Spatial and temporal genetic structure of Asclepias verticillata (whorled milkweed) among prairie patches in a forested landscape

We examined the spatial scale of genetic structure of Asclepias verticillata L. (whorled milkweed) populations in a landscape where suitable habitat is fragmented by woodlands to determine if the distance between patches influences genetic differentiation. In addition, we sampled over 2 years to determine if there are temporal genetic differences. This forb is found in prairie habitat in the Edge of Appalachia Preserve System, Ohio. Prairie patches have a clumped distribution with patches within a region located < 150 m apart and regions located > 1.5 km apart. Allozyme electrophoresis was used to collect genetic data from reproductive individuals in nine patches representing four regions. Observed heterozygosity was not significantly different among patches or between years, even though population sizes varied. Rogers' genetic distance and hierarchical F-statistics indicated that there was little genetic differentiation among patches within a region. Although genetic differentiation was greater among regions, it was relatively low. Between years, genetic differentiation within a patch was as great as genetic differentiation between patches. Conditional allele frequencies suggest that loss of one patch within a region will increase genetic differentiation within a region. Data suggest that prairie management could focus on a few regions with large numbers of patches. Keywords: habitat fragmentation, allozyme, genetic diversity, prairie management.