MITOCHONDRIAL-DNA ANALYSIS OF ATLANTIC COAST, CHESAPEAKE BAY, AND DELAWARE BAY POPULATIONS OF THE TELEOST FUNDULUS-HETEROCLITUS INDICATES TEMPORALLY UNSTABLE DISTRIBUTIONS OVER GEOLOGIC TIME

The estuarine teleost Fundulus heteroclitus exhibits some of the most remarkable genetic and morphologic transitions of any species along th e Atlantic coast of the United States. Abrupt changes in gene frequenc ies at a variety of allelic isozymes have been identified at or near t he Hudson River, and these changes are mirrored in morphologic charact ers. Analysis of mitochondrial DNA variation of four Atlantic coast po pulations identified two major mtDNA haplotype assemblages with a tran sition zone somewhere along the coast of New Jersey, but the concordan ce of patterns of mtDNA distributions and those evident in other genet ic and morphologic features is unclear owing to the limited number of populations examined. In this study we extend the analysis of mtDNA va riation to include 740 individual F. heteroclitus from 29 populations along the Atlantic coast and within the Chesapeake and Delaware Bays. The intent was to define more clearly the transition zone from souther n to northern populations in reference to those already identified in allelic isozymes and morphologic characters. In addition, this study e xamined the distribution of mtDNA variation within two major estuaries (Chesapeake Bay and Delaware Bay) to determine if glacial relect popu lations of the northern form might still inhabit the upper reaches of these systems. The results indicate an abrupt transition in mtDNA hapl otype frequencies in northern New Jersey, which is somewhat south of t he transition zone indicated by allelic isozymes and morphologic featu res. The northern mtDNA haplotypes were also detected in the upper rea ches of Chesapeake and Delaware Bays, but the frequency distribution w as more clinal than that identified along the coast. On the basis of t hese data, we suggest that a single ancestral contact zone previously existed at least as far south as the mouth of the Chesapeake Bay prior to the last glaciation and that this zone has been unstable during th e past several hundred thousand years.