When east meets west: population structure of a high-latitude resident species, the boreal chickadee (Poecile hudsonicus).

The population genetic structure of northern boreal species has been strongly influenced both by the Quaternary glaciations and the presence of contemporary barriers, such as mountain ranges and rivers. We used a combination of mitochondrial DNA (mtDNA), nuclear microsatellites and spatial distribution modelling to study the population genetic structure of the boreal chickadee (Poecile hudsonicus), a resident passerine, and to investigate whether historical or contemporary barriers have influenced this northern species. MtDNA data showed evidence of eastern and western groups, with secondary admixture occurring in central Canada. This suggests that the boreal chickadee probably persisted in multiple glacial refugia, one in Beringia and at least one in the east. Palaeo-distribution modelling identified suitable habitat in Beringia (Alaska), Atlantic Canada and the southern United States, and correspond to divergence dates of 60-96 kya. Pairwise FST values for both mtDNA and microsatellites were significant for all comparisons involving Newfoundland, though mtDNA data suggest a more recent separation. Furthermore, unlike mtDNA data, nuclear data support population connectivity among the continental populations, possibly due to male-biased dispersal. Although both are significant, the isolation-by-distance signal is much stronger for mtDNA (r(2)=0.51) than for microsatellites (r(2)=0.05), supporting the hypothesis of male-biased dispersal. The population structure of the boreal chickadee was influenced by isolation in multiple refugia and contemporary barriers. In addition to geographical distance, physical barriers such as the Strait of Belle Isle and northern mountains in Alaska are restricting gene flow, whereas the Rocky Mountains in the west are a porous barrier.