A similar phylogeographical structure among sympatric North American birches (Betula) is better explained by introgression than by shared biogeographical history

Aim A comparative analysis of the chloroplast (cp) DNA structure of eastern North American birches (Betula) was undertaken to infer the impacts of Quaternary climate change on the phylogeographical structure of these species. Location Eastern North America. Methods Genetic variation in chloroplast microsatellites (cpSSRs) and the psbA-trnH intergenic spacer in Betula papyrifera, Betula alleghaniensis and Betula lenta was analysed in samples from 65, 80 and 12 populations, respectively. Co-occurring Betula uber, Betula populifolia and Betula cordifolia were also sampled to examine haplotype relationships and account for potential introgression. Haplotype networks, Bayesian analysis and comparisons of R-ST and G(ST) values were used to evaluate the phylogeographical structure. Genetic diversity within and among species was compared using rarefaction analysis. Results The two most widespread species, B. papyrifera and B. alleghaniensis, showed high levels of haplotype diversity, while the Appalachian endemic B. lenta possessed a single haplotype. Bayesian analysis revealed three main phylogeographical groups for B. papyrifera and four groups for B. alleghaniensis, and these two species showed extensive regional haplotype sharing and a high introgression ratio. Main conclusions We postulate that at least three separate refugia contributed to the recolonization of B. papyrifera and B. alleghaniensis within eastern North America, while B. lenta appears to have recolonized from a single refugium. A high haplotype diversity of B. papyrifera and B. alleghaniensis in the Great Lakes region may reflect biogeographical contact between eastern and western lineages, with the potential influence of periglacial refugia. Similar phylogeographical patterns in the distantly related B. papyrifera and B. alleghaniensis represent a geographical turnover of the same locally shared haplotypes, pointing to introgression rather than shared biogeographical history as the mechanism. Although similar phylogeographical patterns are often interpreted as evidence for common biogeographical histories, our study demonstrates that such patterns can also arise through introgression.