Web edition: July 7, 2011
Print edition: July 30, 2011; Vol.180 #3 (p. 5)
All polar bears living today may be able to trace their maternal ancestry back through grandmothers many times great to a female brown bear in Ireland.
Analyzing bits of maternally inherited DNA from 242 bear lineages, both modern and fossil, suggests that polar bears interbred with brown bears in or near the vicinity of Ireland between 20,000 and 50,000 years ago, says evolutionary biologist Beth Shapiro of Pennsylvania State University in University Park.
A major Irish brown bear ancestor “was a complete shock,” says study coauthor geneticist Ceiridwen Edwards, now at the University of Oxford in England.
While working at Trinity College in Dublin, Edwards extracted fragments of DNA from fossil brown bears found in Irish caves. Her data were among those combined to outline the last 100,000 years or so of maternal history of polar and brown bears, reported by an international team of scientists online July 7 in Current Biology.
Biologists have known that even today polar bears and brown bears are closely related enough that they can produce fertile hybrid offspring. Also, genetic studies had hinted at ancient connections of some sort between polar bears and the brown bear populations of three islands off the coast of Alaska. A lingering impact from interbreeding around Ireland was an unexpected twist in the tale.
This wasn’t the only important interbreeding event for polar bears, the study suggests. Evidence for other, similar events could make researchers rethink the role of hybridization in the history of these bears, says conservation geneticist Lisette Waits of the University of Idaho in Moscow.
Polar bears specialize in living on Arctic ice, a habitat shrinking so fast with climate change that bears are now listed as threatened under the Endangered Species Act. Polar bears haven’t left much of a fossil record, but genetic studies and other work have suggested that they arose from coastal brown bears moving north to exploit icy resources.
Polar bears and brown bears can still be classified as distinct species even if they can produce fertile offspring now and then, notes coauthor and evolutionary geneticist Mark Thomas of University College London and Uppsala University in Sweden. The popular notion that different species can’t interbreed and create fertile offspring does not reflect real-world biology, he says. Nor does the notion reflect the current debate among biologists about how to define a species.
What biologists don’t generally expect is for a hybrid here, a hybrid there to create a large, lasting transfer of genetic material from one species to the other. Yet the new genetic analysis of polar bear history suggests that’s what may have happened in Ireland.
The researchers traced DNA inherited only from mothers that's stored in the little cellular powerhouse structures called mitochondria. The team did not look at the DNA from the nucleus of cells (which contains the main genetic blueprints from both mom and dad). Daughters pass along mom’s mitochondrial DNA to their offspring, creating genetic bread crumbs for following back the maternal line.
When some ancient polar bear interbred with an Irish bear, the offspring inherited the brown bear mom’s mitochondria and with it the brown bear mitochondrial DNA. That mitochondrial DNA apparently spread through polar bear descendants until it replaced original polar bear versions. “Today, all living polar bears have this brown bear mitochondrion,” Shapiro says.
Important as that maternal lineage proved, it’s only a small part of the total genetic heritage of polar bears, Thomas notes. For a person this lineage would not be mom’s whole family, just the chain of descent from her mother’s mother, her mother’s mother’s mother and so on. Unraveling the complete picture of polar bear origins will require a study of the nuclear DNA.
More mitochondrial DNA studies would also help in testing the new study’s propositions about polar bear history, says Charlotte Lindqvist of the State University of New York at Buffalo, who has also extracted DNA from bear fossils. The new history comes from fragments of mitochondrial DNA, not the whole mitochondrial genome. Bold claims could use more data, she says. “I’m a little concerned.”
What concerns long-time polar bear researcher Steven Amstrup, though, is the possibility of downplaying the current risks facing the bears from the melting of their sea ice homes. The paper suggests that polar bears hybridize with brown bears when ranges overlap during climate upheavals. “Unfortunately, some readers may interpret this statement, offered prominently in the first section of this paper as a suggestion that hybridization with brown bears may save polar bears from this crisis. That would be a terrible mistake!” says Amstrup, now with the conservation group Polar Bears International of Bozeman, Mont.
If polar bears evolved as biologists now think, Amstrup says, the warmest global mean temperatures they experienced were about 1 degree Celsius warmer than now. Predictions have temperatures exceeding that during the next 50 or 60 years. “Crossbreeding or not,” he says, “polar bears will not be able to undo 150,000 years of evolution, or even 20,000, in 50 years.”
Back Story | WHITE STUFF
Even with a bit of DNA that came from a brown-coated relative, polar bears still rank as a separate species with differences in body structure, range and behavior. Their most striking adaptation to life on ice — white fur.
-- The bears’ transparent fur doesn’t have white pigment, or pigment of any color. Bears can look yellow-orange in the light of sunset or even blue in fog.
-- Even though polar bear fur is transparent in visible light, it absorbs ultraviolet wavelengths. A charming proposal that the hairs act like fiber-optic cables transmitting energy from the UV light to the bear’s body has been dismissed by biologists.
-- Underneath that white fur is black skin. A bear that has lost hair swatches looks as if it has black spots.
-- Polar bear fur can even cover the foot pads in winter. Fur engulfs just about everything except the tip of a polar bear’s nose.
C.J. Edwards et al. Ancient hybridization and an Irish Origin for the modern polar bear matriline. Current Biology, Vol. 21, August 9, 2011. Published online July 7, 2011. doi: 10.1016/j.cub.2011.05.058
B.P. Kelly, A. Whiteley and D. Tallmon. The Arctic melting pot. Nature, Vol. 468, 16 December 2010, p. 891. doi: :10.1038/468891a. [Go to]
C. Lindqvist et al. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences, Vol. 107, March 16, 2010, p. 5053. doi:10.1073/pnas.0914266107. [Go to]
S. Milius. Polar bears listed. Science News Online. May 14, 2008. [Go to]