Genetic Variability in the Federal Threatened Mead's Milkweed, Asclepias meadii Torrey (Asclepiacaceae) Determined by Allozyme Electorphoresis
AbstractMost populations of the federal threatened Mead's milkweed, Asclepias meadii Torr. (Asclepiadaceae). occur primarily in prairie haymeadows in Kansas and Missouri, where annual summer mowing prevents seed production. 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 species is self-incompatible. The few remaining small populations in Illinois, Iowa, and northern Missouri persist vegetatively but no longer produce seeds and are vulnerable to stochastic extinction processes. Allozyme electrophoresis was used to measure the amount and distribution of genetic variation in A. meadii and to provide guidance for its recovery and restoration. Samples were obtained from 19 populations encompassing the extant range of the species in Kansas. Missouri. Iowa. and Illinois. Asclepias meadii was genetically variable for most of the 12 loci examined, with a mean of 1.5:l alleles per locus. 40.8% polymorphic loci, and observed heterozygosity of 0. l 58. These values are comparable to published values for other milkweed species. More than half of the total 42 alleles were rare, with 15 alleles unique to single populations. About 74% of the genetic variation in A. meadii occurs within populations (Fw = 0.261), and analyses do not provide conclusive evidence for a geographic pattern in genetic variation among populations. The two fire-managed populations had more genotypes and fewer ramets per genet than the haymeadows. For the latter. inhibition of sexual reproduction may have resulted in clonal spread and attrition of genotypes, thus exacerbating the effects of sexual incompatibility and inbreeding. These factors suggest that multiple propagule sources would maximize genetic diversity for recovering depauperate populations or for restoring new populations; however, sampling from a few larger. genetically diverse populations would provide much of the species' genetic diversity. Such sources would include the large fire-managed populations. Empirical data are needed to determine the population-genetic consequences of long-distance crosses and introductions that are apparently needed to restore viable populations of this species in the eastern part of its range.
Volume, Page Number85, 97-109