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No evi­dence of brown bear ances­try in polar bears, new DNA study says

pub­lished 17 March 2013 | mod­i­fied 08 March 2014

Polar bear arcticUnusual pop­u­la­tion of brown bears on Alaskan islands turns out to have a remark­able and reveal­ing his­tory

At the end of the last ice age, a pop­u­la­tion of polar bears was stranded by the reced­ing ice on a few islands in south­east­ern 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 (UCSC). The find­ings, pub­lished March 14 in PLOS Genet­ics, turn over pre­vail­ing ideas about the evo­lu­tion­ary his­tory of the two species, which are closely related and known to pro­duce fer­tile hybrids.

Pre­vi­ous stud­ies sug­gested that past hybridi­s­a­tion had resulted in all polar bears hav­ing genes that came from brown bears. But the new study indi­cates that episodes of gene flow between the two species occurred only in iso­lated pop­u­la­tions and did not affect the larger polar bear pop­u­la­tion, which remains free of brown bear genes.

This pop­u­la­tion of brown bears [on the ABC Islands] 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
Beth Shapiro, coau­thor, asso­ciate pro­fes­sor of ecol­ogy and evo­lu­tion­ary biol­ogy at UCSC »

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.

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.

ABC-bearA key find­ing is that the polar bear ances­try of ABC Islands brown bears is con­spic­u­ously enriched in the mater­nally inher­ited X chro­mo­some. About 6.5 per­cent of the X chro­mo­somes of the ABC Islands bears came recently from polar bears, com­pared to about 1 per­cent of the rest of their genome. This means that the ABC Islands brown bears share more DNA with polar bear females than they do with polar bear males, Shapiro said.

To under­stand how hybridi­s­a­tion 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
« James Cahill, lead author, grad­u­ate stu­dent in ecol­ogy and evo­lu­tion­ary biol­ogy at UCSC

This sce­nario is con­sis­tent with the known behav­iour of brown bears and polar bears, accord­ing to coau­thor Ian Stir­ling, a biol­o­gist at the Uni­ver­sity of Alberta in Edmon­ton, Canada. Mix­ing of polar bears and brown bears is seen today in the Cana­dian Beau­fort Sea, where adult male brown bears wan­der onto the remain­ing sea ice in late spring and some­times mate with female polar bears, he said. In areas such as west­ern Hud­son Bay and the Russ­ian coast, polar bears are spend­ing more time on land in response to cli­mate warm­ing and loss of sea ice, a behav­iour that could have left polar bears stranded on the ABC Islands at the end of the last ice age.

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 hybridised with polar bears stranded there when the sea ice dis­ap­peared.

“The com­bi­na­tion of genet­ics and the known behav­iour of brown and polar bears hybri­dis­ing 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 coau­thor Richard E. (Ed) Green, an assis­tant pro­fes­sor of bio­mol­e­c­u­lar 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.

The new study looks at the “nuclear DNA” car­ried on the chro­mo­somes in the cell nucleus. It is the lat­est in a series of genetic stud­ies of polar bears pub­lished in recent years, each of which has prompted new ideas about the rela­tion­ship between polar bears and brown bears. A 2010 study of fos­sils and mito­chon­dr­ial DNA sup­ported the idea that polar bears evolved from the ABC Islands brown bears. But a 2011 study of mito­chon­dr­ial DNA from extinct Irish brown bears showed an even closer match to polar bears and sug­gested that polar bears got their mito­chon­dr­ial DNA from hybridi­s­a­tion with Irish bears. Shapiro, a coau­thor of that study, said she now thinks the Irish brown bears may be another exam­ple of what hap­pened in the ABC Islands, but she can’t say for sure until she stud­ies their nuclear DNA.

“In ret­ro­spect, I think we were wrong about the direc­tion­al­ity of the gene flow between polar bears and Irish brown bears,” she said.

Two stud­ies pub­lished in 2012 sought to deter­mine when the polar bear lin­eage diverged from the brown bear lin­eage using nuclear DNA data. The first, pub­lished in April in Sci­ence, put the split at 600,000 years ago and con­cluded that polar bears carry brown bear mito­chon­dr­ial DNA due to past hybridi­s­a­tions. The sec­ond, pub­lished in July in Pro­ceed­ings of the National Acad­emy of Sci­ences, sug­gested that brown bears, black bears, and polar bears diverged around 4 to 5 mil­lion years ago, fol­lowed by repeated episodes of hybridi­s­a­tion between polar bears and brown bears.

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 hybridised with chimps. You would get very strange results until you fig­ured that out,” he said.

(Source: UCSC Press Release, 14.03.2013)

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