DNA study clarifies how Polar bears and brown bears are related25/03/2013 00:01:17 "In retrospect, I think we were wrong about the directionality of the gene flow between polar bears and Irish brown bears," she said.
March 2013. At the end of the last ice age, a population of polar bears was stranded by the receding ice on a few islands in south-eastern Alaska. Male brown bears swam across to the islands from the Alaskan mainland and mated with female polar bears, eventually transforming the polar bear population into brown bears.
Evidence for this surprising scenario emerged from a new genetic study of polar bears and brown bears led by researchers at the University of California, Santa Cruz. The findings, published March 14 in PLOS Genetics, upend prevailing ideas about the evolutionary history of the two species, which are closely related and known to produce fertile hybrids.
At the centre of the confusion is a population of brown bears that live on Alaska's Admiralty, Baranof and Chicagof Islands, known as the ABC Islands. These bears--clearly brown bears in appearance and behaviour--have striking genetic similarities to polar bears.
"This population of brown bears stood out as being really weird genetically, and there's been a long controversy about their relationship to polar bears. We can now explain it, and instead of the convoluted history some have proposed, it's a very simple story," said co-author Beth Shapiro, associate professor of ecology and evolutionary biology at UC Santa Cruz.
Shapiro and her colleagues analysed genome-wide DNA sequence data from seven polar bears, an ABC Islands brown bear, a mainland Alaskan brown bear, and a black bear. The study also included genetic data from other bears that was recently published by other researchers. Shapiro's team found that polar bears are a remarkably homogeneous species with no evidence of brown bear ancestry, whereas the ABC Islands brown bears show clear evidence of polar bear ancestry.
More in common with female Polar bears than males
To understand how hybridization could lead to this unexpected result, the team ran simulations of various demographic scenarios. "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," said first author James Cahill, a graduate student in ecology and evolutionary biology at UC Santa Cruz.
Cross mating still occurs occasionally
Young male brown bears tend to leave the area where they were born in search of new territory. They may well have dispersed across the water from the Alaskan mainland to the ABC Islands and hybridized with polar bears stranded there when the sea ice disappeared."The combination of genetics and the known behaviour of brown and polar bears hybridizing in the wild today tells us how the ABC Islands bears came to be: they are the descendants of many male brown bear immigrants and some female polar bears from long ago," Stirling said.
The findings suggest that continued climate warming and loss of arctic sea ice may lead to the same thing happening more broadly, said co-author Richard E. (Ed) Green, an assistant professor of bio molecular engineering in UCSC's Baskin School of Engineering. "As the ice melts in the Arctic, what is going to happen to the polar bears? In the ABC Islands, the polar bears are gone. They're brown bears now, but with polar bear genes still present in their genomes," he said.
The first genetic studies of ABC Islands brown bears looked at their mitochondrial DNA, which is separate from the chromosomes and is inherited only through the female lineage. The mitochondrial DNA of ABC Islands brown bears matches that of polar bears more closely than that of other brown bears, which led some scientists to think that the ABC Islands brown bears gave rise to modern polar bears.
"In retrospect, I think we were wrong about the directionality of the gene flow between polar bears and Irish brown bears," she said.
The new study does not address the question of how long ago polar bears diverged from brown bears, but it may help sort out the conflicting results of recent studies. "It's a good step in the right direction of understanding what really happened," Shapiro said.
The study does indicate that the divergence of polar bears from brown bears was only half as long ago as the split between the brown bear and black bear lineages, said Cahill. "We can tell how long brown bears and polar bears have been separate species as a proportion of how long ago they separated from more distantly related species, but putting a year on it is very difficult," he said.
Green noted that efforts to understand the relationship between polar bears and brown bears has been complicated by the unusual case of the ABC Islands brown bears. "It's as if you were studying the relationship between humans and chimpanzees and your analysis included DNA from some weird population of humans that had hybridized with chimps. You would get very strange results until you figured that out," he said.
In addition to Cahill, Green, Shapiro, and Stirling, the co-authors of the new paper include postdoctoral researchers Tara Fulton and Mathias Stiller, undergraduate Rauf Salamzade, and graduate student John St. John at UC Santa Cruz; Flora Jay and Montgomery Slatkin at UC Berkeley; and Nikita Ovsyanikov at the Wrangel Island State Nature Reserve in Russia. Green and Shapiro direct the UCSC Paleogenomics Lab. This research was funded by the Searle Scholars Program.