Molecular genetic markers have made extant populations increasingly useful as sources of information for drawing valid inferences about the post-glacial natural history of temperate species. With respect to patterns of past range expansion, we are using microsatellites to investigate the distribution of Napaea dioica L. populations along small rivers throughout its range in the upper mid-western United States. Using four-locus microsatellite genotype frequency estimates for ten sampled (n = 32 to 40) populations, AMOVA reveals significant genetic structure (FCT = 0.20, P = 0.001, 16 002 permutations) among three riparian population groups in N. dioica. In locus-wise analyses, only one locus is significantly structured (FCT = 0.44, P = 0.0005), though a second locus harbors nearly significant structure (FCT = 0.28, P = 0.058). A majority-rule (100 bootstrap replicate) ML tree drawn from estimated population allele frequencies supports the riparian population groups that we used in our AMOVA design. All of 45 population pairwise FST estimates ranging from 0.09 to 0.60 are significant (P < 0.00001, 16 002 permutations). Mantel's test indicates a correlation (r = 0.70, P = 0.0002, 10 000 permutations) between log(FST) and distance along rivers (48 to 2785 km, log-transformed) interconnecting sampled populations of N. dioica. When we remove the correlation (r = 0.64, P = 0.0003, 10 000 permutations) between log(FST) and direct geographic distance (28 to 975 km, log-transformed), isolation-by-distance with respect to interconnecting rivers remains significant (partial r = 0.38, P = 0.0196, 10 000 permutations). From these results, we hypothesize that riparian corridors may have played a role in directing the post-glacial range expansion of N. dioica. Our results also suggest that riparian corridors may be important factors in the population dynamics of N. dioica, a hypothesis that we will test in ongoing studies utilizing additional microsatellites and fine-scale population samples.

Key words: genetic structure, isolation-by-distance, Malvaceae, microsatellite, Napaea dioica, range expansion