Does the Mediterranean killifish Aphanius fasciatus (Teleostei: Cyprinodontidae) fit the one-dimensional stepping-stone model of gene flow?

Five samples (30 individuals each) of the Mediterranean cyprinodontid fish, Aphanius fasciatus, were analysed by allozyme electrophoresis for a study concerning genetic structure and gene flow among populations. Forty-three loci, twelve of which were polymorphic at P_0.99 level, were analysed. A. fasciatus showed low levels of genetic polymorphism, with expected heterozygosity values ranging from 0.027 (SE = 0.013) to 0.064 (SE = 0.023). The modified Rogers' genetic distances ranged from 0.053 to 0.202. The observed pattern of genetic differentiation among populations, obtained by multidimensional scaling of the modified Rogers' genetic distances, was consistent with geographic distribution. Coancestry coefficient ( = 0.302, SE = 0.045) indicated a very high degree of genetic subdivision among populations and the estimate of gene flow (the effective number of migrants per generation, Nm = 0.445), computed by Wright's method, showed very restricted gene flow among populations. The regression analysis carried out using log-transformed values of geographic distance against M^ (pairwise values of Nm) gave a regression coefficient, b = –1.03, indicating that a restricted migration rate among populations can occur according to the one-dimensional stepping-stone model. The amount and modality of gene flow in A. fasciatus inferred from the results of the present study are consistent with the absence of dispersal stages and with the fragmented nature of brackish-water habitats along the coasts.     

Characteristics of dispersal in the eastern mosquitofish Gambusia holbrooki

The demographic and genetic characteristics of dispersing Gambusia holbrooki were examined relative to changes in stream flow velocity, flow direction, and rapid saltwater inundation. Dispersal of all G. holbrooki sex classes decreased as flow velocity increased, dispersal for a given flow velocity being greater in female, than male and juvenile fish respectively. Juveniles and males dispersed at low (9 mm s⁻¹) but not high (109 mm s ¹) flow. Females dispersed at both flow rates. Dispersing adults were not larger than non-dispersing adults of the same sex. Direction of dispersal was sex and size dependent, but independent of flow velocity. Larger females (>22 mm) moved preferentially downstream, while smaller females (17–22 mm) remained stationary or moved in the upstream direction. Males dispersed equally in both directions. No relationship was found between dispersal characteristics and the presence of high salt concentrations. Movement patterns at high flow were independent of glucose-6-phosphate isomerase ( GPI-2* ) alleles carried by dispersing individuals. Significant non-random dispersal of sex or size cohorts, and the interaction of dispersal with flow velocity have significant implications for the maintenance of genetic diversity in stream ecosystems, particularly where localized selection or genetic drift may alter the genetic constitution of specific sex or size cohorts. The applicability of these results to field populations is discussed.     

Protein polymorphism in natural populations of Diachasmimorpha longicaudata (Hymenoptera: Braconidae) in Thailand

Diachasmimorpha longicaudata (Ashmead) is a larval parasitoid of tephritid flies and is widely used as a classical biological control agent. We have used allozyme electrophoresis to evaluate the genetic relationships of six populations of D. longicaudata in Thailand. Twelve loci were examined of which 11 were polymorphic in at least some populations, especially that of Nakornratchasima. We observed a complete lack of heterozygotes for seven of the 10 polymorphic loci in the Nakornratchisima female population, and a significant deficiency of heterozygotes at a further two loci. We discuss possible hypotheses for these findings in light of the haplo-diploid sex determination system of these wasps. This revised version was published online in July 2006 with corrections to the Cover Date.     

POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE MEDITERRANEAN LAND SNAIL ALBINARIA CORRUGATA (PULMONATA: CLAUSILIIDAE)

The amount of gene flow among local populations partly determines the relative importance of genetic drift and natural selection in the differentiation of such populations. Land snails, because of their limited powers for dispersal, may be particularly likely to show such differentiation. In this study, we directly estimate gene flow in Albinaria corrugata, a sedentary, rock-dwelling gastropod from Crete, by mark-recapture studies. In the same area, 23 samples were taken and studied electrophoretically for six polymorphic enzyme loci. The field studies indicate that the population structure corresponds closely to the stepping-stone model: demes are present on limestone boulders that are a few meters apart, and dispersal takes place mainly between adjacent demes. Average deme size (N) is estimated at 29 breeding individuals and the proportion of migrants per generation at 0.195 (Nm = 5.7). We find no reason to assume long-distance dispersal, apart from dispersal along occasional stretches of suitable habitat. Genetic subdivision of the population, as derived from F_ST values, corresponds to the direct estimate only at the lowest spatial level (distance between sample sites < 10 m), where values for Nm of 5.4 and 17.6 were obtained. In contrast, at the larger spatial scales, F_ST values give gene-flow estimates that are incompatible with the expected amount of gene flow at these scales. We explain these discrepancies by arguing that gene flow is in fact extremely limited, making correct estimates of Nm from F_ST impossible at the larger spatial scales. In view of these low levels of gene flow, it is concluded that both genetic drift and natural selection may play important roles in the genetic differentiation of this species, even at the lowest spatial scales.     

Allozyme diversity and population genetic structure of the caddisfly Orthopsyche fimbriata and the mayfly Acanthophlebia cruentata in New Zealand streams

1. Allozymes were used to measure genetic variation within and among regional populations of the caddisfly Orthopsyche fimbriata and the mayfly Acanthophlebia cruentata in North Island New Zealand streams.
2. High levels of genetic differentiation were recorded in populations of O. fimbriata within and among catchments separated by more than 100 km, but little or no differentiation in populations separated by around 10 km. The Auckland isthmus appears to be a major barrier to north–south gene flow, with nearly fixed allelic differences at one locus. Genotype frequencies conformed to Hardy–Weinberg equilibrium.
3.  Acanthophlebia cruentata had low levels of genetic variation; the results are unexpected given that O. fimbriata apparently has greater potential for dispersal. The limited genetic data for A. cruentata provided evidence for genetic differentiation among populations separated by around 100 km, or more, within catchments and some differentiation between catchment populations separated by only 25 km.     

Genetic structure in a montane mayfly Baetis bicaudatus (Ephemeroptera: Baetidae), from the Rocky Mountains, Colorado

1. Populations of a number of sub‐tropical stream insect species have been found to show unexpected patterns of genetic variation, with more differences between samples from the same stream than between whole streams or between subcatchments. Many samples also showed deviations from Hardy–Weinberg proportions. It has been proposed that these patterns result from limited numbers of matings contributing to a given stream reach, because adults emerge throughout the year, and low levels of larval drift between reaches. These patterns may be less likely in a northern hemisphere montane species with synchronous emergence of adults and high levels of drift. We tested the hypothesis that patterns of genetic variation in a montane mayfly from the Rocky Mountains, Colorado, would reflect a pattern of ‘isolation by distance’ with samples from the same creek being more similar than samples from different creeks and that deviations from Hardy–Weinberg proportions would be minimal. 2. Based on allozyme variation, the hypothesis of minimal deviations from Hardy–Weinberg proportions was not supported and there was no evidence of isolation by distance. Nevertheless the levels of differentiation among samples from within the same stream were less than those reported for most subtropical species. 3. Results from analysis of a fragment of the cytochrome oxidase gene (subunit 1) revealed contrasting patterns. The levels of genetic differentiation were an order of magnitude higher between streams than among samples within streams. In addition, although there was no significant isolation by distance effect overall, a nested clade analysis provided evidence for restricted gene flow with isolation by distance for some clades. 4. We suggest that these contrasting results may reflect the differences in male and female dispersal patterns. While differentiation at nuclear gene markers (allozymes) give information about both male and female dispersal, mitochondrial DNA markers reflect only female dispersal. We suggest that in this species, female dispersal may be more restricted, perhaps mostly along stream channels, whereas male dispersal is more widespread. An alternative explanation for the different results is the different evolutionary rates of the mitochondrial and nuclear markers.     

How far is too close? restricted,sex‐biased dispersal in black‐capped vireos

Understanding the interplay of dispersal and how it translates into gene flow is key to understanding population processes, and especially so for endangered species occupying fragmented habitats. In migratory songbirds, there is evidence that long‐distance movement capabilities do not translate well into observed dispersal. Our objectives were to (i) define the fine‐scale spatial genetic structure in endangered black‐capped vireos to characterize dispersal patterns and (ii) to correlate dispersal dynamics to overall population genetic structure using a simulation approach. We sampled 160 individuals over 2 years to (i) describe the fine‐scale genetic structuring and (ii) used this information to model scenarios to compare with actual data on change in population structuring over a 100‐year interval. We found that black‐capped vireos exhibit male philopatry and restricted dispersal distances, relative to females. Our simulations also support a sex‐biased dispersal model. Additionally, we find that fragmentation related changes in rates of dispersal might be a likely cause for increasing levels of population structure over a 100‐year period. We show that restricted sex‐biased dispersal can explain population structuring in this species and that changes in dispersal rates due to fragmentation may be a continuing threat to genetic viability in this species.