redOrbit Staff & Wire Reports – Your Universe Online
Mutations in genes involved in cardiovascular function allowed polar bears to rapidly evolve the ability to consume a fatty, blubber-rich diet without developing high rates of heart disease, an international team of scientists reported this week in the journal Cell.
Moreover, the study revealed that polar bears diverged from brown bears less than 500,000 years ago – much more recently than estimates based on previous genomic data.
The researchers obtained 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 revealed that the polar bear is a much younger species than previously believed, and also uncovered several genes that may be involved in the polar bears’ extreme adaptations to life in the high Arctic.
The genes identified in 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 also provide insight into how humans might be protected from the ill effects of a high-fat diet, the researchers said.
“For polar bears, profound obesity is a benign state,” said Eline Lorenzen, a postdoctoral fellow at the University of California, Berkeley, and one of the lead authors of the study, in a recent statement.
“We wanted to understand how they are able to cope with that.”
The genome analysis comes at a time when the global polar bear population, estimated at between 20,000 and 25,000 individuals, is declining and its Arctic habitat 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, which is one-tenth the evolutionary distance between chimpanzees and humans,” said study author Rasmus Nielsen.
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,” Nielsen said.
These adaptations include not only a change from brown to white fur and the development of a sleeker body, but significant 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,” Nielsen said.
The genome comparison revealed 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. 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 said.
“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.”
Lorenzen noted that the evolution of a new metabolism to deal with high dietary fat must have happened fairly fast, 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, although 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. However, when the warm interlude ended and a glacial cold period set in, 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 at the University of Copenhagen, in a separate statement from BGI Shenzhen.
“The evolution history of polar bear demonstrated how quickly an organism could adapt to the evolutionary challenges,” said Jun Wang, who sequenced the genomes and analyzed the data together with UC Berkeley researchers.
“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.”
Lorenzen said the “key that allowed us to unlock the door of polar bear evolution” was a method devised by UC Berkeley mathematics graduate student Kelley Harris, which has been used to estimate human demographic history. That approach, referred to as the identity by state (IBS) tract method, has proved powerful in estimating when ancient human populations diverged, their past population sizes and when and how much they interbred.