GLOBAL SURVEY OF MITOCHONDRIAL-DNA SEQUENCES IN THE THREESPINE STICKLEBACK - EVIDENCE FOR RECENT MIGRATIONS

Phylogenetic analyses of mitochondrial DNA (mtDNA) sequences were used to assess the matriarchal genetic structure of the threespine stickle back, Gasterosteus aculeatus. A 747 basepair (bp) fragment of the cyto chrome b was sequenced from 36 individuals collected from 25 localitie s in Europe, North America, and Japan. Two major divergent clades were revealed: one widespread in Japan but with representatives in some Al askan and British Columbian lakes and the other common in Europe and N orth America. A simple diagnostic test using the polymerase chain reac tion (PCR) and a restriction enzyme was used to assay additional indiv iduals, confirming the absence of the Japanese clade in the Atlantic b asin. Geographic distribution of mtDNA variation suggests (1) a recent origin of the Atlantic populations, and (2) support for previous hypo theses about the existence of Pleistocene refugia for freshwater fishe s in Alaska and British Columbia. Silent substitution rates were used to date the colonization of the Atlantic at 90,000 to 260,000 yr befor e present, which conflicts with earlier dates implied by the fossil re cord. The recent replacement of Atlantic mitochondrial lineages sugges ted by our data may be explained by severe reduction or extinction of northern Atlantic populations during the Pleistocene, followed by a re cent reinvasion from the Pacific. With a global perspective of the dis tribution of genetic variation as a framework, meaningful comparisons at a smaller geographical scale will now be possible.