A comparison of the genomes of polar bears and brown bears reveals that the polar bear is a much younger species than previously believed, having diverged from brown bears less than 500,000 years ago.
The analysis also uncovered several genes that may be involved in the polar bears’ extreme adaptations to life in the high Arctic. The species lives much of its life on sea ice, where it subsists on a blubber-rich diet of primarily marine mammals.
The genes pinpointed by the study are related to fatty acid metabolism and cardiovascular function, and may explain the bear’s ability to cope with a high-fat diet while avoiding fatty plaques in their arteries and the cardiovascular diseases that afflict humans with diets rich in fat. These genes may provide insight into how to protect humans from the ill effects of a high-fat diet.
The study was a collaboration between Danish researchers, led by Eske Willerslev and including Rune Dietz, Christian Sonne and Erik W. Born, who provided polar bear blood and tissue samples; and researchers at BGI in China, including Shiping Liu, Guojie Zhang and Jun Wang, who sequenced the genomes and analyzed the data together with a team of UC Berkeley researchers, including Eline Lorenzen, Matteo Fumagalli and Rasmus Nielsen. Nielsen is a UC Berkeley professor of integrative biology and of statistics.
“For polar bears, profound obesity is a benign state,” said Lorenzen, one of the lead authors and a UC Berkeley postdoctoral fellow. “We wanted to understand how they are able to cope with that.”
“The promise of comparative genomics is that we learn how other organisms deal with conditions that we also are exposed to,” said Nielsen, a member of UC Berkeley’s Center for Theoretical Evolutionary Genomics. “For example, polar bears have adapted genetically to a high fat diet that many people now impose on themselves. If we learn a bit about the genes that allows them to deal with that, perhaps that will give us tools to modulate human physiology down the line.”
The findings accompany the publication of the first assembled genome of the polar bear as the cover story in the May 8 issue of the journal Cell.
The genome analysis comes at a time when the polar bear population worldwide, estimated at between 20,000 and 25,000, is declining and its habitat, Arctic sea ice, is rapidly disappearing. As the northern latitudes warm, the polar bear’s distant cousin, the brown or grizzly bear (Ursus arctos), is moving farther north and occasionally interbreeding with the polar bear (Ursus maritimus) to produce hybrids dubbed pizzlies.
Their ability to interbreed is a result of this very close relationship, Nielsen said, which is one-tenth the evolutionary distance between chimpanzees and humans. Previous estimates of the divergence time between polar bears and brown bears ranged from 600,000 to 5 million years ago.
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