From Our 2011 Archives

Female Ancestor of All Living Polar Bears Was Brown

THURSDAY, July 7 (HealthDay News) -- The female ancestor of all living polar bears was a brown bear that lived in the vicinity of present-day Britain and Ireland 20,000 to 50,000 years ago, according to an international team of scientists.

It's likely that climate changes that affected the North Atlantic ice sheet led to periodic overlaps in bear habitats and interbreeding between brown bears and polar bears. This resulted in maternal DNA from brown bears being introduced into polar bears, explained team co-leader Beth Shapiro, an associate professor of biology at Penn State University.

Previous research suggested that the ancient female ancestor of modern polar bears lived on three Alaskan islands (Admiralty, Baranof, and Chichagof) only 14,000 years ago.

Polar bears and brown bears such as Grizzlies and Kodiaks are markedly different species in terms of behavior and physical features such as body size, skin and coat color, fur type, tooth structure.

"Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years," Shapiro said in a Penn State news release. "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."

For this study, Shapiro and her colleagues analyzed 242 brown bear and polar bear mitochondrial lineages from a number of regions. The specific population of brown bears that contributed maternal DNA to modern polar bears has been extinct for about 9,000 years, but there's clear genetic evidence that these brown bears and polar bears were in contact long before brown bears vanished from the British Isles.

Learning more about the polar bear's genetic history and its response to previous environmental changes may help guide efforts to conserve the dwindling population of polar bears, Shapiro said.

The study was published July 7 in the journal Current Biology.

-- Robert Preidt

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SOURCE: Penn State University, news release, July 7, 2011