LINKAGE DISEQUILIBRIUM AND A CONTACT ZONE IN PLETHODON CINEREUS ON THE DEL-MAR-VA PENINSULA.

A contact zone between two electrophoretically differentiated forms of Plethodon cinereus extends across the Del-Mar-Va Peninsula. Based on the presumed nature of regional climates during the late Pleistocene, it is suggested that the climate was inhospitable for P. cinereus on the Del-Mar-Va Peninsula at the Wisconsin maximum. It is postulated that, after the Laurentide ice sheet receded about 14,000 years BP, the peninsula was invaded by populations of P. cinereus from the north and south. When the two forms of P. cinereus met, a secondary zone of contact was produced. This contact zone is now apparent in an area in which six electrophoretic loci have concordant clinal changes in allele frequency. The lack of evidence for heterozygote deficiencies, the broad width of the contact zone, and the overall symmetry of introgression suggest that fusion of the populations is occurring freely. Two esterase loci are in linkage disequilibrium in four of eight populations to the north of the contact zone. As all populations of P. cinereus studied to date have disequilibrium coefficients of similar sign, selection is likely important in the maintenance of this linkage disequilibrium. Within the contact zone, the incidence of the linkage disequilibrium increases, but exchange of genes between the two introgressing populations may account for the higher incidence of linkage disequilibrium.