WENGER, JONATHAN P.^{1,2}* and JOHN C. LA DUKE^{1}. ^{1}Department of Biology, University of North Dakota, PO Box 9019, Grand Forks, ND 58201, USA; ^{2}Department of Natural Science and Mathematics, Concordia University, 275 Syndicate St N, Saint Paul, MN 55104, USA. - __Genetic structure and isolation-by-distance suggest a role for riparian corridors in the post-glacial natural history of __*Napaea dioica* L. (Malvaceae).

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