Polar bears linked to Irish cross-species mating

They look to be pure white, but the polar bears inhabiting Canada’s Arctic apparently have faint traces of brown — and a dash of shamrock green — in their DNA. An international team of researchers has linked the ancestry of every polar bear in the world — including Canada’s population of about 15,000 — to the skeletal remains of an extinct species of brown bear from Ireland. (PHOTO BY JANE GEORGE)

RANDY BOSWELL
Postmedia News

They look to be pure white, but the polar bears inhabiting Canada’s Arctic apparently have faint traces of brown — and a dash of shamrock green — in their DNA.

An international team of researchers has linked the ancestry of every polar bear in the world — including Canada’s population of about 15,000 — to the skeletal remains of an extinct species of brown bear from Ireland.

The scientists have found that the genetic material in every living polar bear contains a telltale marker indicating they all descended from a single mating about 50,000 years ago between a prehistoric male member of the species and a female Eurasian brown bear on the Emerald Isle.

The finding, the researchers say, sheds important new light on the way bears adapted to habitat upheaval during the Ice Age and on how present-day climate change could also lead to more inter-species breeding — examples of which are already being observed in Canada, where grizzly-polar bear hybrids have been reported in recent years.

While polar bears and grizzly bears are known to share an ancient genetic history, the much larger, seal-eating Arctic bear — which can grow to 800 kilograms — has evolved to become an expert swimmer and lord of its icy domain. Meanwhile, salmon-loving grizzlies, which can weigh up to 500 kilograms, have adapted to the warmer habitats found on the northwest Pacific Coast and the mountain wilderness of the Canadian Rockies.

“Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years,” team leader Beth Shapiro, a Penn State University biologist, said in a summary of the study published in the journal Current Biology.

“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, Shapiro added, “it was unclear just when modern polar bears acquired their mitochondrial genome in its present form.”

Genetic material for the study was assembled from museum collections representing hundreds of individual polar bear specimens spanning tens of thousands of years and locations throughout the northern hemisphere.

The 18-member research team included biologists and paleontologists from the U.S., Britain, Ireland, Denmark, Belgium, Russia, Spain and Sweden.

The scientists concluded that “opportunistic mating” between polar bears and brown bears may have occurred regularly in Ireland and Great Britain during the ebb and flow of glaciers that occurred before the end of the last Ice Age about 10,000 years ago.

“Today, the Arctic climate is again changing rapidly, and the habitat of brown and polar bears is once again beginning to overlap, providing the opportunity for the two species to hybridize,” the team notes in the Current Biology paper, adding that conservation measures aimed at protecting the polar bear should pay special attention to the role hybrids may play in the species’ survival over time.

“It may be appropriate to reconsider protection of hybrids, as they may play an underappreciated role in the survival of species,” the researchers state. “Our results suggest that, although the genetic mixing observed in bears today may be an important component of the long-term evolution of the polar bear, brown and polar bears have remained evolutionarily distinct lineages over geological time, suggesting that they are likely to remain as such in the future.”