ALLOZYME DIVERSITY AND GENETIC-STRUCTURE IN SOUTHERN APPALACHIAN POPULATIONS OF POKE MILKWEED, ASCLEPIAS-EXALTATA

We investigated levels of genetic diversity, genetic population structure, and gene flow in poke milkweed, Asclepias exaltata. Horizontal starch-gel electrophoresis was performed on leaves collected from 846 plants in 18 populations from the southern Appalachians. Enzyme polymorphisms were observed at 16 of the 18 loci examined and averaged 2.36 +/- 0.32 (mean +/- standard deviation) alleles per polymorphic locus. Mean population heterozygosity (H(o) = 0.202 +/- 0.036), expected heterozygosity (H(e) = 0.182 +/- 0.023), and total gene diversity (H(T) = 0.235) were substantially higher than average values reported for other insect-pollinated, outcrossing herbs. Of this total genetic variation, less than 10% was partitioned among populations (G(ST) = 0.093). Indirect estimates of the number of migrants per generation (Nm = 2.43, calculated from G(ST); Nm = 3.46, calculated from the frequency of 11 alleles unique to single populations) suggest that gene flow is extensive in A. exaltata. These data are consistent with the view that the comose, wind-dispersed seeds and the durable, insect-dispersed pollinia of milkweeds promote high levels of gene flow, resulting in low levels of genetic differentiation between populations.