Determining the phylogenetic and taxonomic relationships among allopatric populations can be difficult, especially when divergence is recent and morphology is conserved.Weused mitochondrial sequence data from the control region and three protein-coding genes (1253 bp in total) and genotypes determined at 13 microsatellite loci to examine the evolutionary relationships among Australia’s largest freshwater fish, the Murray cod, Maccullochella peelii peelii, from the inland Murray–Darling Basin, and its allopatric sister taxa from coastal drainages, the eastern freshwater cod, M. ikei, and Mary River cod, M. peelii mariensis. Phylogenetic analyses provided strong support for taxon-specific clades, with a clade containing both of the eastern taxa reciprocally monophyletic to M. peelii peelii, suggesting a more recent common ancestry between M. ikei and M. peelii mariensis than between the M. peelii subspecies. This finding conflicts with the existing taxonomy and suggests that ancestral Maccullochella crossed the Great Dividing Range in the Pleistocene and subsequently diverged in eastern coastal drainages. Evidence from the present study, in combination with previous morphological and allozymatic data, demonstrates that all extant taxa are genetically and morphologically distinct. The taxonomy of Maccullochella is revised, with Mary River cod now recognised as a species, Maccullochella mariensis, a sister species to eastern freshwater cod, M. ikei. As a result of the taxonomic revision, Murray cod is M. peelii.
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