Population genomics study provides insights into how polar bears adapt to the Arctic

(Press-News.org) May 8, 2014, Shenzhen, China – In a paper published in the May 8 issue of the journal Cell as the cover story, researches from BGI, University of California, University of Copenhagen and other institutes presented the first polar bear genome and their new findings about how polar bear successfully adapted to life in the high Arctic environment, and its demographic history throughout the history of its adaptation.

Polar bears are at the top of the food chain, and spend most of their lifetimes on the sea ice largely within the Arctic Circle. They were well known to the worldwide people for their white coat, big body size, thick layer of body fat and the ability of long distance swimming. However, the current living situation of polar bear is not that optimistic as people used to think. They have been facing the challenges and threats from the climate change in recently years, and the key danger posed by climate change is the unceasing lost habitats. Therefore, polar bear is always serving as an important indicator of climate change due to their dependence on sea ice in the Arctic.

The analysis in the study included blood and tissue samples from 79 Greenlandic polar bears and 10 brown bears from Sweden, Finland, Glacier National Park in Alaska and the Admiralty, Baranof, and Chichagof (ABC) Islands off the Alaskan coast.

The results reveal that the polar bear is a much younger species than previously believed, having diverged from brown bears less than 500,000 years ago, and also uncovered several genes that may be involved in the polar bears' extreme adaptations to life in the high Arctic.

The genes pinpointed by this 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.

"For polar bears, profound obesity is a benign state," said Eline Lorenzen, one of the lead authors of the study. "We wanted to understand how they are able to cope with that."

The genome analysis comes at a time when the polar bear population worldwide, estimated at between 20,000 and 25,000 individuals, is declining and its habitat, Arctic sea ice, is rapidly disappearing. As the northern latitudes warm, its distant cousin the brown or grizzly bear (Ursus arctos) is moving farther north and occasionally interbreeding with the polar bear (U. maritimus) to produce hybrids dubbed pizzlies.

"Their ability to interbreed is a result of this very close relationship", Rasmus Nielsen, one of the corresponding authors, 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.

"It's really surprising that the divergence time is so short. All the unique adaptations polar bears have to the Arctic environment must have evolved in a very short amount of time," he said. These adaptations include not only a change from brown to white fur and development of a sleeker body, but big physiological and metabolic changes as well. "There has been a lot of debate about it, but I think we really nailed down what the divergence time is between them, and it is surprisingly recent."

The genome comparison reveals that over several hundred thousand years, natural selection drove major changes in genes related to fat transport in the blood and fatty acid metabolism. One of the most strongly selected genes is APOB, which in mammals encodes the main protein in LDL (low density lipoprotein), known widely as "bad" cholesterol. Changes or mutations in this gene reflect the critical nature of fat in the polar bear diet and the animal's need to deal with high blood levels of glucose and triglycerides, in particular cholesterol, which would be dangerous in humans. Fat comprises up to half the weight of a polar bear.

"The life of a polar bear revolves around fat," Lorenzen added. "Nursing cubs rely on milk that can be up to 30 percent fat and adults eat primarily blubber of marine mammal prey. Polar bears have large fat deposits under their skin and, because they essentially live in a polar desert and don't have access to fresh water for most of the year, rely on metabolic water, which is a byproduct of the breakdown of fat."

She noted that the evolution of a new metabolism to deal with high dietary fat must have happened very quickly, in just a few hundred thousand years, because we know that polar bears already subsisted on a marine diet 100,000 years ago.

What drove the evolution of polar bears is unclear, though the split from brown bears (dated at 343,000–479,000 years ago) coincides with a particularly warm 50,000-year interglacial period known as Marine Isotope Stage 11. Environmental shifts following climate changes could have encouraged brown bears to extend their range much farther north. When the warm interlude ended and a glacial cold period set in, however, a pocket of brown bears may have become isolated and forced to adapt rapidly to new conditions.

"The next step will be to look back in time by sequencing genomes from ancient polar bears, to address the evolutionary mode and nature of these adaptations", said professor Eske Willerslev from Centre for GeoGenetics, University of Copenhagen.

"The evolution history of polar bear demonstrated how quickly an organism could adapt to the evolutionary challenges ", said Jun Wang, "This is one of the exciting animal genome projects that initiated by BGI. With genomic data released for more species, we hope to reveal a more complete picture of how animals adapt to environment and how biodiversity forms".

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