Genetic Variability in the Federal Threatened Mead's Milkweed, Asclepias meadii Torrey (Asclepiadaceae), as Determined by Allozyme Electrophoresis

Abstract

Most populations ofthe federal threatened Mead's milkweed, Ascleplas meadii Torn (Asclepiadaeeae), 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. Phis 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 1.53 alleles per locus. 40.8% polymoqmic loci, and observed heterozygosity of 0.158. 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 (F sr = 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.

An Adaptive Decision Framework for the Conservation of a Threatened Plant

“…Similarly, when flowering occurred in the previous year, flowering was commonplace (93%) in burn years, but uncommon (11% and 30%) in years without burning (table 1 of Alexander et al [1997]). In general, burning is associated with more flowering plants and more flowers per plant (Kettle et al 2000), as well as more genotypes among smaller clones (Tecic et al 1998;Hayworth et al 2001). Additionally, above-average rainfall in previous years may increase the probability of transitioning to the flowering stage in the current year (Kettle et al 2000).…”

“…An individual may produce numerous vegetative clones, but sexual reproduction is thought to be crucial for maintaining high genetic diversity, providing some insurance of long-term population maintenance in general and protection against chance extirpation in particular (Tecic et al 1998). Viable seed is only produced by outbreeding in most milkweeds, because they are either sexually self-incompatible or highly sensitive to inbreeding depression (Kephart 1981;Shannon and Wyatt 1986;Kahn and Morse 1991;Sage and Williams 1991;Broyles and Wyatt 1993).…”

“…Typically, spring and summer mowing takes place on privately managed lands. Annual mowing virtually halts sexual reproduction in Mead's milkweed by removing the seeds before they mature (Freeman 2006), and some evidence suggests that mowing promotes vegetative growth over reproductive development Tecic et al 1998).…”

“…Burning is believed to increase flowering and seed production, stimulate dormant root stock, increase seedling and juvenile survival, and increase genotypic diversity of Mead's milkweed Tecic et al 1998). The effects are thought to be enhanced in wet years, but do not appear in years following mowing (Kettle et al 2000;Grman and Alexander 2005).…”

“…Detection of juvenile or nonflowering adult stages is not reliable with current field methods at any practical scale (Alexander et al 1997), and many plants apparently remain undetected in their dormant underground stage. Furthermore, only laboratory analysis can confirm whether two stems represent genetically distinct individuals (Tecic et al 1998). In the field, genetic individuality can be roughly inferred by the spacing between stems, but clonal spread (and thus, spacing) is often a function of management treatment ; Supplemental Material, Reference S3; http://dx.doi.org/10.3996/ 012011-JFWM-007.S3; Alexander et al 1997;USFWS 2003).…”