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July 08, 2011

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Polar bears may have Irish ancestor thanks to interbreeding

July 8, 2011
Courtesy of Penn State Uni­vers­ity
and World Science staff

All living po­lar bears can claim as an an­ces­tor a brown bear that lived near pre­s­ent-day Ire­land 20,000 to 50,000 years ago, sci­en­tists say.

Beth Sha­pi­ro, a bi­ol­o­gist at Penn State Uni­vers­ity and one of the lead­ers of a stu­dy point­ing to the odd fam­i­ly con­nec­tion, said cli­mate changes af­fect­ing the North At­lant­ic ice sheet have probably caused per­i­od­ic over­laps in bear habi­tats. These, in turn, led to in­ter­breed­ing. In one such event, just be­fore the the last ice age peak­ed, ma­ter­nal DNA from brown bears found its way in­to po­lar bears.

Credit: Dan­iel J. Cox/Na­tural­Ex­po­sures.com
The find­ings were pub­lished July 7 in the jour­nal Cur­rent Bi­ol­o­gy.

Po­lar and brown bears are very dif­fer­ent in size, skin and coat col­or, fur type and tooth struc­ture, as well as be­hav­ior. Po­lar bears are ex­pert swim­mers adapted to a spe­cial­ized Arc­tic lifestyle, while brown bears—a spe­cies that in­cludes Griz­zlies and Ko­di­ak­s—are climbers that pre­fer moun­tain forests, wil­der­ness re­gions, and riv­er val­leys. 

“De­spite these dif­fer­ences, we know that the two spe­cies have in­ter­bred op­por­tunis­tic­ally and probably on many oc­ca­sions dur­ing the last 100,000 years,” Sha­pi­ro said. “Most im­por­tant­ly, pre­vi­ous re­search has in­di­cat­ed that the brown bear con­tri­but­ed ge­net­ic ma­te­ri­al to the po­lar bear’s mi­to­chon­d­rial lin­eage—the ma­ter­nal part of the ge­nome, or the DNA that is passed ex­clu­sively from moth­ers to off­spring. But, un­til now, it was un­clear just when mod­ern po­lar bears ac­quired their mi­to­chon­d­rial ge­nome in its pre­s­ent for­m.”

Al­though pre­vi­ous re­search­ers had sug­gested that the an­cient fe­male an­ces­tor of mod­ern po­lar bears lived on the ABC Is­land­s—the Alas­kan is­lands of Ad­mi­ral­ty, Bara­nof, and Chicha­gof—only 14,000 years ago, Sha­pi­ro’s team found ev­i­dence of a much ear­li­er in­ter­breed­ing event. Be­cause of this, the mod­ern po­lar bear’s mi­to­chon­d­rial DNA probably un­der­went fixa­t­ion—a dras­tic re­duc­tion in ge­net­ic varia­t­ion and a tran­si­tion to a state in which the en­tire gene pool in­cludes only one form of a par­tic­u­lar gene. 

Af­ter per­form­ing ge­net­ic anal­y­ses of 242 brown-bear and po­lar-bear mi­to­chon­d­rial lin­eages sam­pled through­out the last 120,000 years and across mul­ti­ple ge­o­graph­ic ranges, Sha­pi­ro’s team found that the fixa­t­ion of the mi­to­chon­d­rial ge­nome likely oc­curred dur­ing or just be­fore the peak of the last ice age near pre­s­ent-day Ire­land. Sha­pi­ro added that the spe­cif­ic popula­t­ion of brown bears that shared its ma­ter­nal DNA with po­lar bears has now been ex­tinct for some 9,000 years.

Sha­pi­ro ex­plained that, al­though both po­lar bears and brown bears have ex­pe­ri­enced long pe­ri­ods of ge­o­graph­ic sta­bil­ity, episodes of both warm­ing and cool­ing dur­ing the last 500,000 years or more likely led to en­vi­ron­men­tal con­di­tions fa­vor­a­ble to hy­brid­iz­a­tion be­tween the two bear spe­cies. “Po­lar and brown bears likely came in­to con­tact in­ter­mit­tent­ly, in par­tic­u­lar in coast­al re­gions where the ef­fects of cli­mate change may have been more pro­nounced,” Sha­pi­ro said. “When­ever they come in­to con­tact, there seems to be lit­tle bar­ri­er to their mat­ing.”

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All polar bears can claim as an ancestor a brown bear that lived near present-day Ireland 20,000 to 50,000 years ago, scientists say. Beth Shapiro, a biologist at Penn State University and one of the leaders of a new study, said climate changes affecting the North Atlantic ice sheet probably caused periodic overlaps in bear habitats and, in turn, interbreeding. In one such event, just before the the last ice age peaked, maternal DNA from brown bears found its way into polar bears. The research is expected to help guide future conservation efforts for polar bears, which are listed as threatened under the Endangered Species Act. The results of the study were published July 7 in the journal Current Biology. Polar and brown bears are very different in size, skin and coat color, fur type and tooth structure, as well as behavior. Polar bears are expert swimmers that have adapted to a highly specialized, arctic lifestyle, while brown bears—a species that includes Grizzlies and Kodiaks—are climbers that prefer mountain forests, wilderness regions, and river valleys. “Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years,” Shapiro said. “Most importantly, previous research has indicated that the brown bear contributed genetic material to the polar bear’s mitochondrial lineage—the maternal part of the genome, or the DNA that is passed exclusively from mothers to offspring. But, until now, it was unclear just when modern polar bears acquired their mitochondrial genome in its present form.” Although previous researchers had suggested that the ancient female ancestor of modern polar bears lived on the ABC Islands—the Alaskan islands of Admiralty, Baranof, and Chichagof—only 14,000 years ago, Shapiro’s team found evidence of a much earlier interbreeding event. Because of this, the modern polar bear’s mitochondrial DNA probably underwent fixation—a drastic reduction in genetic variation and a transition to a state in which the entire gene pool includes only one form of a particular gene. After performing genetic analyses of 242 brown-bear and polar-bear mitochondrial lineages sampled throughout the last 120,000 years and across multiple geographic ranges, Shapiro’s team found that the fixation of the mitochondrial genome likely occurred during or just before the peak of the last ice age near present-day Ireland. Shapiro added that the specific population of brown bears that shared its maternal DNA with polar bears has now been extinct for some 9,000 years. Shapiro explained that, although both polar bears and brown bears have experienced long periods of geographic stability, episodes of both warming and cooling during the last 500,000 years or more likely led to environmental conditions favorable to hybridization between the two bear species. “Polar and brown bears likely came into contact intermittently, in particular in coastal regions where the effects of climate change may have been more pronounced,” Shapiro said. “Whenever they come into contact, there seems to be little barrier to their mating.”