTIVE ISOLATION AND INTROGRESSION BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (FAMILY CYPRINIDAE): COMPARISON OF MORPHOLOGY,ALLOZYMES, AND MITOCHONDRIAL DNA

Hybrid zones in fluvial fishes may be heterogeneous from drainage to drainage. The comparison of data from morphology, allozymes, and mitochondrial DNA (mtDNA) indicates variability in the causes and degree of restriction of gene flow between Notropis cornutus and Notropis chrysocephalus. Allozyme marker loci show frequency-dependent introgression; i.e., the rarer species, whichever it is at a particular locality, tends to exhibit a higher proportion of introgressed alleles. Unlike allozymes, introgression of mtDNA haplotypes varies geographically. In westward-flowing Michigan drainages, N. cornutus mtDNA haplotypes are more common in F₁ hybrids and backcrosses, independent of parental frequencies. In eastward-flowing Michigan drainages, N. chrysocephalus mtDNA is more common in F₁ hybrids and backcrosses; this pattern may be due to local ecological effects or frequency-dependent introgression. Morphological data alone are not sufficient to distinguish all classes of hybrids. The lack of concordance of morphological, allozymic, and mtDNA introgression patterns implies operation of one or two factors: 1) geographically variable patterns of selection against different hybrid and backcross combinations or 2) genetic differences between Michigan populations inhabiting eastward- and westward-flowing drainage systems accumulated during historical isolation.     

From Wolves to Dogs,and Back: Genetic Composition of the Czechoslovakian Wolfdog

The Czechoslovakian Wolfdog is a unique dog breed that originated from hybridization between German Shepherds and wild Carpathian wolves in the 1950s as a military experiment. This breed was used for guarding the Czechoslovakian borders during the cold war and is currently kept by civilian breeders all round the world. The aim of our study was to characterize, for the first time, the genetic composition of this breed in relation to its known source populations. We sequenced the hypervariable part of the mtDNA control region and genotyped the Amelogenin gene, four sex-linked microsatellites and 39 autosomal microsatellites in 79 Czechoslovakian Wolfdogs, 20 German Shepherds and 28 Carpathian wolves. We performed a range of population genetic analyses based on both empirical and simulated data. Only two mtDNA and two Y-linked haplotypes were found in Czechoslovakian Wolfdogs. Both mtDNA haplotypes were of domestic origin, while only one of the Y-haplotypes was shared with German Shepherds and the other was unique to Czechoslovakian Wolfdogs. The observed inbreeding coefficient was low despite the small effective population size of the breed, possibly due to heterozygote advantages determined by introgression of wolf alleles. Moreover, Czechoslovakian Wolfdog genotypes were distinct from both parental populations, indicating the role of founder effect, drift and/or genetic hitchhiking. The results revealed the peculiar genetic composition of the Czechoslovakian Wolfdog, showing a limited introgression of wolf alleles within a higher proportion of the dog genome, consistent with the reiterated backcrossing used in the pedigree. Artificial selection aiming to keep wolf-like phenotypes but dog-like behavior resulted in a distinctive genetic composition of Czechoslovakian Wolfdogs, which provides a unique example to study the interactions between dog and wolf genomes.     

Historic hybridization and introgression between two iconic Australian anemonefish and contemporary patterns of population connectivity

Endemic species on islands are considered at risk of extinction for several reasons, including limited dispersal abilities, small population sizes, and low genetic diversity. We used mitochondrial DNA (D-Loop) and 17 microsatellite loci to explore the evolutionary relationship between an endemic anemonefish, Amphiprion mccullochi (restricted to isolated locations in subtropical eastern Australia) and its more widespread sister species, A. akindynos. A mitochondrial DNA (mtDNA) phylogram showed reciprocal monophyly was lacking for the two species, with two supported groups, each containing representatives of both species, but no shared haplotypes and up to 12 species, but not location-specific management units (MUs). Population genetic analyses suggested evolutionary connectivity among samples of each species (mtDNA), while ecological connectivity was only evident among populations of the endemic, A. mccullochi. This suggests higher dispersal between endemic anemonefish populations at both evolutionary and ecological timeframes, despite separation by hundreds of kilometers. The complex mtDNA structure results from historical hybridization and introgression in the evolutionary past of these species, validated by msat analyses (NEWHYBRIDS, STRUCTURE, and DAPC). Both species had high genetic diversities (mtDNA h > 0.90, π = 4.0%; msat genetic diversity, gd > 0.670). While high gd and connectivity reduce extinction risk, identifying and protecting populations implicated in generating reticulate structure among these species should be a conservation priority.     

Introgression and selection shaped the evolutionary history of sympatric sister‐species of coral reef fishes (genus: Haemulon)

Closely related marine species with large overlapping ranges provide opportunities to study mechanisms of speciation, particularly when there is evidence of gene flow between such lineages. Here, we focus on a case of hybridization between the sympatric sister‐species Haemulon maculicauda and H. flaviguttatum, using Sanger sequencing of mitochondrial and nuclear loci, as well as 2422 single nucleotide polymorphisms (SNPs) obtained via restriction site‐associated DNA sequencing (RADSeq). Mitochondrial markers revealed a shared haplotype for COI and low divergence for CytB and CR between the sister‐species. On the other hand, complete lineage sorting was observed at the nuclear loci and most of the SNPs. Under neutral expectations, the smaller effective population size of mtDNA should lead to fixation of mutations faster than nDNA. Thus, these results suggest that hybridization in the recent past (0.174–0.263 Ma) led to introgression of the mtDNA, with little effect on the nuclear genome. Analyses of the SNP data revealed 28 loci potentially under divergent selection between the two species. The combination of mtDNA introgression and limited nuclear DNA introgression provides a mechanism for the evolution of independent lineages despite recurrent hybridization events. This study adds to the growing body of research that exemplifies how genetic divergence can be maintained in the presence of gene flow between closely related species.     

Notes on a discrepancy in mitochondrial DNA and allozyme differentiation in a pond frog Rana nigromaculata

Analyses of complete 1143-base pair sequence of the mitochondrial cytochrome b gene demonstrated a sister relationship between Japanese R. nigromaculata and Korean R. plancyichosenica, but not with Korean R. nigromaculata, while the allozyme data strongly supported the monophyly of the Korean and Japanese populations of R. nigromaculata. We surmise this discordance to be the result of the inheritance of introduced mtDNA and the dilution of introduced nuclear DNA in mixed lineages after past hybridization and genome introgression between the two species, although the direction of introgression is unknown.     

Y‐chromosome evidence supports widespread signatures of three‐species Canis hybridization in eastern North America

There has been considerable discussion on the origin of the red wolf and eastern wolf and their evolution independent of the gray wolf. We analyzed mitochondrial DNA (mtDNA) and a Y‐chromosome intron sequence in combination with Y‐chromosome microsatellites from wolves and coyotes within the range of extensive wolf–coyote hybridization, that is, eastern North America. The detection of divergent Y‐chromosome haplotypes in the historic range of the eastern wolf is concordant with earlier mtDNA findings, and the absence of these haplotypes in western coyotes supports the existence of the North American evolved eastern wolf (Canis lycaon). Having haplotypes observed exclusively in eastern North America as a result of insufficient sampling in the historic range of the coyote or that these lineages subsequently went extinct in western geographies is unlikely given that eastern‐specific mtDNA and Y‐chromosome haplotypes represent lineages divergent from those observed in extant western coyotes. By combining Y‐chromosome and mtDNA distributional patterns, we identified hybrid genomes of eastern wolf, coyote, gray wolf, and potentially dog origin in Canis populations of central and eastern North America. The natural contemporary eastern Canis populations represent an important example of widespread introgression resulting in hybrid genomes across the original C. lycaon range that appears to be facilitated by the eastern wolf acting as a conduit for hybridization. Applying conventional taxonomic nomenclature and species‐based conservation initiatives, particularly in human‐modified landscapes, may be counterproductive to the effective management of these hybrids and fails to consider their evolutionary potential.     

Frequent cytoplasmic exchanges between oak species that are not closely related: Quercus suber and Q. ilex in Morocco

Chloroplast (cp) and mitochondrial (mt) DNA variation were studied in 97 populations of cork oak (Quercus suber) in Morocco; in 31 of these populations, holm oak (Quercus ilex), a clearly distinct species, also occurred and was compared with Q. suber. Three cpDNA and one mtDNA primer pairs were used in the survey, each in combination with one restriction enzyme. Six haplotypes belonging to two very divergent lineages were detected; one lineage predominates in each species, and is probably ancestral, as inferred from comparisons with other oak species. In the mixed-species populations, cytoplasmic genomes were frequently shared across species, as indicated by an introgression ratio of 0.63. This index is a new measure of the propensity of species to share locally genetic markers, varying from zero (complete differentiation) to one (no differentiation). By contrast, more closely related deciduous oak species (Q. robur, Q. petraea and Q. pubescens) have introgression ratios varying from 0.82 to 0.97. The introgression events appear to have been more frequent in the direction Q. ilex (female) x Q. suber (male), a finding which seems attributable to the flowering phenology of these two species. This asymmetry may have favoured immigration of Q. suber beyond its main range, in regions already colonized by Q. ilex. There, rare hybridization and further introgression through long distance pollen flow have established populations that are morphologically indistinguishable from Q. suber but that have cytoplasmic genomes originating from the local Q. ilex populations.     

Nuclear loci and coalescent methods support ancient hybridization as cause of mitochondrial paraphyly between gadwall and falcated duck (Anas spp.)

Many species have mitochondrial DNA lineages that are phylogenetically intermixed with other species, but studies have rarely tested the cause of such paraphyly. In this study, we tested two hypotheses that could explain mitochondrial paraphyly of Holarctic gadwalls (Anas strepera) with respect to Asian falcated ducks (A. falcata). First, hybridization could have resulted in falcated duck mitochondrial DNA (mtDNA) introgressing into the gadwall gene pool. Second, gadwalls and falcated ducks could have diverged so recently that mtDNA lineages have not sorted to reciprocal monophyly. We used coalescent analyses of three independent loci to distinguish between these two hypotheses. Two lines of evidence support introgression. First, analyses of the three loci combined show that some introgression is necessary to explain current genetic diversity in gadwalls. Second, we generated alternative predictions regarding time since divergence estimated from mtDNA: falcated ducks and gadwalls would have diverged between 65,000 and 700,000 years before present (ybp) under the introgression hypothesis and between 11,000 and 76,000 ybp under the incomplete lineage sorting hypothesis. The two independent nuclear introns indicated that these species diverged between 210,000 and 5,200,000 ybp, which did not overlap the predicted time for incomplete lineage sorting. These analyses also suggested that ancient introgression ( approximately 14,000 ybp) has resulted in the widespread distribution and high frequency of falcated-like mtDNA (5.5% of haplotypes) in North America. This is the first study to use a rigorous quantitative framework to reject incomplete lineage sorting as the cause of mitochondrial paraphyly.     

Rapid diversification, incomplete isolation, and the "speciation clock" in North American salamanders (Genus Plethodon): testing the hybrid swarm hypothesis of rapid radiation

The history of life has been marked by several spectacular radiations, in which many lineages arise over a short period of time. A possible consequence of such rapid splitting in the recent past is that the intrinsic barriers that prevent gene flow between many species may have too little time to develop fully, leading to extensive hybridization among recently evolved lineages. The salamander genus Plethodon in eastern North America has been proposed as a possible example of this scenario, but without explicit statistical tests. In this paper, we present a nearly comprehensive phylogeny for the 45 extant species of eastern Plethodon, based on DNA sequences of mitochondrial (two genes, 1335 base pairs) and nuclear genes (two genes, up to 3481 base pairs). We then use this phylogeny to examine rates and patterns of diversification and hybridization. We find significantly rapid diversification within the glutinosus species group. Examining patterns of natural hybridization in light of the phylogeny shows considerable hybridization within this clade, including introgression between species that are morphologically distinct and distantly related. Reproductive isolation increases over time and may be very weak among the most recently diverged species. These results suggest that the origin of species and the evolution of intrinsic reproductive isolating mechanisms, rather than being synonymous, may be decoupled in some cases (i.e., rapid origin of lineages outstrips the "speciation clock"). In contrast to the conclusions of a recent review of adaptive radiation and hybridization, we suggest that extensive hybridization sometimes may be a consequence, rather than a cause, of rapid diversification.     

Genetic analysis of sable (<Emphasis Type="Italic">Martes zibellina</Emphasis>) and pine marten (<Emphasis Type="Italic">M. martes</Emphasis>) populations in sympatric part of distribution area in the northern Urals

Analysis of nucleotide sequences of the mitochondrial DNA (mtDNA) control region (495 bp) of sables (Martes zibellina) and pine martens (M. martes) from allopatric parts of the species ranges has shown a considerable interspecific genetic distance (>3%). In sympatric populations of these species in the northern Urals, differences between two species-specific mtDNA lineages are still large; however, classification of each individual nucleotide sequence with one of the two lineages is not correlated with whether the given animal is phenotypically a sable, a pine marten, or a potential hybrid (the so-called “kidas”). This indicates a high degree of reciprocal introgression of the sable and pine marten mtDNA in the northern Urals and suggests that their interspecific hybridization is common in the sympatric zone.     

Phylogeography of a parasitoid wasp (Diaeretiella rapae): no evidence of host-associated lineages

The exceptional diversity of insects is often attributed to the effects of specialized relationships between insects and their hosts. Parasite-host interactions are influenced by current natural selection and dispersal, in addition to historical effects that may include past selection, vicariance, and random genetic drift. Both current and historical events can lead to reduced fitness on some hosts. If trade-offs in fitness on alternate hosts are common, adaptation to one host can prevent adaptation to another, giving rise to genetic differentiation among host-associated lineages. Previous studies of Diaeretiella rapae (Hymenoptera: Aphidiidae), a parasitoid of aphids, have revealed additive genetic differences in performance between populations that parasitize different aphid host species. To determine whether D. rapae populations collected from different aphid hosts have diverged into genetically independent lineages, we constructed a haplotype network based on sequence variation in mitochondrial DNA (mtDNA). We used single strand conformation polymorphism (SSCP) analysis to examine 2041 base pairs of mtDNA and to identify nucleotide sequences of 42 unique SSCP haplotypes. We found no association between mtDNA haplotypes and host species in either the ancestral range (Europe, Mediterranean region, Middle East, Asia) or part of the introduced range (western North America). Haplotypes likely to be ancestral were geographically widespread and found on both hosts, suggesting that the ability to use both hosts evolved prior to the diversification of the mtDNA. Ongoing gene flow appears to prevent the formation of host races.     

Introgressive hybridization between Dolly Varden Salvelinus malma and white‐spotted charr Salvelinus leucomaenis on Hokkaido Island, Japan

Intra‐ and interspecific phylogenetic analysis of Dolly Varden Salvelinus malma and white‐spotted charr Salvelinus leucomaenis throughout Hokkaido Island was conducted using nucleotide sequences of the mitochondrial DNA control region and three nuclear DNA markers [ribosomal DNA (ITS‐1), microsatellite ( u ‐85), SINE ( Fok ‐223)]. A total of 19 mtDNA haplotypes from 271 individuals of Dolly Varden, nine haplotypes from 134 white‐spotted charr were defined, and identified two well‐supported monophyletic clades for each species. Two haplotypes of Dolly Varden, however, were clustered together in the white‐spotted charr clade. Those haplotypes were found only from several rivers in the Shiretoko Peninsula, and a neighbouring river of that region. Analyses of ribosomal DNA and Fok ‐223 loci revealed that both species are characterized by having completely species‐specific diagnostic sequence and fragment patterns. Dolly Varden of the Shiretoko populations are typically allopatric in distribution where white‐spotted charr do not currently occur. In addition, incongruence in genetic relationships between mtDNA and nuclear DNA markers give strong evidence of historical mtDNA introgression between Dolly Varden and white‐spotted charr. Some white‐spotted charr diagnostic alleles in a nuclear microsatellite locus ( u ‐85) were found in some Shiretoko Dolly Varden populations, suggesting that introgressive hybridization might have also occurred in relatively recent contact and potentially ongoing evolutionary event. The present study presents an example of historical hybridization and introgression at the southernmost distribution limits of Dolly Varden.     

Competition at the range boundary in the slimy salamander: using reciprocal transplants for studies on the role of biotic interactions in spatial distributions

1. Determining the factors that influence the distribution of species has been a longstanding goal in the field of ecology. New techniques such as ecological niche modelling have the potential to aid in addressing many broad questions in ecology, evolutionary biology, and behavioural ecology. 2. This study combines broad-scale ecological niche models with fine-scaled studies of biotic interactions to examine how abiotic and biotic interactions affect the spatial distribution of the terrestrial salamander species Plethodon glutinosus (northern slimy salamander), in a potential contact zone shared with Plethodon mississippi (Mississippi slimy salamander). 3. The core habitat in the interior portion of the range of P. glutinosus and the contact zone are distributed in unique environmental niche space. 4. The form of competition, inter- or intraspecific, significantly affected mass loss of adult salamanders. Salamanders lost more mass when interacting with a heterospecific. 5. Abiotic conditions strongly influenced the impact of competition on salamanders. Under stressful environmental conditions at the field site located in the contact zone, salamanders lost more mass than at the field site located in the interior of the range. 6. Furthermore, adult salamanders from range-edge populations and core populations (from the interior of the range) differed in their respective abilities to compete under the abiotic conditions in the contact zone.