MOLECULAR GENETIC ANALYSIS OF A STEPPED MULTILOCUS CLINE IN THE AMERICAN OYSTER (CRASSOSTREA VIRGINICA).

Gulf of Mexico versus Atlantic populations of several coastal species in the southeastern United States are known to differ sharply in genetic composition, but most transitional zones have not previously been examined in detail. Here we employ molecular markers from mitochondrial and nuclear loci to characterize cytonuclear genetic associations at meso- and microgeographic scales along an eastern Florida transitional zone between genetically distinct Atlantic and Gulf populations of the American oyster, Crassostrea virginica. The single- and multilocus cytonuclear patterns display: (1) a cline extending along 340 km of the east Florida coastline; (2) a pronounced step in the cline centered at Cape Canaveral (shifts in allelic frequencies by 50-75% over a 20 km distance); (3) a close agreement of observed genotypic frequencies with Hardy-Weinberg expectations within locales; and (4) mild or nonexistent nuclear and cytonuclear disequilibria in most local population samples. These results imply: (1) considerable restrictions to interpopulational gene flow along the eastern Florida coastline; (2) within locales, free interbreeding (as opposed to mere population admixture) between Gulf and Atlantic forms of oysters; and (3) localized population recruitment in the transition zone localities. These findings demonstrate that marine organisms with high dispersal potential via long-lived pelagic larvae can nonetheless display pronounced spatial population genetic structure, and more generally they exemplify the utility of pronounced genetic transition zones for the study of population level processes. © 1996 The Society for the Study of Evolution.