Genome-Wide Perspectives On Admixture and Paleodemographic Footprints In Polar Bears

Hybridization is a widespread evolutionary phenomenon that can play a role in diversification, especially among closely related taxa. Whereas hybridization is well known in plants, natural animal hybrids are considered much less common, although recent genome-wide investigations of canids and hominids suggest that admixture may have shaped the evolution of many mammal species. Recent advances in next-generation sequencing have opened doors to population genomic studies not just in model organisms but also wildlife species, for which only limited genetic resources exist. We have used a next-generation sequencing strategy to detect more than 13 million genome-wide variants in the polar bear, brown bear and American black bear. While nuclear DNA markers reflect a species tree consistent with expectation, showing polar and brown bears to be sister species, we estimate that not only the mitochondrial genome of the enigmatic brown bears native to Alaska’s Alexander Archipelago, but also 5–10% of their nuclear genome is more closely related to polar bears than to other brown bears, indicating ancient admixture between the two species. Coalescent analyses suggest ancient splits among polar, brown and black bears providing sufficiently independent evolutionary histories to leave imprints in the polar bear nuclear genome associated with ecological adaptation to the Arctic environment. Further, paleodemographic estimates reveal that polar bears in particular experienced a prolonged and dramatic decline in its effective population size during the last few hundred thousand years. As such, bears provide an excellent system for investigating the evolutionary impact of  range contractions and expansions during times of climatic fluctuation.