title: Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change.
abstract: Polar bears (PBs) are superbly adapted to the extreme Arctic environment and have become emblematic of the threat to biodiversity from global climate change. Their divergence from the lower-latitude brown bear provides a textbook example of rapid evolution of distinct phenotypes. However, limited mitochondrial and nuclear DNA evidence conflicts in the timing of PB origin as well as placement of the species within versus sister to the brown bear lineage. We gathered extensive genomic sequence data from contemporary polar, brown, and American black bear samples, in addition to a 130,000- to 110,000-y old PB, to examine this problem from a genome-wide perspective. Nuclear DNA markers reflect a species tree consistent with expectation, showing polar and brown bears to be sister species. However, for the enigmatic brown bears native to Alaska's Alexander Archipelago, we estimate that not only their mitochondrial genome, but also 5-10% of their nuclear genome, is most closely related to PBs, indicating ancient admixture between the two species. Explicit admixture analyses are consistent with ancient splits among PBs, brown bears and black bears that were later followed by occasional admixture. We also provide paleodemographic estimates that suggest bear evolution has tracked key climate events, and that PB in particular experienced a prolonged and dramatic decline in its effective population size during the last ca. 500,000 years. We demonstrate that brown bears and PBs have had sufficiently independent evolutionary histories over the last 4-5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment.
authors: Center for Comparative Genomics and Bioinformatics, Pennsylvania State University, University Park, PA 16802, USA.
authors: Webb
authors: W
authors: Miller
authors: Stephan C
authors: SC
authors: Schuster
authors: Andreanna J
authors: AJ
authors: Welch
authors: Aakrosh
authors: A
authors: Ratan
authors: Oscar C
authors: OC
authors: Bedoya-Reina
authors: Fangqing
authors: F
authors: Zhao
authors: Hie Lim
authors: HL
authors: Kim
authors: Richard C
authors: RC
authors: Burhans
authors: Daniela I
authors: DI
authors: Drautz
authors: Nicola E
authors: NE
authors: Wittekindt
authors: Lynn P
authors: LP
authors: Tomsho
authors: Enrique
authors: E
authors: Ibarra-Laclette
authors: Luis
authors: L
authors: Herrera-Estrella
authors: Elizabeth
authors: E
authors: Peacock
authors: Sean
authors: S
authors: Farley
authors: George K
authors: GK
authors: Sage
authors: Karyn
authors: K
authors: Rode
authors: Martyn
authors: M
authors: Obbard
authors: Rafael
authors: R
authors: Montiel
authors: Lutz
authors: L
authors: Bachmann
authors: Olafur
authors: O
authors: Ingólfsson
authors: Jon
authors: J
authors: Aars
authors: Thomas
authors: T
authors: Mailund
authors: Oystein
authors: O
authors: Wiig
authors: Sandra L
authors: SL
authors: Talbot
authors: Charlotte
authors: C
authors: Lindqvist
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics*; D000818:Animals; D001110:Arctic Regions; D001483:Base Sequence; D057231:Climate Change / Q000266:history*; D019143:Evolution, Molecular; D005819:Genetic Markers / Q000235:genetics; D005828:Genetics, Population; D016678:Genome / Q000235:genetics*; D049690:History, Ancient; D008969:Molecular Sequence Data; D011156:Population Density; D011157:Population Dynamics; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D001503:Ursidae / Q000235:genetics*
chemical_list: D005819:Genetic Markers