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Study Provides Fresh Take on Polar Bear History

NEW YORK (GenomeWeb News) – A PLOS Genetics study out last night offers a new explanation for the genetic similarities between polar bears and brown bears living on the Admiralty, Baranof, and Chichagof, or ABC, islands — pointing to a more straightforward view of polar bear evolution than theories proposed in the past.

"This population of [ABC] brown bears stood out as being really weird genetically, and there's been a long controversy about their relationship to polar bears," senior author Beth Shapiro, an ecology and evolutionary biology researcher at the University of California, Santa Cruz, said in a statement.

"We can now explain it," she continued, "and instead of the convoluted history some have proposed, it's a very simple story."

Shapiro and colleagues from the US, Russia, and Canada performed shotgun sequencing on genomic DNA from 10 polar, brown, or black bears. Using this and other genetic data, they came up with a model suggesting polar bears ventured south to the ABC island area during an ice age. There, female polar bears — potentially marooned by changing ice patterns — apparently met and mated with male brown bears from mainland Alaska.

Over time, this population of ABC island bears took on traits that were decidedly brown bear-like, the team proposed, though the animals share closer-than-usual genetic ties to polar bears, particularly in the mitochondrial genome and on the X chromosome.

If that theory holds up to future scrutiny, it suggests that while ABC brown bears have polar bear ancestry, most present-day polar bears probably carry little in the way of brown bear DNA — a prediction that fits with the limited genetic diversity detected in polar bears.

"These data reveal remarkable genetic homogeneity within polar bears and clear evidence of past hybridization with brown bears," Shapiro and co-authors wrote. "Hybridization, however, appears to be limited to habitat islands, where isolated populations of polar bears are gradually converted into brown bears via male-mediated dispersal and sex-biased gene flow."

Studies of polar bear evolution have produced a range of sometimes contradictory theories about the animals' history and ancestry, depending on the type of genetic analysis used and the bear populations considered.

Just last year, for instance, one study based on intronic nuclear DNA sequences suggested that polar bear and brown bear species diverged from one another some 600,000 years ago, while genomic sequence data from an independent team put the split even further back, at around four or five million years ago. Similarly, there has been debate about the extent to which admixture between the species has affected modern-day bear populations.

For the new study, researchers used Illumina's HiSeq 2000 to do random, shotgun sequencing on genomic DNA from 10 bears: seven polar bears and one representative each from ABC brown bear, mainland brown bear, and black bear populations.

Using composite haplotype profiles cobbled together from the resulting low-coverage sequence data for each animal — coupled with published genetic profiles — researchers saw signs that polar bear-related sequences were over-represented in certain parts of the ABC brown bear genome, namely the X chromosome.

An estimated 6.5 percent of sequences on the ABC brown bear's X chromosome are descended from polar bear DNA, they reported. In contrast, the ABC brown bear autosomal chromosomes carry around 1 percent polar bear sequence.

That X chromosome over-representation, coupled with the mitochondrial similarities between polar bears and the ABC brown bears, are consistent with more pronounced polar bear ancestry on the maternal side of the ABC brown bear family tree, authors of the study said.

Also, during their follow-up analyses, the researchers found that the genetic data were predicting gene flow in only one direction: from the polar bear population into the ABC brown bears via female polar bear and male brown bear mixing.

"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," the study's first author James Cahill, a graduate student in Shapiro's UCSC ecology and evolutionary biology lab, said in a statement.

If so, it means polar bear and brown bear species must have split some time prior to this encounter on the ABC islands, perhaps during the Pleistocene period, though the details of that divergence still need to be worked out.

"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," co-author Ian Stirling, a biologist at the University of Alberta, said in a statement.

"But, for the moment at least, we are back to not being certain where in the Arctic polar bears first diverged from brown bears," he said.

The new findings are also expected to force a re-interpretation of a mitochondrial DNA-based study that Shapiro and her colleagues published in Current Biology in 2011.

At the time, the group speculated that a shared maternal ancestry between polar bears and ancient Irish brown bears was an indicator of polar bear that could be traced back to present-day Ireland and Britain.

But Shapiro and co-authors on the current study now say that the patterns detected in the Irish brown bear may have been introduced through a model similar to that proposed for the ABC brown bears — in other words, as a consequence of gene flow from polar bears into brown bears in an area where the two already separated species came into contact.

"In retrospect, I think we were wrong about the directionality of the gene flow between polar bears and Irish brown bears," Shapiro said in a statement.