Extensive genetic divergence among populations of the Australian freshwater fish, Pseudomugil signifer (Pseudomugilidae), at different hierarchical scales

The 'island' nature of river systems limits the possibility of dispersal in freshwater fish species, with many displaying extensive genetic subdivision across their ranges. We examined the population genetic structure of the predominantly freshwater fish, Pseudomugil signifer, at fine, medium and broad scales using allozyme and mitochondrial DNA techniques. Extensive genetic subdivision across the range (FST = 0.6) at six polymorphic allozyme loci was partitioned mostly among river-system drainages (42%). Patterns of mitochondrial DNA sequences, which mirrored previous taxonomic designations and a terrestrial biogeographic barrier, identified two major groups. The level of divergence between the groups implied a considerable period of isolation. At a more localized scale, significant genetic differences between adjacent drainages implied that flood events are not effective transport mechanisms and indicate that populations of P. signifer in different drainages are essentially independent units. The history of P.�signifer appears to be dominated, at different spatial scales, by long periods of isolation among drainages mediated by changes in sea-level, which merge and isolate drainages haphazardly over time. The chance nature of dispersal, coupled with isolation and potential population size fluctuations, probably leads to the observed complex patterns of genetic divergence.