Insights into bear evolution from a Pleistocene polar bear genome

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Lan, Tianying, Leppälä, Kalle Matias, Tomlin, Crystal, Talbot, Sandra L., Sage, George K., Farley, Sean D., Shideler, Richard T., Bachmann, Lutz, Wiig, Øystein, Albert, Victor A., Salojärvi, Jarkko, Mailund, Thomas, Drautz-Moses, Daniela I., Schuster, Stephan C., Herrera-Estrella, Luis, Lindqvist, Charlotte
• Other Authors: Organismal and Evolutionary Biology Research Programme, University of Helsinki
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: 1181 Ecology; evolutionary biology; bear evolution; climate change; comparative genomics; hybridization; Ursus; SEA-ICE; PHYLOGEOGRAPHY; POPULATION HISTORY; ANCIENT ADMIXTURE; REVEAL; GENE FLOW; BROWN BEARS; URSUS-ARCTOS; GRIZZLY BEARS; Arctic; Svalbard; Svalbard Archipelago; Archipelago; Climate change; Sea ice; Ursus arctos; Ursus maritimus; Alaska
• Online Access: http://hdl.handle.net/10138/348204

The polar bear (Ursus maritimus) has become a symbol of the threat to biodiversity from climate change. Understanding polar bear evolutionary history may provide insights into apex carnivore responses and prospects during periods of extreme environmental perturbations. In recent years, genomic studies have examined bear speciation and population history, including evidence for ancient admixture between polar bears and brown bears (Ursus arctos). Here, we extend our earlier studies of a 130,000- to 115,000-y-old polar bear from the Svalbard Archipelago using a 10x coverage genome sequence and 10 new genomes of polar and brown bears from contemporary zones of overlap in northern Alaska. We demonstrate a dramatic decline in effective population size for this ancient polar bear's lineage, followed by a modest increase just before its demise. A slightly higher genetic diversity in the ancient polar bear suggests a severe genetic erosion over a prolonged bottleneck in modern polar bears. Statistical fitting of data to alternative admixture graph scenarios favors at least one ancient introgression event from brown bears into the ancestor of polar bears, possibly dating back over 150,000 y. Gene flow was likely bidirectional, but allelic transfer from brown into polar bear is the strongest detected signal, which contrasts with other published work. These findings may have implications for our understanding of climate change impacts: Polar bears, a specialist Arctic lineage, may not only have undergone severe genetic bottlenecks but also been the recipient of generalist, boreal genetic variants from brown bears during critical phases of Northern Hemisphere glacial oscillations. Peer reviewed