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Molecular and Population Genetic Aspects of Mitochondrial DNA Variability in the Diamondback Terrapin, Malaclemys terrapin


Diamondback terrapins (Malaclemys terrapin) occupy brackish waters along North America's Atlantic and Gulf coasts. Despite nearly continuous distribution, terrapin populations exhibit extensive geographic variation, with seven subspecies recognized. To assess population-genetic structure in Malaclemys, we used 18 restriction enzymes to assay mitochondrial DNA (mtDNA) genotypes in 53 terrapins collected from Massachusetts to western Louisiana. MtDNA size polymorphism and heteroplasmy were observed, attributable to variation in copy number of a 75-bp tandem repeat. In terms of restriction sites, mtDNA genotypic diversity (G = 0.582) and divergence levels (p < 0.004) were exceptionally low. Only one restriction site polymorphism appeared geographically informative, clearly distinguishing populations north versus south of Florida's Cape Canaveral region. Nonetheless, the probable zoogeographic significance of this single site change is underscored by its (1) perfect concordance with the distribution of a key morphological character and (2) striking agreement with phylogeographic patterns observed for mtDNA profiles of several other coastal marine species. The possible isolation of Atlantic and Gulf terrapin populations during late-Pleistocene glacial maxima conceivably accounts for the observed patterns of mtDNA (and morphological) variation. © 1992 The American Genetic Association.

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