Population differentiation in plethodontid salamanders: divergence of allozymes and sexual compatibility among populations of Desmognathus imitator and D. ochrophaeus (Gaudata: Plethodontidae)

Plethodontid salamanders of the genus Desmognathus exhibit varying levels of genetic differentiation among and within both allopatric and sympatric taxa. This provides excellent opportunities to study population differentiation and speciation. Two morphologically similar species in this genus, D. imitator and D. ochrophaeus, are genetically well-differentiated from one another and occur in sympatry with no evidence of hybridization and introgression. We report that the degree of sexual isolation between these two species is very high, regardless of whether the populations under comparison are allopatric or sympatric with one another. Neither reinforcement nor reproductive character displacement are required to explain the evolution of sexual incompatibility in sympatry. Sexual behaviour apparently diverges while populations are allopatric with one another. Preliminary study indicates that D. imitator consists of populations among which there may be significant sexual isolation in the absence of detectable genetic differentiation.     

Population and Species Divergence of Chemical Cues that Influence Male Recognition of Females in Desmognathine Salamanders

Growing evidence indicates that males may be more discriminating of mating partners than often has been assumed. In the North American Ocoee dusky salamander, Desmognathus ocoee (Plethodontidae: Desmognathinae), sexual incompatibility among conspecific populations is high in encounters staged in the laboratory, at least in part because males fail to recognize ‘other’ females as appropriate targets for courtship. I used Y-mazes to test the hypothesis that males of D. ocoee discriminate between substrate-borne chemical cues produced by ‘own’ (homotypic) and ‘other’ (heterotypic) females. Males of four populations discriminated in favor of substrates soiled by homotypic females over clean (control) substrates (expt 1), suggesting that females produce chemical cues of sociosexual significance to males. Furthermore, males from these populations discriminated in favor of substrates soiled by homotypic females vs. substrates soiled by heterotypic females (expt 2), both conspecific and heterospecific (D. carolinensis and D. orestes). Thus, differences among populations and species in female chemical cues appear to affect the chemotactic responses of males. I suggest that, together with differences in behavioral signals and responses exhibited during courtship, differences in female chemical cues likely contribute to sexual incompatibility among populations and taxa of desmognathine salamanders.     

Falling apart and merging: diversification of slender salamanders (Plethodontidae: Batrachoseps) in the American West

The plethodontid genus Batrachoseps , the slender salamanders, is the most diverse clade of salamanders in western North America, but it has posed taxonomic difficulties because it contains many morphologically cryptic species. A segment of the mitochondrial DNA gene cytochrome b was studied for 278 individuals densely sampled from throughout the range of all 18 described species and several undescribed species. Phylogenetic analyses of the mtDNA data identify six major clades, one corresponding to the subgenus Plethopsis and five within a monophyletic subgenus Batrachoseps. All major clades and most species within these clades display strong phylogeographic structuring. Comparisons of mtDNA and allozyme data show that several allozymically cohesive groups are not monophyletic with respect to mtDNA. We suggest that this phenomenon results from fragmentation of populations, divergence in allopatry, and then recontact and gradual merging of units caused predominantly by male-mediated gene flow. The mtDNA offers evidence that populations were once more isolated than they are now, while the patterns of allozyme variation reflect recent and current interactions among populations. The complex patterns of morphological, allozymic and mtDNA variation associated with the constantly changing geological landscape give insight into the nature of processes responsible for species formation in Batrachoseps .  © 2002 The Linnean Society of London. Biological Journal of the Linnean Society , 2002, 76 , 361–391.     

Hybrid queen butterflies from the cross Danaus chrysippus (L.) × D. gilippus (Cramer):confirmation of species status for the parents and further support for Haldane's Rule

A cross between queen butterflies of the Palaeotropical species Danaus chrysippus and the Neotropical D. gilippus was achieved with difficulty in both directions. Only one progeny ( N  = 70) was reared comprising sterile males and inviable females in a precisely 1 : 1 ratio. Both prezygotic and postzygotic barriers to gene flow are strong. The result supports Haldane's Rule, to which we propose a minor amendment. The F₁ hybrids were intermediate for background colour between the brown (genotype BB ) of gilippus and orange (genotype bb ) of chrysippus . Most F₁ pattern characters were also intermediate. In polymorphic chrysippus populations, because Bb heterozygotes are brown, or nearly so, we suggest the B allele may have evolved towards dominance in sympatry. Hybrid males show positive heterosis for body size. The close similarity of male genitalia between the allopatric, genetically distant species chrysippus  and gilippus , compared to their divergence between gilippus and its largely sympatric sister species eresimus , suggest that reinforcement of sexual isolation or reproductive character displacement have evolved in sympatry.  © 2002 The Linnean Societyof London, Biological Journal of the Linnean Society , 2002, 76 , 535–544.     

Seasonality, microhabitat and cryptic variation in tropical salamander reproductive cycles

In many salamanders, most aspects of reproduction coincide with seasonal changes in abiotic factors such as temperature and humidity. However, while these reproductive patterns have been well documented for temperate salamanders, detailed data for tropical species are relatively sparse. I used histological techniques to examine temporal variation in the spermatogenetic cycles of four species of Guatemalan bolitoglossine salamanders ( Bolitoglossa occidentalis , B. rostrata , Dendrotriton bromeliacia and Pseudoeurycea goebeli ) from different microhabitats along an elevational gradient. All four species have mature sperm present in the testis throughout the year, irrespective of patterns of courtship and egg-laying. Additionally, the two species from more seasonal habitats ( B. occidentalis and B. rostrata ) exhibit significant but cryptic levels of variation in the amount of spermatozoa present in the testis not detectable by external appearance or the presence of secondary sexual characteristics. For the two species from less seasonal cloud forests ( D. bromeliacia and P. goebeli ), there were no detectable patterns to variation within the testis. Regional variation in climate undoubtedly influences reproductive cycles, however, microhabitat and the immediate environment are also important determinants of reproductive strategy.  © 2003 The Linnean Society of London . Biological Journal of the Linnean Society , 2003, 78 , 489–496.     

Terrestrial versus aquatic phenotypes correlate with hydrological predictability of habitats in a semiterrestrial salamander (Urodela, Plethodontidae)

Terrestrial adaptations by small, desmognathine salamanders in Appalachian stream communities have been viewed as evolved responses to large, aquatic, predatory congeners. We tested the role of moisture in the development of adaptive phenotypes in the small, semiterrestrial Desmognathus ocoee . We measured indicators of relative terrestrial vs. aquatic phenotypes (i.e. tolerance of desiccation, anatomical depression of body/tail) using individuals from stream habitats having different flow regimes within each of three geographical locations. Salamanders from intermittent, first-order streams exhibited greater desiccation tolerance, took longer to reach tolerance limits, and had more anatomically depressed bodies/tails than those from second-order, perennial streams, regardless of location. Phenotypes of salamanders from second-order streams showed aquatic tendencies. Phenotypic variation was consistent with relative habitat stability and not with relative abundance of large congeners. MtDNA analysis revealed that individuals grouped together based on geographical location and not habitat type, implying that the observed phenotypic differences arose in situ at each geographical location. Recent phylogenetic reconstructions of the Desmognathus indicate that large, aquatic forms are derived, and their appearance corresponds with the late Cenozoic development of modern Appalachian topography. We propose that radiation of desmognathine lifestyles and associated phenotypes tracked the formation of moisture-stability gradients associated with the creation of cool, wet, montane habitats.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 227–238.     

Divergence of mate recognition systems among conspecific populations of the plethodontid salamander Desmognathus santeetlah (Amphibia: Plethodontidae)

The disjunct geographic distribution of plethodontid salamanders of the genus Desmognathus provides excellent opportunities to test whether populations diverge in mate recognition systems so as to result in sexual incompatibility. Previous work has shown that sexual incompatibility may exist both among conspecific populations and between different species in this genus. Populations of the Santeetlah dusky salamander, Desmognathus santeetlah , are found at higher elevations in three mountain ranges in the southern Appalachian Mountains of eastern North America, where they are sympatric with congeneric species of similar body size. We report that statistically significant levels of sexual incompatibility exist among populations of D. santeetlah collected from across the species' range. Contrary to popular belief, but in accord with a growing body of empirical literature, aspects of mate recognition systems can diverge among conspecific populations of salamanders, resulting in incidental sexual incompatibility.     

Rapid sexual and genomic isolation in sympatric Drosophila without reproductive character displacement

The rapid evolution of sexual isolation in sympatry has long been associated with reinforcement (i.e., selection to avoid maladaptive hybridization). However, there are many species pairs in sympatry that have evolved rapid sexual isolation without known costs to hybridization. A major unresolved question is what evolutionary processes are involved in driving rapid speciation in such cases. Here, we focus on one such system; the Drosophila athabasca species complex, which is composed of three partially sympatric and interfertile semispecies: WN, EA, and EB. To study speciation in this species complex, we assayed sexual and genomic isolation within and between these semispecies in both sympatric and allopatric populations. First, we found no evidence of reproductive character displacement (RCD) in sympatric zones compared to distant allopatry. Instead, semispecies were virtually completely sexually isolated from each other across their entire ranges. Moreover, using spatial approaches and coalescent demographic simulations, we detected either zero or only weak heterospecific gene flow in sympatry. In contrast, within each semispecies we found only random mating and little population genetic structure, except between highly geographically distant populations. Finally, we determined that speciation in this system is at least an order of magnitude older than previously assumed, with WN diverging first, around 200K years ago, and EA and EB diverging 100K years ago. In total, these results suggest that these semispecies should be given full species status and we adopt new nomenclature: WN—D. athabasca, EA—D. mahican, and EB—D. lenape. While the lack of RCD in sympatry and interfertility do not support reinforcement, we discuss what additional evidence is needed to further decipher the mechanisms that caused rapid speciation in this species complex.     

Species-specific song convergence in a moving hybrid zone between two passerines

Moving hybrid zones are receiving increasing attention. However, so far little is known about the proximate mechanisms underlying these movements. Signalling behaviour, by individuals engaged in interspecific sexual and aggressive interactions, may play a crucial role. In this study, we investigated song variation within a moving hybrid zone between two warblers, Hippolais polyglotta and H. icterina . In these species, song is involved in interspecific territoriality and, probably, in mixed pairings. We showed that allopatric populations of the two species are clearly acoustically differentiated. However, interspecific differences faded out in sympatry as a result of an overall pattern of convergence. Unexpectedly, the two species converged for different song parameters, namely temporal parameters for H. icterina and syntax for H. polyglotta . Hybridization and interspecific competition could explain convergence in H. icterina . Instead, in H. polyglotta we suggest that local adaptation to habitat and interspecific learning might contribute to convergence. We particularly stress that cross-species learning, by maintaining high levels of interspecific interactions, may influence the movement of the zone.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 507–517.     

Systematics of dusky salamanders, Desmognathus (Caudata: Plethodontidae), in the mountain and Piedmont regions of Virginia and North Carolina, USA

We analysed mitochondrial (cytochrome  b ) nucleotide sequences, nuclear allozyme markers, and morphometric characters to investigate species boundaries and phylogenetic relationships among dusky salamanders ( Desmognathus ) in the southern Blue Ridge and adjacent Piedmont Physiographic Provinces of Virginia and North Carolina. Our results revealed four distinct mitochondrial DNA clades that are also characterized by distinct allozyme markers. One clade consists of sequences derived from populations distributed from New England to south-western Virginia that are referable to Desmognathus fuscus Rafinesque, 1820, although there is considerable sequence and allozyme divergence within this clade. A second clade consists of sequences derived from populations referable to Desmognathus planiceps Newman, 1955, a form that we resurrect from its long synonymy under D. fuscus . Desmognathus planiceps and D. fuscus also differ in mandibular tooth shape. Two other cytochrome  b sequences recovered from populations along the Blue Ridge escarpment in southern Virginia are quite divergent from those of the previous two clades, and these populations may represent yet another distinct species. Sequences from a population in the Brushy Mountains in the Piedmont of northern North Carolina are similar to those of Desmognathus carolinensis . Population groupings indicated by allozyme data generally correspond to the cytochrome  b clades. Cryptic diversity in Appalachian desmognathan salamanders clearly requires further study. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society , 2008, 152 , 115–130.     

Implications of contrasting patterns of genetic variability in two vespertilionid bats from the Indonesian archipelago

Wallacean island populations of two Vespertilioninae bats, Myotis muricola and Scotophilus kuhlii , which have similar geographical ranges, showed marked contrast in the amount and pattern of genetic variation. Within islands, genetic variation was on average much higher in M. muricola but declined from west to east, whereas all populations of S. kuhlii had uniformly low levels of genetic variation by mammalian standards. S. kuhlii showed little genetic differentiation between islands and estimates of gene flow were substantial whereas island populations of M. muricola differed markedly and there was a strong isolation-by-distance effect associated with the extent of the sea crossing between islands. Furthermore, the lower mean heterozygosity and small genetic distances between eastern island populations of M. muricola is evidence that there has been a bottleneck associated with the colonization of this area. The attenuation of genetic diversity to the east is also seen in some other mammalian species and may indicate limits to dispersal and have implications for species management. The patterns of variability in S. kuhlii may be a consequence of its strong dispersal capacity and close association with human activity, which, together with other factors, suggest a panmictic population.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 421–431.     

Hybrids between Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) do not show endogenous postzygotic isolation

Studies of prezygotic and postzygotic isolation in Drosophila have shown in general that species in sympatry tend to evolve prezygotic barriers earlier than do species in allopatry. However, postzygotic barriers tend to evolve at the same evolutionary rate in both sympatric and allopatric species. In contrast to these observations, the grasshoppers Chorthippus parallelus parallelus and C. p. erythropus show complete hybrid male sterility but only limited prezygotic isolation after an estimated 0.5 millions years of divergence. Like their congeners, C. brunneus and C. jacobsi form a hybrid zone where their ranges meet in northern Spain. However, the hybrid zone is mosaic and bimodal and, in contrast to the high levels of postzygotic isolation between C. parallelus subspecies, these two species showed no significant reduction in hybrid fitness in F₁ or backcross generations relative to the parental generations. The level of prezygotic isolation in laboratory tests was comparable to that between C. parallelus subspecies. These results suggest that endogenous postzygotic isolation does not play an important role in the reproductive isolation between C. brunneus and C. jacobsi , or in determining the structure of the hybrid zone. Exogenous postzygotic isolation may be present and should be tested in future studies.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 195–203.     

Time in a bottle: the evolutionary fate of species discrimination in sibling Drosophila species

Disadvantageous hybridization favors the evolution of prezygotic isolating behaviors, generating a geographic pattern of interspecific mate discrimination where members of different species drawn from sympatric populations exhibit stronger preference for members of their own species than do individuals drawn from allopatric populations. Geographic shifts in species' boundaries can relax local selection against hybridization; under such scenarios the fate of enhanced species preference is unknown. Lineages established from populations in the region of sympatry that have been maintained as single-species laboratory cultures represent cases where allopatry has been produced experimentally. Using such cultures dating from the 1950s, we assess how Drosophila pseudoobscura and D. persimilis mate preferences respond to relaxed natural selection against hybridization. We found that the propensity to hybridize generally declines with increasing time in experimental allopatry, suggesting that maintaining enhanced preference for conspecifics may be costly. However, our data also suggest a strong role for drift in determining mating preferences once secondary allopatry has been established. Finally, we discuss the interplay between populations in establishing the presence or absence of patterns consistent with reinforcement.     

Population genetic structure of New Zealand's endemic corophiid amphipods: evidence for allopatric speciation

Allozyme electrophoresis was used to examine population genetic structure at inter- and intraspecific levels for the New Zealand endemic corophiid amphipods, Paracorophium lucasi and P. excavatum. Individuals were collected from estuarine and freshwater habitats from North, South and Chatham Islands. Analyses of genetic structure among interspecific populations indicated clear allelic differentiation between the two Paracorophium species (Nei's genetic distance, D  = 1.62), as well as considerable intraspecific substructuring ( D  = 0.15–0.65). These levels of divergence are similar to interspecific levels for other amphipods and it is proposed that at least two groups from the P. lucasi complex and three from the P. excavatum complex correspond to sibling species. In most cases allopatry can account for the differentiation among the putative sibling species. For populations that share a common coastline we found low levels of differentiation and little or no correlation with geographical distance, suggesting that gene flow is adequate to maintain homogeneous population genetic structure. By contrast, populations on separate coastlines (i.e. isolated by land) showed moderate levels of geographical differentiation indicating restricted gene flow. The juxtaposition of population genetic and biogeographical data for Paracorophium in conjunction with the geological record infers past histories of glacial extirpation, and possible isolating effects of sea-level and landmass changes that have occurred throughout the Plio-Pleistocene.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 119–133.     

Spatio-temporal genetic variation in sympatric and allopatric Mediterranean Cicada species (Hemiptera, Cicadidae)

Cicada barbara (Stål) and Cicada orni L. are two Mediterranean cicadas, very similar in morphology, that produce distinct acoustic mating signals and that have partially overlapping distribution ranges in the Iberian Peninsula, occurring in sympatry in several locations. In the present study, six microsatellite loci were analysed in C. barbara , four of which were also analysed in C. orni . Geographical and temporal genetic variation in these species was studied. No evidence of hybridization was found, enabling us to infer that the isolating barriers between these species are efficient. Partitioning of geographic variation in each species, revealed the following patterns: Iberian Peninsula and Northwestern African populations of C. barbara showed higher differentiation between than within each region, supporting C. barbara subspecific taxonomy ( C. barbara lusitanica in the Iberian Peninsula and C. barbara barbara in Northwestern Africa) and highlighting isolation coincident with the presence of physical barriers to gene-flow; differentiation between populations of C. orni from both sides of the Pyrenees was very low, and this mountain range may not constitute a significant barrier for the dispersal of this species; Greek populations of C. orni were found to be highly differentiated from Western European populations; and no pattern of isolation-by-distance was found in either species within the Iberian Peninsula.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 249–265.     

Hybridization, developmental stability, and functionality of morphological traits in the ground beetle Carabus solieri (Coleoptera, Carabidae)

The assessment of developmental stability in hybrids can provide valuable information in the study of species formation because it allows an evaluation of the degree of incompatibility of genetic systems that control developmental processes. The present study assessed the impact of two hybridization events, assumed to have occurred at different times, on developmental instability in the ground beetle Carabus solieri . Developmental instability was estimated in 678 individuals from 27 populations from the fluctuating asymmetry (FA) levels of four morphological traits: the tibia length of middle and hind legs, which are functional structures, and the length and the proximal width of the hind wings, which are vestigial and thus nonfunctional structures. Significant variations of FA levels between populations were shown only for the wing width. For this trait, FA levels in hybrids were higher than in their parental entities for both hybridization events, indicating a significant divergence of the gene systems controlling development between the parental entities in the two hybridization cases. As expected, wing traits exhibited FA levels at least three times higher than leg trait. Finally, the potential interest of vestigial traits in the particular context of hybridization is discussed.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 151–158.     

Genetic and morphological differentiation patterns between sister species: the case of Onthophagus taurus and Onthophagus illyricus (Coleoptera, Scarabaeidae)

The present study investigated the morphological and genetic differentiation pattern between two sympatric dung beetle sister species, Onthophagus taurus and Onthophagus illyricus . The geometric morphometric approach coupled with the use of molecular markers allowed examination of the nature of interspecific relationships and an outline of the evolutionary and geographical processes that might have led to interspecific differentiation and the present-day partial sympatric and syntopic pattern of distribution. Geometric morphometrics failed to discrimininate the two species on the ground of external morphological traits, but revealed interspecific differences when the shape of the primary sexual traits was taken into account. The use of two different molecular markers (cytochrome oxidase subunit I gene and amplified fragment-length polymorphism) demonstrates that the two species are genetically well differentiated, forming two distinguishable lineages probably diverged during the Pliocene by allopatric speciation. No evidence of past or recent introgression and no support for hybridization were found, suggesting that sympatry was established subsequently, when speciation was accomplished. Both molecular markers and genital shape indicate that phenotypically intermediate individuals, despite their ambiguous appearance, are members of O. illyricus species rather than hybrids.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 197–211.     

Evolution of mosaic hybrid zone between invasive and endemic species of Cyprinidae through space and time

Hybridization phenomena can be considered in different ways. Hybridization studies can be used to elucidate some aspects of speciation and adaptation. However, hybridization may be a threat to endemic species. Here, we studied bidirectional introgression between two sympatric species of Cyprinidae: Chondrostoma toxostoma toxostoma and C. nasus nasus . Analysis of morphology, allozymes and mtDNA sequences revealed that the hybridization between these two cyprinids takes different forms in the same river. The mosaic hybrid zone (so called because of the absence of a simple cline) appeared to be partitioned due to the proportions of the two species along a spatial scale. The proportion of each hybrid group in this zone was unstable over a 5-year period. We propose various protection/conservation states for the C. t. toxostoma populations based on the level of introgressive hybridization and the genetic structure of both species in 'pure' populations.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 135–155.