Abstract

This study examines phylogeography, population genetics, and introgression in birches (Betula spp.) of eastern North America with a specific focus on B. papyrifera Marsh., B. alleghaniensis Britt., and B. lenta L. Betula papyrifera (2n=56, 70, 84)occupies a wide variety of habitats across the transcontinental boreal zone and into northeastern temperate forests. Betula alleghaniensis (2n=84) is a mesophytic species occurring from southeastern Canada and New England to the southern Appalachians. Betula lenta (2n=28) is endemic to the Appalachian region of the United States, where it occurs primarily on moist, cool sites. Phylogeographic analyses based on chloroplast DNA (cpDNA) markers were used for inferring locations of glacial refugia and patterns of post-glacial migration. Nuclear microsatellite markers were used for investigating
allelic richness, gene diversity, and genetic structure of populations of each species. The haplotype sharing among species was examined to elucidate patterns of cpDNA
introgression, and nuclear microsatellite allele sharing was investigated to determine the incidence of nuclear genomic introgression. The cpDNA analyses revealed phylogeographic structuring of eastern and western populations of B. alleghaniensis and B. papyrifera suggesting postglacial colonization from separate refugia. The geographically structured patterns of regional cpDNA haplotype sharing between B. alleghaniensis and B. papyrifera suggest widespread introgression between species during the postglacial recolonization. In contrast, cpDNA markers were monomorphic in B. lenta and no haplotype sharing between B. lenta and other species were detected. Nuclear microsatellite data revealed low levels of population genetic substructure in each of the species with significant differentiation at larger spatial scales due to isolation by
distance. The low level of population differentiation over relatively long distances probably reflects the long distance pollen and seed dispersal of these species. The
analysis of nuclear microsatellite allele-sharing revealed that despite moderate to high proportions of shared alleles, each of the species was significantly genetically
differentiated. No strong evidence of genetic introgression was found for B. alleghaniensis and B. lenta, or B. papyrifera and B. lenta. Admixture proportions were
higher between B. alleghaniensis and B. papyrifera in the western Great Lakes, which is a previously-documented zone of hybridization based on the occurrence of morphological
intermediates.