Planet Earth

Study Shakes the Polar Bear Family Tree

D-briefBy Gemma TarlachMar 14, 2013 8:00 PM

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Scientists received a surprising answer when they asked an isolated population of brown bears in southeastern Alaska “who’s your daddy?” Although previous research suggested the brown bears living on the Admiralty, Baranof and Chichagof (ABC) Islands were the ancestors of modern polar bears, the ABC bears are actually descendants of ancient polar bears, according to the latest study

, published today in PLOS Genetics. The first ABC bears, the study suggests, were polar bears stranded on the islands as ice receded at the end of the last ice age, more than 10,000 years ago. Hybridization with wandering brown bears resulted in today’s genetically distinct population of brown bears carrying a high proportion of polar bear DNA, concluded researchers.

An ABC brown bear. Image courtesy of Michael Dobson The ABC brown bear population was the focus of a study

published in 2010 that suggested, based on fossil records and mitochondrial DNA, that ABC bears gave rise to the modern polar bear. Mitochondrial DNA is separate from chromosomes and inherited only through the female lineage, however, which may have skewed the earlier study’s findings. The study published today examined “nuclear DNA”---DNA on the chromosomes in the cell nucleus---in seven polar bears, one ABC brown bear, a mainland Alaska brown bear and a black bear. The polar bears’ DNA showed no evidence of brown bear ancestry, which refutes the findings of the 2010 study. In addition, tests found that about 6.5 percent of the ABC bears' X chromosome came from modern polar bears, compared to about 1 percent of the rest of the ABC bear genome, leading the new study’s research team to conclude that the ABC bear population shared more DNA with modern polar bear females than with polar bear males. The team compared a number of scenarios that might have explained why the ABC brown bears showed a higher percentage of polar bear X chromosome. According to the study’s lead author James Cahill, “Of all the models we tested, the best supported was the scenario in which male brown bears wandered onto the islands and gradually transformed the population from polar bears into brown bears.” This scenario is consistent with modern brown bear behavior, since young males typically leave the area in which they were born to establish a new territory. Aside from redefining our understanding of modern polar bear ancestry, this finding may predict the ultimate fate of the species: as the loss of arctic sea ice and natural polar bear habitat continues, increased hybridization similar to what happened on the ABC islands could result in a widespread population of brown bears with polar bear DNA. Top image courtesy Vladimir Melnik / Shutterstock

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