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A study appear­ing in the cur­rent issue of the jour­nal “Sci­ence” reveals that polar bears evolved as early as some 600,000 years ago. An inter­na­tional team of scientists

led by researchers from the Ger­man Bio­di­ver­sity and Cli­mate Research Cen­tre (BiK-​F) shows the largest arc­tic car­ni­vore to be five times older than pre­vi­ously rec­og­nized. The new find­ings on the evo­lu­tion­ary his­tory of polar bears are the result of an analy­sis of infor­ma­tion from the nuclear genome of polar and brown bears, and shed new light on con­ser­va­tion issues regard­ing this endan­gered arc­tic specialist.

Polar bears are uniquely spe­cial­ized for life in the arc­tic. This fact is undis­puted, and sup­ported by a range of mor­pho­log­i­cal, phys­i­o­log­i­cal and behav­ioural evi­dence. How­ever, con­duct­ing research on the evo­lu­tion­ary his­tory of polar bears is dif­fi­cult. The arc­tic giant spends most of its life on sea ice, and typ­i­cally also dies there. Its remains sink to the sea floor, where they get ground up by glac­i­ers, or remain undis­cov­ered. Fos­sil remains of polar bears are there­fore scarce. Because the genetic infor­ma­tion con­tained in each organ­ism car­ries a lot of infor­ma­tion about the past, researchers can study the his­tory of the species by look­ing at the genes of today’s polar bears.

Analy­sis of the genetic infor­ma­tion in the cell nucleus
Recent stud­ies had sug­gested that the ances­tor of polar bears was a brown bear that lived some 150,000 years ago, in the late Pleis­tocene. That research was based on DNA from the mito­chon­dria — organelles often called the ‘pow­er­houses of the cell’. Researchers from the Ger­man Bio­di­ver­sity and Cli­mate Research Cen­tre (BiK-​F), together with sci­en­tists from Spain, Swe­den and the USA, now took an in-​depth look at the genetic infor­ma­tion con­tained in the cell nucleus. Frank Hailer, BiK-​F, lead author of the study explains: “Instead of the tra­di­tional approach of look­ing at mito­chon­dr­ial DNA we stud­ied many pieces of nuclear DNA that are each inde­pen­dently inher­ited. We char­ac­ter­ized those pieces, or genetic mark­ers, in mul­ti­ple polar and brown bear individuals”.

Polar bears had much more time for adap­ta­tion and spe­ci­a­tion than pre­vi­ously assumed
This genetic sur­vey was well worth the effort — the infor­ma­tion obtained from nuclear DNA indi­cates that polar bears actu­ally evolved in the mid Pleis­tocene, some 600,000 years ago. This pro­vides much more time for the polar bear ances­tors to col­o­nize and adapt to the harsh con­di­tions of the arc­tic. Based on stud­ies of mito­chon­dr­ial DNA, polar bears had ear­lier been con­sid­ered an exam­ple of sur­pris­ingly rapid adap­ta­tion of a mam­mal to colder cli­mates. The polar bear’s spe­cific adap­ta­tions, includ­ing its black skin, white fur, and fur-​covered feet now seem less sur­pris­ing. “In fact, the polar bear genome har­bours a lot of dis­tinct genetic infor­ma­tion”, says Hailer, “which makes a lot of sense, given all the unique adap­ta­tions in polar bears”.

Mater­nally inher­ited (mito­chon­dr­ial) DNA was show­ing a biased pic­ture
Pre­vi­ous stud­ies of mito­chon­dr­ial DNA had indi­cated that polar bears are much younger as a species. The authors of the new paper in “Sci­ence” explain this appar­ent dis­crep­ancy with past events of hybridiza­tion between polar and brown bears — a process recently observed in the Cana­dian arc­tic. After their ini­tial spe­ci­a­tion, polar bears and brown bears came into con­tact again, maybe due to past cli­matic fluc­tu­a­tions, as was shown in a recent study (more infor­ma­tion here). The mito­chon­dr­ial DNA found in polar bears today was prob­a­bly inher­ited from a brown bear female that hybridized with polar bears at some point in the late Pleis­tocene. It appears that much of the nuclear genome remained unaf­fected by hybridiza­tion, so polar bears retained their genetic dis­tinc­tive­ness. “Each part of the genome tells its own story. In our study we analysed nuclear DNA that is inher­ited from both par­ents. It pro­vides a more detailed and accu­rate pic­ture of the evo­lu­tion­ary his­tory of a species than mito­chon­dr­ial DNA that is inher­ited only from the mother”, says Axel Janke, BiK-​F, senior author on the study who also headed the recent sequenc­ing of the brown bear genome. He goes on to say: “Infer­ring a species’ evo­lu­tion­ary his­tory based on mito­chon­dr­ial DNA alone is like solv­ing a puz­zle with only a few of the many avail­able pieces. You need to study many genetic mark­ers (loci) to put together the full picture.”

« Frank Hailer, lead author of the cur­rent study

Genome car­ries evi­dence of past cli­mate fluc­tu­a­tions
The new genetic data indi­cate that polar bears went through tough times over the course of their 600,000 year-​old evo­lu­tion­ary his­tory. Polar bears show much less genetic diver­sity than brown bear. This is prob­a­bly due to dra­matic reduc­tions in pop­u­la­tion size in the past. Maybe those times coin­cided with phases of cli­matic warm­ing. Whether polar bears will be able to sur­vive the cur­rent phase of sea ice melt­ing is not clear. Firstly, human impacts are accel­er­at­ing the rate of cli­mate change, and the arc­tic could reach higher tem­per­a­tures than in pre­vi­ous inter­glacial warm phases. In addi­tion, numer­ous human-​related issues are threat­en­ing the polar bear today. Polar bears col­o­niz­ing coastal regions due to sea ice melt­ing fre­quently encounter human habi­tat, and many bears are killed. Besides per­se­cu­tion, polar bears are also fac­ing other — evo­lu­tion­ar­ily novel — threats, includ­ing pol­lu­tion by per­sis­tent chem­i­cals in the food chain.

The above news item is reprinted from mate­ri­als avail­able at Senck­en­berg World of Bio­di­ver­sity (Research Insti­tute and Nat­ural His­tory Museum) via Alpha­Galileo. Orig­i­nal text may be edited for con­tent and length.

(Source: Senck­en­berg, 20.04.2012)