Mitochondrial DNA evidence of distinct glacial refugia for brown bullhead (Ameiurus nebulosus) in the Great Lakes

Restriction fragment length polymorphisms were used to survey the mitochondrial genome of Ameiurus nebulosus for nucleotide sequence variation. Two hundred and forty-nine individuals were analyzed from 12 populations across the Great Lakes drainage and 3 populations in possible refugial drainages. Fifteen restriction endonucleases revealed 50 distinct haplotypes among these fish. Two major phylogenetic assemblages, A and B, were revealed with an average 3.22% sequence divergence. Both assemblages were themselves fragmented into two groups. Strong geographic patterning was observed in the frequency of assemblages and groups across the sampling area: assemblage A was predominant in fish from populations east of Lake Erie, while western populations were dominated by assemblage B. The distribution and phylogenetic divergence of mitochondrial haplotypes indicate that brown bullhead in the Great Lakes originated from two Pleistocene refugia and further provides genetic evidence of subdivision within these refugia. Pleistocene glaciations appear to have had a similar influence on the geographic distribution of mitochondrial DNA lineages of other North American fishes. In contrast with these other species, however, much of the mitochondrial DNA variation and divergence was retained in brown bullhead populations living south of the ice sheets and is represented in extant populations.