Study offers better insight into the tangled evolutionary histories of polar bears and brown bears

Research News

Subfossil jawbone of a polar bear that lived 115,000 to 130,000 years ago in the Norwegian archipelago of Svalbard. A genomic study includes an analysis of DNA extracted from a tooth attached to this jawbone, which is now housed at the University of Oslo Museum of Natural History. Photo: Karsten Sund, Museum of Natural History, University of Oslo

By CHARLOTTE HSU

A new study offers an improved look at the intertwined evolutionary histories of polar bears and brown bears.

Becoming separate species did not completely prevent these animals from mating. Scientists have known this for a while, but the new research relies on an expanded dataset – including DNA from an ancient polar bear tooth – to get more details.

The emerging story reveals complexities similar to those that complicate the story of human evolution.

“The formation and maintenance of species can be a messy process,” says Charlotte Lindqvist, associate professor of biological sciences at the College of Arts and Sciences and an expert in bear genetics. “What happened with polar bears and brown bears is a good analog to what we learn about human evolution: that the division of species can be incomplete.

“As more and more ancient genomes have been recovered from ancient human populations, including Neanderthals and Denisovans, we find that there was ongoing multidirectional genetic mixing as different groups of archaic humans were mating with ancestors of modern humans. Polar bears and brown bears are another system where you see this happening.

“We find evidence of interbreeding between polar bears and brown bears that predates an ancient polar bear that we studied,” she says. “And, further, our results demonstrate a complicated and intertwined evolutionary history in brown and polar bears, with the primary direction of gene flow from polar bears to brown bears. This reverses a hypothesis suggested by other researchers that the gene flow was unidirectional and passed to brown bears at the height of the last ice age.

A mother polar bear and her 2-year-old cubs in northwest Greenland. Photo: Øystein Wiig

The study was published June 6 in the Proceedings of the National Academy of Sciences. It was led by Lindqvist; Luis Herrera-Estrella of the National Genomics Laboratory for Biodiversity (LANGEBIO) in Mexico and Texas Tech University; and Kalle Leppälä from the University of Oulu in Finland. Tianying Lan, a former UB postdoctoral researcher now at Daicel Arbor Biosciences, was co-first author with Leppälä.

The concept of arctic-adapted polar bears capturing the genetic material of brown bears, which are adapted to life in lower latitudes, is one of many discoveries that may be of interest to scientists concerned with the impacts of climate change on endangered species.

As the world warms and Arctic sea ice shrinks, polar bears and brown bears may encounter each other more frequently in places where their ranges overlap. This makes their shared evolutionary history a particularly intriguing subject of study, says Lindqvist.

An adult male polar bear in northwest Greenland. Photo: Øystein Wiig

Species splitting can be a messy process

As Lindqvist explains, scientists once thought that modern humans and Neanderthals simply separated into separate species after evolving from a common ancestor. Then the researchers found Neanderthal DNA in modern Eurasians, implying that modern human populations received an influx of Neanderthal genes at some point in their shared evolutionary history, she says.

It was only later that scientists realized that this genetic mixing also supplemented Neanderthal populations with modern human genes, adds Lindqvist. In other words, miscegenation can be complex, not necessarily one-sided, she says.

The new bear study reveals a remarkably similar story: Analysis finds evidence of hybridization in the genomes of polar bears and brown bears, with polar bears in particular bearing a strong signature of an influx of DNA from brown bears, according to the researchers. Previous research only offered the inverse model, Lindqvist says.

“It’s exciting to see how DNA can help reveal the history of ancient life. The direction of gene flow is more difficult to determine than its mere presence, but these patterns are key to understanding how past adaptations transferred between species to give modern animals their current characteristics,” says Leppälä, postdoctoral researcher at the mathematical sciences research unit of the University. from Oulu.

“Population genomics is an increasingly powerful toolkit for studying the evolution of plants and animals, as well as the effects of human activity and climate change on endangered species,” says Herrera. -Estrella, President Emeritus Professor of Plant Genomics and Director of the Genomics Institute for Abiotic Crops. Stress Tolerance at Texas Tech Department of Plant and Soil Science. He is also professor emeritus at LANGEBIO. “Bears provide no more simple speciation stories than human evolution. This new genomic research suggests that groups of mammalian species may hide complicated evolutionary histories.

Subfossil jawbone of a polar bear that lived 115,000 to 130,000 years ago in the Norwegian archipelago of Svalbard. Photo: Karsten Sund, Museum of Natural History, University of Oslo

Evidence for modern bear genomes – and DNA from an ancient tooth

The study analyzed the genomes of 64 modern polar and brown bears, including several new genomes from Alaska, a state where both species are found.

The team also produced a new, more complete genome for a polar bear that lived 115,000 to 130,000 years ago in the Norwegian archipelago of Svalbard. The ancient polar bear’s DNA was extracted from a tooth attached to a subfossil jawbone, which is now in the University of Oslo Museum of Natural History.

Using this dataset, the researchers estimate that polar bears and brown bears began becoming separate species around 1.3 to 1.6 million years ago, updating earlier assessments made by some from the same scientists. The age of the split has been and remains a matter of scientific debate, with past interbreeding and limited fossil evidence of ancient polar bears among factors that make the timing difficult to determine, Lindqvist says.

In any case, after becoming their own species, polar bears suffered a dramatic population decline and a prolonged genetic bottleneck, leaving these bears with far less genetic diversity than brown bears, the new study concludes. The results confirm previous research indicating the same trends and add evidence to support this hypothesis.

Genomes analyzed in a new bear study include that of this bear, pictured here in 1995 on Alaska’s North Slope. Scientists wondered if this bear could be a brown bear-polar bear hybrid, but the new research reveals that “this bear is not a hybrid, just a light-colored brown bear,” says the biologist from the UB Charlotte Lindqvist. Photo: Richard Shideler, Division of Wildlife Conservation, Alaska Department of Fish and Game

Coupled with gene flow analysis, these findings provide new insights into the messy and intertwined evolutionary history of polar bears and brown bears.

The international research team included scientists from UB, LANGEBIO, Texas Tech, University of Oulu, Far Northwestern Institute of Art and Science, Alaska Department of Fish and Game , the Natural History Museum of the University of Oslo, Nanyang Technological University, University of Helsinki and University of Aarhus.

The research was funded by the National Fish and Wildlife Foundation, the US National Science Foundation, the Alaska Department of Fish and Game, and the US Geological Survey.

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