POPULATION-GENETICS OF THE RARE ASCLEPIAS-TEXANA AND ITS WIDESPREAD SISTER SPECIES, A-PERENNIS

We investigated the levels and distribution of genetic diversity in tw o presumed sister species of milkweeds: the widespread Asclepias peren nis and the narrowly distributed A. texana. Horizontal starch-gel elec trophoresis was performed on leaf tissue from 942 individuals from 18 populations of A. perennis and 357 individuals representing 11 populat ions of A. texana from throughout their respective ranges. All 16 loci were polymorphic in A. perennis, with a mean of 3.63 alleles per locu s at the species level, and 1.57 +/- 0.29 (mean +/- standard error) al leles per locus at the population level. In A. texana 15 loci were pol ymorphic, with a species average of 3.00 alleles per locus and 1.51 +/ - 0.17 alleles per locus at the population level. Overall species-leve l mean heterozygosity was slightly higher in A. perennis (H(es)BAR = 0 .076 +/- 0.035) than A. texana (H(es)BAR = 0.064 +/- 0.020). Mean expe cted heterozygosity within populations was the same for both species ( H(es)BAR = 0.061 +/-0.009). Total gene diversity was low (A. perennis, H(T)BAR = 0.076; A. texana, H(T)BAR = 0.069), with most of this varia tion found within populations (A. perennis, H(S)BAR = 0.058; A. texana , H(S)BAR = 0.060). Among-population diversity relative to total diver sity was very low (A. perennis, G(ST)BAR = 0.082; A. texana, G(ST)BAR = 0.068). Genetic identities within species averaged 0.95 for A. peren nis and 0.98 for A. texana, but these taxa were quite distinct from ea ch other (IBAR = 0.79). These species were even more distinct from ano ther milkweed, A. exaltata (for A. perennis, IBAR = 0.63; for A. texan a, IBAR = 0.39). Using several lines of evidence, we estimate that A. perennis and A. texana diverged 1-2 x 10(6) years B.P.