July 8, 2011
Arctic Polar Bears Really Irish
A team of international scientists has found that all living polar bears descend from a single mama brown bear that lived 20,000 to 50,000 years ago in modern-day Ireland.
The scientists from Britain, Ireland and the U.S. analyzed DNA samples from polar bears in Alaska, Canada, Greenland, Norway and Russia, and compared them to DNA obtained from teeth and skeletons of 17 ancient brown bears that were found at eight cave sites across Ireland.
The results revealed that every individual Polar bear lineage could be traced back to an Irish forebear.
The analysis also showed that brown and polar bears mated periodically over the last 100,000 years, raising the possibility that such cross-species mingling played a role in their recent evolution, the researchers said.
This hybridization, or interbreeding, which caused maternal DNA from brown bears to be introduced into polar bears, was likely made possible by climate changes affecting the North Atlantic ice sheet that caused periodic overlaps in bear habitats, they said.
Polar and brown bears differ vastly in terms of body size, skin and coat color, fur type, tooth structure, and many other physical features.
Behaviorally, they are also quite distinct: Polar bears are expert swimmers that have adapted to a highly specialized, arctic lifestyle, while brown bears -- a species that includes Grizzlies and Kodiaks -- are climbers that prefer the mountain forests, wilderness regions, and river valleys of Europe, Asia, and North America.
"Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years," said Beth Shapiro, an Associate Professor of Biology at Penn State University and one of the study's team leaders.
"Most importantly, previous research has indicated that the brown bear contributed genetic material to the polar bear's mitochondrial lineage -- the maternal part of the genome, or the DNA that is passed exclusively from mothers to offspring. But, until now, it was unclear just when modern polar bears acquired their mitochondrial genome in its present form."
Although previous researcher had suggested that the ancient female ancestor of modern polar bears lived on the Alaskan islands of Admiralty, Baranof, and Chichagof just 14,000 years ago, Shapiro's team found evidence of a much earlier hybridization event.
Because of this event, the modern polar bear's mitochondrial DNA probably underwent something known as fixation -- a dramatic reduction in genetic variation -- and a transition to a state in which the entire gene pool includes only one form of a particular gene.
After conducting genetic analyses of 242 brown-bear and polar-bear mitochondrial lineages sampled throughout the last 120,000 years and across multiple geographic ranges, the researchers found that the fixation of the mitochondrial genome likely occurred during or just before the peak of the last ice age, possibly as early as 50,000 years ago, near present-day Ireland.
Shapiro noted that the specific population of brown bears that shared its maternal DNA with polar bears has been extinct for roughly 9,000 years. However, her data offer clear genetic evidence that the two species were in contact long before the brown bear's disappearance from the British Isles.
Shapiro explained that, although both polar bears and brown bears have experienced long periods of geographic stability, episodes of both warming and cooling during the last half a million years or more likely led to environmental conditions favorable to hybridization between the two bear species.
"Polar and brown bears likely came into contact intermittently, in particular in coastal regions where the effects of climate change may have been more pronounced," Shapiro said.
"Whenever they come into contact, there seems to be little barrier to their mating."
Such climate changes included large fluctuations in the amount and distribution of habitats in the North Atlantic region, which would have caused the geographic ranges of polar and brown bears to overlap temporarily.
For example, during a warming period, elevated air temperatures, melting glacial ice, and rising sea levels likely forced polar bears to spend more time onshore in search of food, and thus closer to their brown-bear neighbors. Likewise, during a cooling glacial period, brown bears living farther from the coast may have been forced into habitat normally occupied by polar bears.
The British-Irish Ice Sheet reached its maximum extent about 20,000 years ago, with major tidewater glaciers on the western shelf and down the Irish Sea Basin into the Celtic Sea. During this period, parts of Ireland were probably uninhabitable because of glaciation, pushing brown bears toward ice shelves and land exposed by lower sea levels.
"The bottom line is that the two species bumped up against one another for extended periods of time on different occasions, sharing both habitats and genes," Shapiro said.
Shapiro hopes to design future studies of the polar bear's DNA by concentrating on other parts of the animal's genome.
"Until now we have focused our efforts on the polar bear's mitochondrial DNA, which traces only the mother's side of the family tree," she said.
"But there is much to be learned from the nuclear genome -- the genetic material contained within the nucleus of the cell, which has been passed to offspring from both mothers and fathers."
A more complete investigation of this part of the genetic story could answer deeper questions about how interactions with other species and environmental changes affected polar bears in the distant past, how frequently hybridizations between species actually occurred, and how these hybridizations affected the genetic diversity of the polar bear generally, she said.
The study is expected to help future conservation efforts for polar bears, which are listed as threatened under the Endangered Species Act.
The research was published July 7 in the journal Current Biology. A summary can be viewed at http://www.cell.com/current-biology/abstract/S0960-9822(11)00645-2.
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