DCU researchers have contributed to an international research project comparing genomes of polar and brown bears which reveal 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 has uncovered several genes that may be involved in the polar bear's extreme adaptations to life in the high Arctic.
Dr Mary O’Connell and Dr Claire Morgan, along with colleagues in NUI Maynooth, were involved in the international collaboration to sequence and analyse the polar bear, assembling large datasets of genes to assess what pathways and proteins underwent selective pressure to change in response to the colonisation of this extreme environment.
"Studying the typical model organisms such as mouse and rat has yielded lots of important insights that have progressed our understanding of genetics and life sciences - but non-model organisms such as polar bear can have secrets locked away in their genomes that are valuable to explore and understand. These secrets can not only increase our fundamental understanding of life but they also hold promise for disease biology and biomedical sciences", says Dr O’Connell.
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, which is published today in Cell, the international research journal, 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.
Dr O’Connell continued, "I am driven to identify and understand species-specific DNA changes that convey resistance to certain diseases and that make each species unique. For this reason, the polar bear was of particular interest to me. Obesity and high fat-high cholesterol diets are standard for a polar bear - how is this possible without being lethal? What changes have occurred in its genome to allow this to be a benign and indeed advantageous state?"
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 long 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 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.
"The polar bear has been considered an indicator species for some time, that is to say, how well this species is doing is a direct reflection of how well our planet is. And so polar bears and climate change have become synonymous, yet we didn’t know where they came from, when they emerged or how they survive their harsh habitat", says Dr O’Connell.
"Understanding when and how this species emerged, and what genomic changes were required to facilitate adaptation to the extreme High Arctic environment are important questions to address - the answers give us insight into how quickly a species can become adapted to a new environment – we estimate it was within ~20,000 generations. Using genomic data from 89 individual bears we have been able to determine where polar bears came from, when they emerged and we have been able to see that their split from brown bears was a messy one with some subsequent interbreeding ", says Dr O’Connell, "and in addition, we have been able to determine the changes in their DNA that occurred in their cardiovascular system to permit their high fat and high cholesterol diets".
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; researchers at BGI in China, including Shiping Liu, Guojie Zhang and Jun Wang, who sequenced the genomes and analyzed the data with a team of researchers at UC Berkeley, including Eline Lorenzen, Matteo Fumagalli and Rasmus Nielsen.