Ancient skull reveals how all grizzly bears carry genes from polar cousins

100,000-year-old polar bear skull reveals mix with grizzly bears (Danny Lawson/PA) (PA File)

All grizzly bears today have polar bear ancestry due to genetic mixing over 100,000 years ago.

The researchers obtained ancient DNA from the skull of a young polar bear that was found in 2009 off the coast of the Beaufort Sea in Arctic Alaska.

They nicknamed the bear Bruno, although DNA analysis later showed it to be a female.

Analysis of ancient 100,000-year-old polar bear DNA revealed that extensive hybridization between polar bears and brown bears occurred during the last warm interglacial period of the Pleistocene.

It shouldn’t surprise us to see mixing happening again today as the climate changes and these species overlap and meet again in the wild.

Professor Beth Shapiro, UC Santa Cruz

This left a surprising number of polar bear ancestors in the genomes of all living brown bears, the researchers say.

Corresponding author Beth Shapiro, a professor of ecology and evolutionary biology at UC Santa Cruz, said the team’s genomic analyzes show that Bruno belonged to a population of polar bears that was ancestral to living polar bears.

He added that at some point, probably after about 125,000 years ago, the polar bear lineage that led to Bruno and the brown bear lineage that led to all living brown bears interbred and hybridized.

As a result of this, polar bear ancestry accounts for up to 10% of the genomes of grizzly bears living today.

Professor Shapiro said: “We would never have seen this without Bruno’s genome, because all living brown bears have that mix as part of their genomes.”

Although polar bears and brown bears are distinct species with differences in appearance, behavior, and habitat, they are closely related and can easily interbreed when their ranges overlap.

Experts say reports of hybrids have increased in recent years as the climate warms and disappearing sea ice forces polar bears to live in coastal areas of the Arctic, while grizzly bears expand their home range. distribution to the north.

Previous studies of ancient DNA have shown that admixture has occurred in certain brown bear populations at least four different times between about 15,000 and 25,000 years ago.

In all cases, the direction of gene flow was from polar bears to brown bears.

While the new study found some evidence of possible gene flow from grizzly bears to Bruno’s lineage, the absence of admixture in polar bears today supports the idea that grizzly bear ancestry reduces a bear’s fitness. for life as a polar bear.

After splitting from brown bears about 500,000 years ago, polar bears became highly specialized hunters of marine mammals on Arctic sea ice.

Brown bears are generalists that range widely across North America, Europe, and Asia.

Professor Shapiro added: “It shouldn’t surprise us to see mixing happening again today as the climate changes and these species overlap and meet again in the wild.

“Climate change allows gene flow to occur between what we consider to be different species.”

The findings are published in Nature Ecology & Evolution.

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