Postglacial migration and the loss of allozyme variation in northern populations of Asclepias exaltata (Asclepiadaceae)

The recent Wisconsin glaciation has provided opportunities for examining the effects of postglacial recolonization on the population genetics of plant and animal communities. In this study allozyme variation was examined in 19 populations of the herbaceous perennial Asclepias exaltata occurring in previously glaciated regions of North America. These northern populations of A. exaltata possess significantly fewer polymorphic loci (46.3 +/- 2.7; mean +/- 1 SD), alleles per polymorphic locus (1.84 +/- 0.24), and expected heterozygosity (0.133 +/- 0.031) than populations found in the Pleistocene refugium in the southern Appalachians. Population-level allozyme diversity decreased linearly from south to north and from east to west. Nineteen uncommon alleles previously observed in southern Appalachian populations were undetected in the northern region. Seven common alleles exhibited a clinal change in allele frequency. Of these, only Pgd-1a and Mnr-1c were at low-frequency in the southern Appalachians and increased significantly with increasing latitude and longitude, respectively. Despite this loss of allozyme diversity following postglacial migration, northern populations of A. exaltata have higher allozyme diversity and lower population differentiation (G(ST) = 0.117) than mean values for other long-lived herbaceous perennials. Increased habitat fragmentation in northern regions and potential habitat loss in the southern Appalachians are likely to reduce the historically rich gene pool that has provided the genetic stock for postglacial recoveries.