Genetic structure and isolation by distance in Napaea dioica L. (Malvaceae): An analysis of microsatellite DNA variation

Napaea dioica L. (Malvaceae) is a robust, dioecious perennial flowering plant that is endemic to the upper midwestern United States. Occurring exclusively on the floodplains of small rivers, populations of N. dioica are distributed discretely on a network of riparian habitat throughout its range which is coincident with the southernmost margin of the Wisconsinan continental glacier. Ten population of N. dioica were sampled across the species geographic range and were assayed at four high resolution microsatellite marker loci to assess genetic diversity and population structure in relation to the riparian habitat network and the ecological and organismal characteristics of the species.; Multilocus gene diversity was found to be uniformly high within populations and there was little evidence of significant reduction in heterozygosity consistent with the putative outcrossing mating system of N. dioica. A west-to-east decrease in mean allelic richness was detected suggesting that N. dioica may have colonized its present range from a western refugium. All but three pairwise population comparisons across the species range were found to exhibit significant genetic differentiation. Consistent with a hypothesis of low capacity for dispersal and a gradual stepping stone mode of post-glacial colonization, results of Mantel tests indicated significant evidence for isolation-by-distance in N. dioica. This finding also suggests that contemporary gene flow in N. dioica is probably restricted to neighboring populations. A maximum likelihood estimation of phylogeography in N. dioica performed over the microsatellite allele frequencies identified two distinct population groups. These groups, which also exhibited significant hierarchical genetic structure in AMOVA tests, were maximally separated on the riparian habitat network suggesting that N. dioica may have colonized its present range via two flanks of expansion along distinct portions of the riparian network.; These results illustrate the usefulness of high resolution microsatellite markers for studies of genetic diversity and structure in plants and suggest that microsatellites may be a useful tool to address the methodologically problematic study of plant phylogeography. Additionally, the potential of plants in general and N. dioica in particular as useful models for exploring biogeographical, ecological, and evolutionary genetic questions is supported by the results of this study.