Range-wide comparisons of northern leatherside chub populations reveal historical and contemporary patterns of genetic variation

Patchily distributed species are those taxa whose populations occupy geographically insular habitats and their conservation often depends on an understanding of the relationship among disjunct populations. The objective of our study was to use molecular data and analytical techniques to separate the effects of historical and contemporary processes influencing the distribution of a high-desert minnow, the northern leatherside chub (Lepidomeda copei). Individuals from 23 populations were sequenced for 1,140 base pairs of the cytochrome B gene of the mitochondrial genome and genotyped at seven nuclear microsatellite loci. We estimated gene flow and examined population structure using both microsatellite and mtDNA data. Low sequence divergence and the distribution of shared haplotypes in multiple watersheds suggest historical connectivity between populations over a large geographic area. In contrast, patterns of microsatellite diversity indicate that populations of leatherside chub are isolated from one another with low levels of contemporary gene flow between populations. Our results suggest that populations of leatherside chub were historically more widely inter-connected and have recently been isolated, likely through a combination of natural and anthropogenic habitat fragmentation. As populations become increasingly isolated, they are more vulnerable to extirpation as a result of stochastic events. For northern leatherside chub, recent isolation and lack of gene flow among populations may affect their long-term survival in the arid landscapes of the Great Basin and surrounding watersheds because of widespread and increasing habitat alteration and fragmentation.     

Microsatellite variation and structure of 28 populations of the common wetland plant, Lychnis flos-cuculi L., in a fragmented landscape

Habitat fragmentation is known to cause genetic differentiation between small populations of rare species and decrease genetic variation within such populations. However, common species with recently fragmented populations have rarely been studied in this context. We investigated genetic variation and its relationship to population size and geographical isolation of populations of the common plant species, Lychnis flos-cuculi L., in fragmented fen grasslands. We analysed 467 plants from 28 L. flos-cuculi populations of different sizes (60 000-54 000 flowering individuals) in northeastern Switzerland using seven polymorphic microsatellite loci. Genetic differentiation between populations is small (F(ST) = 0.022; amova; P < 0.001), suggesting that gene flow among populations is still high or that habitat fragmentation is too recent to result in pronounced differentiation. Observed heterozygosity (H(O) = 0.44) significantly deviates from Hardy-Weinberg equilibrium, and within-population inbreeding coefficient F(IS) is high (0.30-0.59), indicating a mixed mating breeding system with substantial inbreeding in L. flos-cuculi. Gene diversity is the only measure of genetic variation which decreased with decreasing population size (R = 0.42; P < 0.05). While our results do not indicate pronounced effects of habitat fragmentation on genetic variation in the still common L. flos-cuculi, the lower gene diversity of smaller populations suggests that the species is not entirely unaffected.     

Limited effect of anthropogenic habitat fragmentation on molecular diversity in a rain forest skink, Gnypetoscincus queenslandiae

To examine the effects of recent habitat fragmentation, we assayed genetic diversity in a rain forest endemic lizard, the prickly forest skink (Gnypetoscincus queenslandiae), from seven forest fragments and five sites in continuous forest on the Atherton tableland of northeastern Queensland, Australia. The rain forest in this region was fragmented by logging and clearing for dairy farms in the early 1900s and most forest fragments studied have been isolated for 50-80 years or nine to 12 skink generations. We genotyped 411 individuals at nine microsatellite DNA loci and found fewer alleles per locus in prickly forest skinks from small rain forest fragments and a lower ratio of allele number to allele size range in forest fragments than in continuous forest, indicative of a decrease in effective population size. In contrast, and as expected for populations with small neighbourhood sizes, neither heterozygosity nor variance in allele size differed between fragments and sites in continuous forests. Considering measures of among population differentiation, there was no increase in FST among fragments and a significant isolation by distance pattern was identified across all 12 sites. However, the relationship between genetic (FST) and geographical distance was significantly stronger for continuous forest sites than for fragments, consistent with disruption of gene flow among the latter. The observed changes in genetic diversity within and among populations are small, but in the direction predicted by the theory of genetic erosion in recently fragmented populations. The results also illustrate the inherent difficulty in detecting genetic consequences of recent habitat fragmentation, even in genetically variable species, and especially when effective population size and dispersal rates are low.     

Habitat fragmentation affects genetic diversity and differentiation of the Yarkand hare

The Yarkand hare, Lepus yarkandensis, is an endemic, endangered species restricted to the Tarim Basin of the Xinjiang Uygur Autonomous Region, China. The Yarkand hare is distributed in scattered oases which are physically isolated by the desert. Its natural fragmentation habitat makes it an ideal object for studying effect of habitat fragmentation on its genetic structure. To evaluate the effects of habitat fragmentation on genetic diversity of the species, we assessed genetic diversity for 20 sampling populations based on control region and Cytb markers. Relatively low levels of gene diversity are found in most of isolated populations in the southern margin of the Taklamakan Desert. Furthermore, a positive correlation is found between gene diversity and the size of historical effective population. Significant genetic differentiation is detected among most populations by pairwise F_ST analyses, which is characterized by an isolation by distance pattern. Additionally, the AMOVA results show highly significant population structure among seven geographical groups. High migration rates are found among continuous populations, while very low levels of migration rates are found among the relatively isolated populations, suggesting that the desert may make an effective barrier against gene flow. Finally, the control region shows four clades by the phylogenetic analyses, three of which are present in nearly all sampling populations. The observed pattern of the lineage mixing, also shown by the Cytb data, may be caused by extensive gene flow among populations, and could be explained by possible demographical expansion of the Yarkand hare during the late Pleistocene interglacial period.     

Differential threshold effects of habitat fragmentation on gene flow in two widespread species of bush crickets

Effects of habitat fragmentation on genetic diversity vary among species. This may be attributed to the interacting effects of species traits and landscape structure. While widely distributed and abundant species are often considered less susceptible to fragmentation, this may be different if they are small sized and show limited dispersal. Under intensive land use, habitat fragmentation may reach thresholds at which gene flow among populations of small-sized and dispersal-limited species becomes disrupted. Here, we studied the genetic diversity of two abundant and widespread bush crickets along a gradient of habitat fragmentation in an agricultural landscape. We applied traditional (G(ST), θ) and recently developed (G'ST', D) estimators of genetic differentiation on microsatellite data from each of twelve populations of the grassland species Metrioptera roeselii and the forest-edge species Pholidoptera griseoaptera to identify thresholds of habitat fragmentation below which genetic population structure is affected. Whereas the grassland species exhibited a uniform genetic structuring (G(ST) = 0.020-0.033; D = 0.085-0.149) along the whole fragmentation gradient, the forest-edge species' genetic differentiation increased significantly from D < 0.063 (G(ST) < 0.018) to D = 0.166 (G(ST) = 0.074), once the amount of suitable habitat dropped below a threshold of 20% and its proximity decreased substantially at the landscape scale. The influence of fragmentation on genetic differentiation was qualitatively unaffected by the choice of estimators of genetic differentiation but quantitatively underestimated by the traditional estimators. These results indicate that even for widespread species in modern agricultural landscapes fragmentation thresholds exist at which gene flow among suitable habitat patches becomes restricted.     

Population genetic structure of Bellamya aeruginosa (Mollusca: Gastropoda: Viviparidae) in China: weak divergence across large geographic distances

Bellamya aeruginosa is a widely distributed Chinese freshwater snail that is heavily harvested, and its natural habitats are under severe threat due to fragmentation and loss. We were interested whether the large geographic distances between populations and habitat fragmentation have led to population differentiation and reduced genetic diversity in the species. To estimate the genetic diversity and population structure of B. aeruginosa, 277 individuals from 12 populations throughout its distribution range across China were sampled: two populations were sampled from the Yellow River system, eight populations from the Yangtze River system, and two populations from isolated plateau lakes. We used seven microsatellite loci and mitochondrial cytochrome oxidase I sequences to estimate population genetic parameters and test for demographic fluctuations. Our results showed that (1) the genetic diversity of B. aeruginosa was high for both markers in most of the studied populations and effective population sizes appear to be large, (2) only very low and mostly nonsignificant levels of genetic differentiation existed among the 12 populations, gene flow was generally high, and (3) relatively weak geographic structure was detected despite large geographic distances between populations. Further, no isolation by linear or stream distance was found among populations within the Yangtze River system and no signs of population bottlenecks were detected. Gene flow occurred even between far distant populations, possibly as a result of passive dispersal during flooding events, zoochoric dispersal, and/or anthropogenic translocations explaining the lack of stronger differentiation across large geographic distances. The high genetic diversity of B. aeruginosa and the weak population differentiation are likely the results of strong gene flow facilitated by passive dispersal and large population sizes suggesting that the species currently is not of conservation concern.     

The genetic status of the violet copper Lycaena helle– a relict of the cold past in times of global warming

Rising temperatures and agricultural changes (intensification and succession on fallow land) during the last few decades have caused a strong decline of moist and cool sites on nutrient-poor grasslands and species depending on these habitats. We tested the effects of habitat deterioration on a local and regional scale in such a species, the highly endangered butterfly Lycaena helle , which was more widely distributed over central Europe during the postglacial period, but has recently become restricted to some remnants. We analysed five polymorphic microsatellite loci in 220 individuals sampled at ten different localities. The study sites in Germany, Luxembourg and Belgium are geographically split into three mountain regions: the Ardennes, the Eifel and the Westerwald; the latter is separated from the other two by the river Rhine. A comparatively high genetic diversity was detected in all local populations and genetic differentiation was found among the Ardennes, the Eifel and the Westerwald (F_CT: 0.084). The genetic differentiation among all populations (F_ST: 0.137) underlines natural and anthropogenic habitat fragmentation. While ongoing gene flow seems to exist among the Eifel populations indicating the only intact metapopulation, a high genetic differentiation in the Ardennes and the Westerwald indicates a disruption of population connectivity. Our genetic data obtained on different spatial scales show the genetic consequence of long-term isolation and should trigger necessary conservation measures at the metapopulation level.     

Gene flow among native bush rat,Rattus fuscipes (Rodentia: Muridae), populations in the fragmented subtropical forests of south‐east Queensland

Abstract Many natural populations in areas of continuous habitat exhibit some form of local genetic structure. Anthropogenic habitat fragmentation can also strongly influence the dynamics of gene flow between populations. We used eight microsatellite markers to investigate the population genetic structure of an abundant forest species, the Australian bush rat (Rattus fuscipes), in the subtropical forests of south‐east Queensland. Five sites were sampled, allowing pairwise comparisons within continuous habitat and across clearings. Weak, but significant population differentiation and a significant pattern of isolation by distance was detected over the small scale (<10 km) of this study. Fine‐scale analysis at a single site (<1 km) showed a significant correlation between individual female genetic distance and geographical distance, but no similar pattern among male individuals. There was no evidence of increased population differentiation across clearings relative to comparisons within continuous forest. This was attributed to dispersal within corridors of remnant and revegetated habitat between the forested areas. We concluded that an inherently restricted dispersal ability, female philopatry and natural habitat heterogeneity play an important part in the development of genetic structure among populations of R. fuscipes. It is important to understand the relationship between landscape features and the pattern of gene flow among continuous populations, as this allows us to predict the impact of fragmentation on natural populations.     

Demographic history and the low genetic diversity in Dipteryx alata (Fabaceae) from Brazilian Neotropical savannas

Genetic effects of habitat fragmentation may be undetectable because they are generally a recent event in evolutionary time or because of confounding effects such as historical bottlenecks and historical changes in species'' distribution. To assess the effects of demographic history on the genetic diversity and population structure in the Neotropical tree Dipteryx alata (Fabaceae), we used coalescence analyses coupled with ecological niche modeling to hindcast its distribution over the last 21 000 years. Twenty-five populations (644 individuals) were sampled and all individuals were genotyped using eight microsatellite loci. All populations presented low allelic richness and genetic diversity. The estimated effective population size was small in all populations and gene flow was negligible among most. We also found a significant signal of demographic reduction in most cases. Genetic differentiation among populations was significantly correlated with geographical distance. Allelic richness showed a spatial cline pattern in relation to the species'' paleodistribution 21 kyr BP (thousand years before present), as expected under a range expansion model. Our results show strong evidences that genetic diversity in D. alata is the outcome of the historical changes in species distribution during the late Pleistocene. Because of this historically low effective population size and the low genetic diversity, recent fragmentation of the Cerrado biome may increase population differentiation, causing population decline and compromising long-term persistence.     

Characterization of chloroplast microsatellite loci from whole chloroplast genome of Camellia taliensis and their utilization for evaluating genetic diversity of Camellia reticulata (Theaceae)

This study characterized chloroplast microsatellite markers for Camellia reticulata, a famous ornamental and edible economic tree species only distributed in Southwestern China. Thirty-two chloroplast microsatellite markers were originally annotated in the whole chloroplast genome of Camellia taliensis, ten polymorphic microsatellite markers were tested in 96 individuals from four natural populations of C. reticulata. Alleles numbered from two to four, and average value of Shannon's Information index, Nei's gene diversity, total genetic diversity, genetic diversity within populations, gene differentiation coefficient and gene flow index were 0.408, 0.225, 0.217, 0.102, 0.530 and 0.443, respectively. Our results showed high genetic differentiation and limited gene flow among the studied populations, which may be explained by recent fragmentation of the remnant populations due to human destruction and disturbance of natural habitats of the species. The identified cpSSR markers will be useful for the future studies on population genetics, conservation biology and phylogeography of C. reticulata.     

Fine-scale spatial genetic structure and clonal distribution of the cold-water coral Lophelia pertusa

Determining the spatial genetic structure within and among cold-water coral populations is crucial to understanding population dynamics, assessing the resilience of cold-water coral communities and estimating genetic effects of habitat fragmentation for conservation. The spatial distribution of genetic diversity in natural populations depends on the species’ mode of reproduction, and coral species often have a mixed strategy of sexual and asexual reproduction. We describe the clonal architecture of a cold-water coral reef and the fine-scale population genetic structure (<35 km) of five reef localities in the NE Skagerrak. This study represents the first of this type of analysis from deep waters. We used thirteen microsatellite loci to estimate gene flow and genotypic diversity and to describe the fine-scale spatial distribution of clonal individuals of Lophelia pertusa. Within-population genetic diversity was high in four of the five reef localities. These four reefs constitute a genetic cluster with asymmetric gene flow that indicates metapopulation dynamics. One locality, the Säcken reef, was genetically isolated and depauperate. Asexual reproduction was found to be a highly important mode of reproduction for L. pertusa: 35 genetic individuals were found on the largest reef, with the largest clone covering an area of nearly 300 m².     

Population structure of an endangered frog (<Emphasis Type="Italic">Babina subaspera</Emphasis>) endemic to the Amami Islands: possible impacts of invasive predators on gene flow

The otton frog (Babina subaspera) is an endangered species endemic to the Amami Islands, Japan. High predation pressure from an introduced carnivore, the mongoose, has caused declines in the frog populations and created a large habitat gap around an urban area. To promote effective conservation, we investigated the genetic status of the species and examined the effect of the habitat gap on gene flow among populations. Using five polymorphic microsatellite loci and mitochondrial DNA sequences, we investigated genetic diversity, genetic structure and gene flow in B. subaspera populations on the islands of Amami-Oshima and Kakeroma-jima. The expected heterozygosity (H _E) within each locality was generally high (range: 0.67–0.85), indicating that B. subaspera maintains high genetic diversity. However, genetic differentiation was observed, and the two populations, TAG and KAR, showed little gene flow with other populations. The clustering and F _ST analyses also predicted that these two populations were clearly distinct. According to the mitochondrial DNA analysis, the observed genetic differentiation occurred relatively recently. Possible barriers such as mountain ridges, rivers or roads did not result in genetic separation of the populations. These data support the hypothesis that the habitat gap created by an introduced predator prevented the gene flow among B. subaspera populations. When developing conservation strategies for B. subaspera, focus should be directed to these two isolated populations; careful monitoring of population size and genetic diversity should be conducted along with the mongoose elimination project ensues.     

Microsatellite diversity and genetic structure of fragmented populations of the rare,fire-dependent shrub Grevillea macleayana

Recent habitat loss and fragmentation superimposed upon ancient patterns of population subdivision are likely to have produced low levels of neutral genetic diversity and marked genetic structure in many plant species. The genetic effects of habitat fragmentation may be most pronounced in species that form small populations, are fully self-compatible and have limited seed dispersal. However, long-lived seed banks, mobile pollinators and long adult lifespans may prevent or delay the accumulation of genetic effects. We studied a rare Australian shrub species, Grevillea macleayana (Proteaceae), that occurs in many small populations, is self-compatible and has restricted seed dispersal. However, it has a relatively long adult lifespan (c. 30 years), a long-lived seed bank that germinates after fire and is pollinated by birds that are numerous and highly mobile. These latter characteristics raise the possibility that populations in the past may have been effectively large and genetically homogeneous. Using six microsatellites, we found that G. macleayana may have relatively low within-population diversity (3.2-4.2 alleles/locus; Hexp = 0.420-0.530), significant population differentiation and moderate genetic structure (FST = 0.218) showing isolation by distance, consistent with historically low gene flow. The frequency distribution of allele sizes suggest that this geographical differentiation is being driven by mutation. We found a lack mutation-drift equilibrium in some populations that is indicative of population bottlenecks. Combined with evidence for large spatiotemporal variation of selfing rates, this suggests that fluctuating population sizes characterize the demography in this species, promoting genetic drift. We argue that natural patterns of pollen and seed dispersal, coupled with the patchy, fire-shaped distribution, may have restricted long-distance gene flow in the past.     

Genetic population structure of the sagebrush Brewer’s sparrow, <Emphasis Type="Italic">Spizella breweri breweri</Emphasis>, in a fragmented landscape at the northern range periphery

Assessing the genetic consequences of habitat fragmentation is a crucial step in conservation planning for species in endangered habitats. We tested for the impact of natural habitat fragmentation on gene flow and genetic diversity in seven northern breeding locations of the sagebrush Brewer’s sparrow, Spizella breweri breweri. Genetic analyses using five highly variable DNA microsatellite loci suggested that individuals sampled within a sagebrush landscape fragmented by natural elements such as coniferous forest, comprise a single genetic population and that gene flow among them is unimpeded. We posit that juvenile dispersal links seemingly isolated breeding locales of this species, and discuss implications of our findings for conservation of migratory songbirds in the northern portion of their ranges in light of potential shifts in distribution due to climate change.     

Microsatellite analysis reveals interpopulation differentiation and gene flow in the endangered tree Changiostyrax dolichocarpa (Styracaceae) with fragmented distribution in central China

Polymorphic simple sequence repeat (SSR) markers were used to investigate the impact of habitat fragmentation on the population structure and gene flow of Changiostyrax dolichocarpa, a critically endangered tree in central China. Intrapopulation genetic diversity, population structure and gene flow in the five extant populations of this species were analysed by eight SSR markers. Intrapopulation genetic diversity results suggest that C. dolichocarpa remnants maintained a relatively high degree of genetic diversity despite severe fragmentation. Low genetic differentiation among populations was found based on Wright's F(ST) and amova analysis. Both the F(ST)-based estimate and private allele method revealed high historical gene flow among the remnant populations. Recent immigrants, detected by assignment tests, tend to decrease from the grandparent generation to the current generation. The potentially highly restricted current gene flow among fragments may render the fragmented populations of C. dolichocarpa at a higher risk of local extinction several generations after fragmentation. Both in situ and ex situ conservation management for the remnant populations of C. dolichocarpa are therefore urgently needed to rescue remaining genetic diversity.     

Similarity of contemporary and historical gene flow among highly fragmented populations of an endangered rattlesnake

Populations of endangered taxa in recently fragmented habitats often show high levels of genetic structure, but the role that contemporary versus historical processes play in generating this pattern is unclear. The eastern massasauga rattlesnake (Sistrurus c. catenatus) is an endangered snake that presently occurs throughout central and eastern North America in a series of populations that are isolated because of habitat fragmentation and destruction. Here, we use data from 19 species-specific microsatellite DNA loci to assess the levels of genetic differentiation, genetic effective population size, and contemporary and historical levels of gene flow for 19 populations sampled across the range of this snake. Eastern massasaugas display high levels of genetic differentiation (overall θ(Fst) = 0.21) and a Bayesian clustering method indicates that each population represents a unique genetic cluster even at regional spatial scales. There is a twofold variation in genetically effective population sizes but little genetic evidence that populations have undergone recent or historical declines in size. Finally, both contemporary and historical migration rates among populations were low and similar in magnitude even for populations located <7 km apart. A test of alternate models of population history strongly favours a model of long-term drift-migration equilibrium over a recent isolation drift-only model. These results suggest that recent habitat fragmentation has had little effect on the genetic characteristics of these snakes, but rather that this species has historically existed in small isolated populations that may be resistant to the long-term negative effects of inbreeding.     

Fragmentation and Genetic Diversity in Romnalda (Dasypogonaceae), a Rare Rain forest Herbaceous Genus from New Guinea and Australia

Rain forests are expected to be amongst the ecosystem types most affected by fragmentation due to their high species diversity, high endemism, complexity of interactions, and contrast with surrounding altered matrix. Due to their shorter life cycles and dependence on canopy cover, rain forest understory herbs are expected to indicate the effects of recent fragmentation more rapidly than canopy trees. This study investigated all four known species of the genus Romnalda , all of which are rare rain forest herbaceous species, to investigate the possible effects of habitat fragmentation and isolation on genetic diversity and gene flow. Allozymes were used as genetic markers and regional remnant vegetation maps were used to compare landscape fragmentation. We found that R. strobilacea populations in a highly fragmented landscape were genetically depauperate compared with those of its congeneric species that are found within continuous rain forest habitats and that allelic diversity decreased with decreasing population size but not geographic distance in R. strobilacea . Given the similarity among the species, our results indicate that all Romnalda species are potentially susceptible to loss of genetic diversity due to habitat fragmentation within relatively short timeframes. The results indicate that populations are not highly genetically differentiated and there is little evidence of genetic provenance where the species have restricted geographic ranges. Thus, species recovery programs would be better to focus on maintaining population size and genetic diversity rather than population differentiation.     

Genetic diversity within and between remnant populations of the endangered calcareous grassland plant <Emphasis Type="Italic">Globularia bisnagarica</Emphasis> L.

Changes in agricultural production methods over the last century have caused a massive reduction and fragmentation of the area of European semi-natural grasslands. It remains unclear whether small and isolated grassland fragments can support viable plant populations in a sustainable way. In our study area in southern Belgium, the extent of calcareous grasslands was reduced from c. 650 ha in 1775 to less than 30 ha in 2004. We used AFLP markers to quantify the effects of present and historical grassland fragmentation on the genetic structure of 27 populations of the rare perennial plant species Globularia bisnagarica. Given the mixed breeding system of the species and the relatively small area of the studied system, the populations were characterized by high genetic differentiation (F _st range: 0.42–0.48; Φ_st=0.53). A Mantel test revealed significant isolation by distance of the populations. Average within population genetic diversity, measured as expected heterozygosity or gene diversity, was low (H _j =0.081) and was negatively related to population isolation. This suggests more gene flow into less isolated populations. Population size and local habitat characteristics did not significantly influence population genetic diversity. Both, high selfing rates in G. bisnagarica and a population genetic response to habitat fragmentation may explain our findings. Finally, a clear geographical clustering was observed, with cluster membership partially explainable by historical grassland connectivity. If populations indeed started to differentiate after fragmentation, this process was not (yet) strong enough to erase the genetic similarity between fragments that historically belonged to the same large grassland fragment.     

Conservation genetics of the endangered terrestrial orchid Liparis japonica in Northeast China based on AFLP markers

Levels of genetic diversity and population genetic structure of the rare, endangered terrestrial orchid Liparis japonica were examined for eight natural populations (n = 185) in Northeast China using six AFLP primer pairs, where this species has experienced severe habitat loss and fragmentation. Based on 406 DNA bands, a high level of genetic diversity was found at the species level with the PPB of 85.47 %, while the genetic diversity at the population level was low (PPB = 47.48 %). A significantly high degree of population differentiation was found with 42.69 % variation existed among populations as measured by AMOVA, indicating potential restricted gene flow. The genetic distances between populations were independent of the corresponding geographic distances, and the genetic relationship of individuals had no significant correlation with their spatial distribution. The restricted gene flow might be impacted by reduced population size, habitat destruction and fragmentation. The results in this study suggested that habitat protection and keeping a stable environment are critical for the conservation of L. japonica species.     

River fragmentation increases localized population genetic structure and enhances asymmetry of dispersal in bullhead (Cottus gobio)

Man-made habitat fragmentation is a major concern in river ecology and is expected to have particularly detrimental effects on aquatic species with limited dispersal abilities, like the bullhead (Cottus gobio). We used ten microsatellite markers to investigate small-scale patterns of gene flow, current dispersal and neutral genetic diversity in a morphologically diverse river where fragmented and unfragmented sections could be compared. We found high genetic differentiation between sampling sites with a maximum F _ST of 0.32 between sites separated by only 35 km. A significant increase of genetic differentiation with geographical distance was observed in the continuous river section as well as in the full dataset which included headwater populations isolated by anthropogenic barriers. Several lines of evidence are consistent with the hypothesis that such barriers completely block upstream movement while downstream dispersal may be little affected. In the unfragmented habitat, dispersal rates were also higher in the direction of water flow than against it. The resulting asymmetry in gene flow likely contributes to the decrease of genetic variation observed from the lower reaches towards the headwaters, which is particularly pronounced in physically isolated populations. Our findings suggest that headwater populations, due to their isolation and low genetic variation, may be particularly vulnerable to extinction.