GENETIC-VARIABILITY IN THE FEDERAL THREATENED MEADS MILKWEED, ASCLEPIAS-MEADII TORREY (ASCLEPIADACEAE), AS DETERMINED BY ALLOZYME ELECTROPHORESIS

Most populations of the federal threatened Mead's milkweed, Asclepias meadii Torr. (Asclepiadaceae), occur primarily in prairie haymeadowsin Kansas and Missouri, where annual summer mowing prevents seed product ion. Exceptions are large populations in fire-managed habitats at the Rockefeller Prairie, a former Kansas haymeadow, and at Weimer Hill, a glade complex in southeastern Missouri. This perennial rhizomatous spe cies is self-incompatible. The few remaining small populations in Illi nois, Iowa, and northern Missouri persist vegetatively but no longer p roduce seeds and are vulnerable to stochastic extinction processes. Al lozyme electrophoresis was used to measure the amount and distribution of genetic variation in A. meadii and to provide guidance for its rec overy and restoration. Samples were obtained from 19 populations encom passing the extant range of the species in Kansas, Missouri, Iowa, and Illinois. Asclepias meadii was genetically variable for most of the 1 2 loci examined, with a mean of 1.53 alleles per locus, 40.8% polymorp hic loci, and observed heterozygosity of 0.158. These values are compa rable to published values for other milkweed species. More than half o f the total 42 alleles were rare, with 15 alleles unique to single pop ulations. About 74% of the genetic variation in A. meadii occurs withi n populations (FST=0.261), and analyses do not provide conclusive evid ence for a geographic pattern in genetic variation among populations. The two fire-managed populations had more genotypes and fewer ramets p er genet than the haymeadows. For the latter, inhibition in sexual rep roduction may have resulted in clonal spread and attrition of genotype s, thus exacerbating the effects of sexual incompatibility and inbreed ing. These factors suggest that multiple propagule sources would maxim ize genetic diversity for recovering depauperate populations or for re storing new populations; however, sampling from a few larger, genetica lly diverse populations would provide much of the species' genetic div ersity. Such sources would include the large fire-managed populations. Empirical data are needed to determine the population-genetic consequ ences of long-distance crosses and introductions that are apparently n eeded to restore viable populations of this species in the eastern par t of its range.