Polar bears' family secrets revealed with DNA sequencing

Scientists have long struggled to understand the exact nature of the evolutionary relationship between brown bears and their arctic descendants. It is known that 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.

Previous analyses of DNA sequences have yielded conflicting results on this question. At the center of the controversy are a group of brown bears that live on the Admiralty, Baranof and Chichagof (ABC) Islands of southeastern Alaska. These bears -- clearly brown bears in appearance and behavior -- inexplicably carry mitochondrial 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.

"The key to solving this mystery was to analyze DNA from the ABC Islands bears' nuclear genomes, and in particular their X-chromosomes," explained Beth Shapiro, Associate Professor at UC 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 found that around 6.5% of the X chromosomes of the ABC Islands bears had recently come from polar bears. In contrast, only about 1% of the rest of the genome of the ABC Islands brown bears had come from polar bears.

The team simulated various scenarios to determine the most likely evolutionary history for the ABC Islands bears. The results suggested a situation that differs considerably from any previously imagined for these bears. They concluded that the ABC Islands bears descended from polar bears that were gradually converted into brown bears through hybridization with male brown bears dispersing from the Alaskan mainland.

"During a previous ice age, polar bears ranged much farther to the south than they do today, reaching the present-day ABC Islands and Alexander Archipelago," said Ian Stirling, an Adjunct Professor at the University of Alberta in Edmonton, Canada. "As the climate warmed and ice began to retreat, it is possible that some of the animals began to spend progressively longer on land with reduced access to ice. We see the same sort of thing happening today with polar bears in areas such as western Hudson Bay or the Russian coast, in response to continued climate warming and loss of ice."

"One of the most exciting results from this study is that we now finally understand what happened between the polar and brown bears on the ABC islands," said Stirling. "But, for the moment at least, we are back to not being certain where in the Arctic polar bears first diverged from brown bears."