Polar bears (Ursus maritimus) and brown bears (Ursus arctos) are sister species possessing distinct physiological and behavioral adaptations that evolved over the last 500,000 years. In new research, a team of scientists from the United States, the United Kingdom, Canada and Sweden generated and analyzed a paleogenome from an approximately 100,000-year-old polar bear that reveals a massive prehistoric admixture event between these two species, which is evident in the genomes of all living brown bears.
The polar bear (Ursus maritimus) from the Viviparous Quadrupeds of North America (1845) illustrated by John Woodhouse Audubon.
In 2009, the skull of a juvenile polar bear nicknamed Bruno was found on the beach of the Beaufort Sea near Point McLeod in Arctic Alaska.
The geomorphic setting where the skull was found suggests that Bruno lived during the most recent period when the relative sea level was higher than today, possibly between 110,000 and 70,000 years before present when the sea was as much as 10 m higher than today along this portion of the Beaufort Sea coastline.
“The availability of Bruno’s paleogenome has made it possible to detect an ancient admixture event that impacted all living brown bears,” said Dr. Ming-Shan Wang, a postdoctoral researcher in the Howard Hughes Medical Institute and the Department of Ecology and Evolutionary Biology at the University of California Santa Cruz.
In the study, Dr. Wang and colleagues generated the nuclear genome and the high coverage mitochondrial genome using DNA from the root of a tooth extracted from Bruno’s skull.
Their genomic analysis shows that Bruno belonged to a polar bear population that was ancestral to living polar bears.
At some point, probably after around 125,000 years ago, the polar bear lineage leading to Bruno and the brown bear lineage leading to all living brown bears crossed paths and hybridized.
As a result of this ancient admixture, polar bear ancestry accounts for as much as 10% of the genomes of brown bears living today.
“We never would have seen this without Bruno’s genome, because all living brown bears have that admixture as part of their genomes,” said Professor Beth Shapiro, also from the Howard Hughes Medical Institute and the Department of Ecology and Evolutionary Biology at the University of California Santa Cruz.
The study did find some evidence of possible gene flow from brown bears into Bruno’s lineage, but the absence of admixture in polar bears today supports the idea that brown bear ancestry reduces a bear’s fitness for life as a polar bear.
After diverging from brown bears about 500,000 years ago, polar bears evolved into highly specialized hunters of marine mammals on the Arctic sea ice. Brown bears, in contrast, are generalists ranging widely across North America, Europe, and Asia.
Bruno lived during a time of changing climate after the peak of a warm interglacial period when temperatures and sea levels were considerably higher than they are now.
Similar conditions can be expected in the future as a result of rapid climate change driven by the burning of fossil fuels and other human activities.
As Arctic sea ice declines, many polar bear populations are already struggling to survive.
“If the rapid, unnatural, and severe human-caused warming of the Arctic we are documenting today continues unabated, it is uncertain whether polar bears will have a sea ice habitat to return to and survive genetically,” said Dr. Ian Stirling, a researcher in the Department of Biological Sciences and the Environment and Climate Change Canada Department of Biological Sciences at the University of Alberta.
“We shouldn’t be surprised to see admixture happening again today as the climate changes and these species are overlapping and encountering each other again in the wild,” Professor Shapiro said.
“Climate change allows gene flow to occur between what we think of as different species.”
The results appear in the journal Nature Ecology & Evolution.
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MS. Wang et al. A polar bear paleogenome reveals extensive ancient gene flow from polar bears into brown bears. Nat Ecol Evol, published online June 16, 2022; doi: 10.1038/s41559-022-01753-8
Source link: https://www.sci.news/genetics/polar-bear-paleogenome-10914.html
