Genetic evidence of widespread dispersal in a parthenogenetic freshwater ostracod

We used an hierarchical analysis of allozyme variation to investigate for the freshwater ostracod Candonocypris novaezelandiae the relative contributions of sexual and asexual reproduction to recruitment into 42 local populations and to infer patterns of gene flow within and among four geographical regions (watersheds) in south-eastern Australia. Allele frequency variation among local populations was marked (mean F(ST)=0.228) but showed no regional differentiation. The allele frequency differences among local populations probably reflect the effects of stochastic processes, such as founder events, as well as variation in the success, and hence abundance, of particular clonal genotypes within water-bodies. Indeed, local populations were highly clonal, containing only females and displaying relatively low levels of genotypic diversity. Nevertheless, the distribution of genotypes within and among regions was surprising. The bulk of sampled individuals (88 per cent) were represented by just six common genotypes that were shared extensively among local populations and were geographically widespread. Individual samples contained a mean of 4.05, and up to 10, distinct four-locus genotypes and overall we detected a total of 26 electrophoretically distinct genotypes. In combination, our results suggest that either the south-eastern Australian populations of C. novaezelandiae arose through a recent colonization event (perhaps associated with an expansion of agricultural practices) or there is sufficient continuing gene flow between regions to prevent differentiation. However, the exact contributions of sexual and asexual reproduction to dispersal in this ostracod remain unclear.