Brown bears on an Alaskan archipelago are the descendants of an ancient polar bear population rather than being the ancestors of modern polar bears, a new study has found.
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,” Beth Shapiro, Associate Professor at UC Santa Cruz, who led the research, said.
“Focusing on the X gave us a surprising result,” she said.
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 percent of the X chromosomes of the ABC Islands bears had recently come from polar bears.
In contrast, only about 1 percent 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.
The research is published in the journal PLOS Genetics.