Within-drainage population genetic structure of the freshwater fish Pseudomugil signifer (Pseudomugilidae) in northern Australia

Dendritic channel patterns have the potential to isolate populations within drainages, depending on the relative position within the stream hierarchy of the populations. We investigated the extent of genetic subdivision in th e Australian freshwater fish Pseudomugil signifer (Kner) (Pseudomugilidae) from two drainages in northern Queensland, Australia, using allozyme techni ques. The drainages were adjacent and had similar channel patterns each wit h two major subcatchments coalesced to an estuarine confluence. Analysis of 30 sites across the two drainages revealed that although there was signifi cant genetic variation among sites in both drainages, this was not between the two subcatchments in either case. This result did not support predictio ns of the stream hierarchy model (SHM), which would predict higher levels o f variation among subcatchments than within them, nor did it suggest that e stuarine conditions represent a significant barrier to dispersal in this sp ecies. More variation was among sites within each subcatchment. Multidimens ional scaling plots revealed that, although most sites within a drainage we re similar to one another, outlier sites occurred in each drainage, so corr elations between genetic distance and geographic distance were weak. We sug gest that the distance between sites and the probability of connectivity be tween sites may better explain the observed distribution of genetic diversi ty.