MITOCHONDRIAL-DNA EVIDENCE OF DISTINCT GLACIAL REFUGIA FOR BROWN BULLHEAD (AMEIURUS-NEBULOSUS) IN THE GREAT-LAKES

Restriction fragment length polymorphisms were used to sun ey the mito chondrial genome of Ameiurus nebulosus for nucleotide sequence variati on. Two hundred and forty-nine individuals were analyzed from 12 popul ations across the Great Lakes drainage and 3 populations in possible r efugial drainages. Fifteen restriction endonucleases revealed 50 disti nct 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 geograp hic patterning was observed in the frequency of assemblages and groups across the sampling area: assemblage A was predominant in fish from p opulations east of Lake Erie, while western populations were dominated by assemblage B. The distribution and phylogenetic divergence of mito chondrial haplotypes indicate that brown bullhead in the Great Lakes o riginated from two Pleistocene refugia and further provides genetic ev idence of subdivision within these refugia. Pleistocene glaciations ap pear to have had a similar influence on the geographic distribution of mitochondrial DNA lineages of other North American fishes. In contras t with these other species, however, much of the mitochondrial DNA var iation and divergence was retained in brown bullhead populations livin g south of the ice sheets and is represented in extant populations.