Paul R. Cabe

Fine-scale population differentiation and gene flow in a terrestrial salamander ( Plethodon cinereus ) living in continuous habitat

Several recent studies have shown that amphibian populations may exhibit high genetic subdivision in areas with recent fragmentation and urban development. Less is known about the potential for genetic differentiation in continuous habitats. We studied genetic differentiation of red-backed salamanders (Plethodon cinereus) across a 2-km transect through continuous forest in Virginia, USA. Mark-recapture studies suggest very little dispersal for this species, whereas homing experiments and post-Pleistocene range expansion both suggest greater dispersal abilities. We used six microsatellite loci to examine genetic population structure and differentiation between eight subpopulations of red-backed salamanders at distances from 200 m to 2 km. We also used several methods to extrapolate dispersal frequencies and test for sex-biased dispersal. We found small, but detectable differentiation among populations, even at distances as small as 200 m. Differentiation was closely correlated with distance and both Mantel tests and assignment tests were consistent with an isolation-by-distance model for the population. Extrapolations of intergenerational variance in spatial position (σ2<15 m2) and pair-wise dispersal frequencies (4Nm<25 for plots separated by 300 m) both suggest limited gene flow. Additionally, tests for sex-biased dispersal imply that dispersal frequency is similarly low for both sexes. We suggest that these low levels of gene flow and the infrequent dispersal observed in mark-recapture studies may be reconciled with homing ability and range expansion if dispersing animals rarely succeed in breeding in saturated habitats, if dispersal is flexible depending on the availability of habitat, or if dispersal frequency varies across the geographic range of red-backed salamanders.

Effects of roads on patterns of genetic differentiation in red-backed salamanders, Plethodon cinereus

Roads can fragment animal populations by reducing gene flow, which can lead to drift and the loss of genetic diversity. One of the principle signatures of reduced gene flow is increased genetic differentiation in isolated populations, and evidence that roads contribute to such differentiation has been reported for several species. We used microsatellites to examine whether six roads led to increased genetic differentiation in red-backed salamanders (Plethodon cinereus). These six roads included one divided interstate highway, one undivided four-lane highway, and four secondary roads. We found that the genetic distance between plots that were bisected by the interstate highway was significantly greater than the genetic distance between equidistant plots on the same side of the highway. However, for the five smaller roads, plots across the road were no more genetically distinct than were plots on the same side of the road. Bayesian clustering methods also supported both of these findings. The optimal clustering of plots for the interstate highway consisted of two clusters that corresponded to the two sides of highway. For the other five sites, the optimal grouping consisted of a single cluster containing all of the plots. Our findings suggest that gene flow across very large roads is rare and that bisected red-backed salamander populations are likely to diverge from one another. For smaller roads, our results imply that the indirect effects of roads on genetic population structure are probably less of a pressing concern for terrestrial salamanders than are the direct effects of mortality and habitat alteration.

Female philopatry and male-biased dispersal in a direct-developing salamander, Plethodon cinereus.

The local resource competition hypothesis and the local mate competition hypothesis were developed based on avian and mammalian systems to explain sex‐biased dispersal. Most avian species show a female bias in dispersal, ostensibly due to resource defence, and most mammals show a male bias, ostensibly due to male–male competition. These findings confound phylogeny with mating strategy; little is known about sex‐biased dispersal in other taxa. Resource defence and male–male competition are both intense in Plethodon cinereus, a direct‐developing salamander, so we tested whether sex‐biased dispersal in this amphibian is consistent with the local resource competition hypothesis (female‐biased) or the local mate competition hypothesis (male‐biased). Using fine‐scale genetic spatial autocorrelation analyses, we found that females were philopatric, showing significant positive genetic structure in the shortest distance classes, with stronger patterns apparent when only territorial females were tested. Males showed no spatial genetic structure over the shortest distances. Mark–recapture observations of P. cinereus over 5 years were consistent with the genetic data: males dispersed farther than females during natal dispersal and 44% of females were recaptured within 1 m of their juvenile locations. We conclude that, in this population of a direct‐developing amphibian, females are philopatric and dispersal is male‐biased, consistent with the local mate competition hypothesis.

Multiple paternity in a salamander with socially monogamous behaviour.

In the majority of birds and mammals, social monogamy is not congruent with genetic monogamy. No research to date has compared social and genetic monogamy in amphibians. We analysed paternity in clutches of red‐backed salamanders (Plethodon cinereus), a species in which social monogamy has been demonstrated in the laboratory, and 28% of individuals in the forest are found in male‐female pairs in the noncourtship season. We collected 16 clutches of eggs of P. cinereus in the southern Appalachian Mountains of Virginia and collected tail clippings from attending mothers. We genotyped embryos and adults at five microsatellite loci in order to analyse paternity of clutches. Most clutches (84.6%) had multiple sires, with two to three sires per clutch. In this study, 25% of clutches had males in addition to females attending eggs. None of the mothers of these clutches were genetically monogamous. All attending males sired some of the offspring in the clutch that they attended (between 9% and 50%) but never sired a majority in that clutch. We conclude that, at least in this population, social monogamy in P. cinereus is not concomitant with genetic monogamy.

The effects of founding bottlenecks on genetic variation in the European starling ( Sturnus vulgaris ) in North America

Genetic variation in European starlings in North America was examined using enzyme electrophoresis and compared to that in their home range. The effect of the founding bottleneck matched theoretical predictions. Heterozygosity was unaffected, whereas allelic diversity may have decreased. Results from this study and others suggest that theoretical predictions of bottlenecks are robust for allozyme data, and applicable under a wide variety of conditions.

Reproductive success of Acadian Flycatchers in the Blue Ridge Mountains of Virginia

Abstract Conservation strategies for Neotropical migratory birds have emphasized identification and preservation of habitats in which populations are reproducing above replacement rates. In 2000–2001, we monitored 141 Acadian Flycatcher (Empidonax virescens) nests in the extensively forested Blue Ridge Mountains in Virginia to assess the sites potential to host “source” populations of this common species. Our estimates of reproductive success, unlike most studies, incorporated the nesting behavior of marked females followed throughout the breeding season. Despite the presence of Brown-headed Cowbirds (Molothrus ater) in a nearby 15-y-old clearcut and in open areas of a summer camp, we found no parasitized nests. Overall, the predation rate was 41%, 2.5 fledglings were produced per successful nesting attempt, and seasonal fecundity approached two female fledglings per adult female per season. Given return rates (minimum survival rates) of 52% at our study site, our measured levels of reproductive success are sufficient to classify the Blue Ridge Mountains as a potential “source” for Acadian Flycatchers. Our data contrast sharply with those collected from landscapes with high levels of forest fragmentation in the midwestern and southeastern U.S., where even relatively large patches of forest appear to host “sink” populations of Acadian Flycatchers.

Interpreting population differentiation in terms of drift and selection

SummaryMany empirical studies demonstrate some degree of genetic differentiation among populations of the same species. Understanding the relative importance of the processes causing this genetic differentiation has proven to be a difficult task. In particular, population differentiation can be influenced primarily by selection, genetic drift, and migration. We review the effect of drift and migration on patterns of genetic variation, with special reference to the conditions necessary for population differentiation. Conceptually, selection may be implicated in cases of population differentiation if the effect of drift and migration can be shown to be insufficient to cause the observed patterns. We examine some of the pitfalls of this approach when used with allozyme data, and revise a previous conclusion concerning the relative importance of selection in poulations of scale insects.

Dispersal and population structure in the European starling

Dispersal in birds can be estimated in several ways, including the use of banding data and the indirect use of genetic data. This study uses both of these to estimate dispersal and genetic population structure in the European Starling (Sturnus vulgaris) in North America. Banding data imply that natal dispersal is quite high, and this finding is supported by the observed rapid colonization of North America. Genetic data, based on allozyme allele frequcncies from populations in Virginia, Vermont, Colorado, and California, are consistent with a species with large demes and high rates of dispersal.

Consequences of Stocking Headwater Impoundments on Native Populations of Brook Trout in Tributaries

Abstract Understanding the impact of hatchery supplementation on the genetics of wild fish populations is important for designing and evaluating ecologically sound stocking practices. For species such as brook trout Salvelinus fontinalis, which are a high priority for conservation and restoration in their native range, understanding the potential impacts of stocking on the functional diversity of wild populations is critical. We sought to determine whether brook trout stocked in low-order reservoirs colonize impoundment feeder streams and if they naturally reproduce and interbreed with established native populations in these tributaries. Analysis of microsatellite DNA allowed us to distinguish hatchery-origin brook trout and putative native strains among tributaries of three stocked reservoirs and one unstocked stream. Hatchery-origin fish were found in tributaries of all stocked reservoirs, mixed with native populations; none were found in an unstocked reference stream that supported wild brook trout. Ag...

Genetic Diversity and Population Structure of the Endemic Disjunct Species, Helenium virginicum (Asteraceae)

Abstract Helenium virginicum is a narrowly endemic federally protected species, disjunct between Virginia and Missouri. To study the magnitude and distribution of genetic diversity within this species, we determined ISSR fingerprints and cpDNA haplotypes of plants in nine Virginia and eight Missouri populations. We found high ISSR diversity at the species level, with higher diversity in Virginia than Missouri. Additionally, moderately low diversity at the population level and high population structure suggests low gene flow among populations. Mean divergence between Virginia and Missouri populations was greater than between populations in either region. Missouri populations were more structured than Virginia populations. We found six distinct cpDNA haplotypes distributed among H. virginicum populations, with one found in both regions, two only in Virginia, and three only in Missouri. High genetic divergence among populations and between regions, the demographic asynchrony of populations, and self-incompatibility of the species suggest that clusters of populations in Missouri and Virginia be protected. These data will assist federal and state agencies deciding whether to delist and how to manage this species.