Polar bears on family tree of isolated Alaskan brown bears: new research

This brown bear on an Alaskan island chain is descended from an ancient polar bear population, new research says. (HANDOUT PHOTO)

Brown bears on an Alaskan island chain are the descendants of an ancient polar bear population, new research published in PLOS Genetics shows.

Scientists have struggled to understand the nature of the evolutionary relationship between brown bears and today’s polar bears, said a news release about the research.

These two species can mate successfully in captivity and in the wild.

But how much of their genetic histories are the result of past interbreeding has remained a puzzle.

At the center of the controversy: a group of brown bears that live on the Admiralty, Baranof and Chichagof (ABC) Islands of southeastern Alaska.

These bears, brown bears in appearance and behaviour, carry DNA that match polar bears more closely than other brown bears.

This observation led some researchers to conclude that the ABC Islands brown bears gave rise to modern day polar bears.

But research shows that it was the other way around, with the ABC polar bears interbreeding with brown bears in the past.

“The key to solving this mystery was to analyze DNA from the ABC Islands bears’ nuclear genomes, and in particular their X-chromosomes,” said Beth Shapiro from the University of California at Santa Cruz, who led the research. “Focusing on the X gave us a surprising result.”

The team compared the X chromosomes of the ABC Islands brown bears to the X chromosome of brown bears from the Alaskan mainland.

They concluded that the ABC Islands bears descended from polar bears that were gradually converted into brown bears through interbreeding with male brown bears from the Alaskan mainland.

Other scientists have also 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 Ireland.

They 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.

Interbreeding between species has also taken place in zoos, where zookeepers experimented with breeding zebras and donkeys to make “zedonks” and lions and tigers to produce “tigons.” Most were dead-ends and could not reproduce.

But polar bears and grizzly bears were bred more than 100 years ago in German zoos — and unlike most other hybrids, the females were able to give birth to other beige-coloured bears.

A “pizzly,” shot in 2006 on Banks Island, had the head and neck of a polar bear, but was the size of a grizzly. Genetic analysis showed the bear’s mother was a polar bear and its father was a grizzly. Both bears mate several times before the female can become fertile, so the two bears would been together for at least a week, biologists said.