Date of Award

6-12-2002

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

John C. La Duke

Abstract

Napaea dioica L. (Malvaceae) is a robust, dioecious perennial flowering plant that is endemic to the upper midwestem United States. Occurring exclusively on the floodplains o f small rivers, populations o f N. dioica are distributed discretely on a network o f riparian habitat throughout its range which is coincident with the southernmost margin o f the Wisconsinan continental glacier. Ten population o f 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 o f the species. Multilocus gene diversity was found to be uniformly high within populations and there was little evidence o f significant reduction in heterozygosity consistent with the putative outcrossing mating system o f 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 o f low capacity for dispersal and a gradual stepping stone mode o f postglacial colonization, results o f Mantel tests indicated significant evidence for isolationby- 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 o f 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 o f expansion along distinct portions o f the riparian network. These results illustrate the usefulness o f high resolution microsatellite markers for studies o f genetic diversity and structure in plants and suggest that microsatellites may be a useful tool to address the methodologically problematic study o f plant phylogeography. Additionally, the potential o f 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.

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