Reconciling patterns of genetic variation with stream structure, earth history and biology in the Australian freshwater fish Craterocephalus stercusmuscarum (Atherinidae)

We examined the consequences of barriers, stream architecture and putative dispersal capability on levels of genetic differentiation among populations of the freshwater fish Craterocephalus stercusmuscarum. Seven polymorphic allozyme loci and sequences of a 498-bp fragment of the ATPase 6 mitochondr ial DNA (mtDNA) gene were used to assess patterns of genetic variation amon g 16 populations from upland and lowland streams of five drainages in north ern Queensland, Australia. Concordant patterns at both genetic markers reve aled that there were significant levels of genetic subdivision among all po pulations, while an analysis of molecular variation showed that the distrib ution of genetic diversity was not consistent with contemporary drainage st ructure. There were reciprocally monophyletic mtDNA clades and fixed or lar ge frequency differences at allozyme loci either side of instream barriers such as waterfalls. This implied barriers were effective in restricting gen e flow between upland and lowland populations separated by waterfalls. Howe ver, there were two genetically distinct groups in upland areas, even withi n the same subcatchment, as well as high levels of genetic subdivision amon g lowland populations, suggesting barriers alone do not explain the pattern s of genetic diversity. The data revealed a complex phylogeographic pattern , which we interpreted to be the result of one or more invasion events of i ndependent lineages to different sections of each drainage, possibly mediat ed by well documented geomorphological changes. Our results highlight the i mportance of earth structure and history in shaping population genetic stru cture in stream organisms where dispersal capability may be limited, and re veal that the contemporary structure of drainages is not necessarily a good indicator of genetic relationships among populations.