Wynn Addison H
Department of Zoology, University of Maryland, College Park, MD, 20742.
Evolution. 1986 Jan;40(1):44-54. doi: 10.1111/j.1558-5646.1986.tb05716.x.
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.
两种经电泳分化的灰林螈形态之间的接触带横跨德尔马瓦半岛。基于晚更新世时期区域气候的推测性质，有人认为在威斯康星冰期极盛期，德尔马瓦半岛的气候对灰林螈来说不适宜生存。据推测，在约14000年前劳伦泰德冰盖消退后，该半岛被来自北方和南方的灰林螈种群入侵。当两种灰林螈形态相遇时，形成了一个次生接触带。这个接触带现在在一个区域很明显，在该区域六个电泳位点的等位基因频率有一致的渐变。缺乏杂合子缺失的证据、接触带的宽度较宽以及基因渗入的总体对称性表明种群正在自由融合。在接触带以北的八个种群中的四个种群中，两个酯酶位点处于连锁不平衡状态。由于迄今为止研究的所有灰林螈种群的不平衡系数都具有相似的符号，选择可能在维持这种连锁不平衡中起重要作用。在接触带内，连锁不平衡的发生率增加，但两个渗入种群之间的基因交换可能是连锁不平衡发生率较高的原因。
