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Mitochondrial Phylogeny Points to Irish Ancestry for Modern Polar Bears

By a GenomeWeb staff reporter

NEW YORK (GenomeWeb News) – Polar bears have mixed with brown bears several times throughout their evolutionary history and share maternal ancestry with ancient Irish brown bears, according to a phylogenetic study online today in Current Biology.

Based on mitochondrial sequence data from hundreds of brown bear and polar bear samples stretching back roughly 120,000 years, an international team found evidence for several hybridization events occurring between the species. And, they say, their findings suggest modern polar bears share a maternal ancestor with brown bears from the area that's now Ireland and Britain rather than Alaska as previously thought.

"[O]pportunistic mating between these two species as their ranges overlapped has left a strong genetic imprint," co-corresponding author Beth Shapiro, a biologist at Pennsylvania State University, and co-authors wrote. "In particular, a likely genetic exchange with extinct Irish brown bears forms the origin of the modern polar bear matriline."

Changing climate and dwindling sea ice has sparked concern for the polar bear, Ursus maritimus, which uses ice as a stage for their hunting, the study authors noted. For their part, they argue that efforts to protect the animals should be informed by research on the bears' biology and evolutionary history, including interactions with the related brown bear species.

"Future conservation strategies would significantly benefit from an understanding of basic evolutionary information," they wrote, "such as the timing and conditions of their initial divergence from brown bears (U. arctos) or their response to previous environmental change."

While polar bear and brown bear species are physically and behaviorally distinct, they do share some mitochondrial genome patterns, with past studies suggesting present day polar bears are related to brown bears from the Admiralty, Baranof, and Chichagof, or ABC, islands of Alaska.

Still, there is some debate over whether the mitochondrial sequence similarities between these bears are a consequence of inter-breeding between the species or whether they result from recent shared ancestry followed by rapid polar bear adaptations, researchers explained.

Last spring, a Norwegian-led team sequenced the mitochondrial genome of a 110,000-130,000 year-old polar bear using DNA from a jawbone sample found in the Svalbard Archipelago in Norway. Their analyses suggest the ancient polar bear was related to both polar bears and ABC brown bears.

For the current study, researchers brought together sequence data on the hyper-variable region of mitochondrial DNA for 242 brown bear and polar bear samples from across the bears' range spanning roughly 120,000 years.

Among the samples included in the study were the ancient sample from Norway's Svalbard Archipelago and another ancient polar bear sample previously described by researchers from Estonia, Australia, and Norway.

Many of the mitochondrial DNA sequences came from GenBank, though the team also isolated and sequenced the HVR region from 23 ancient Irish bear samples, four polar bear samples from the Holocene period that were thought to be around 8,000 years old, 30 other historical polar bear samples, and 17 modern day polar bear samples.

The researchers found that all of the polar bear samples grouped in one clade as expected when they did phylogenetic analyses of the mitochondrial HVR sequences alone or in combination with whole mitochondrial genome data for 10 bears.

But rather than finding the closest ties between the modern polar bears and ABC brown bears, the researchers found that the day polar bears clustered most closely with ancient brown bears from Ireland.

"[I]n contrast to previous analyses, we found no evidence for reciprocal monophyly between brown and polar bears within this clade, nor do our results support a sister relationship of polar bears with the ABC island brown bears," they wrote. "Instead, the inferred common matrilineal ancestor of modern polar bears falls within the genetic diversity of Irish brown bears."

The phylogenetic data fits several possible models of brown bear and polar bear divergence and inter-breeding, the team explained, but is consistent with a shared maternal ancestor for all modern polar bears within the past 20,000 to 51,000 years.

Moreover, researchers say the new findings hint that hybridizations tend to occur when climate or other environmental circumstances nudge polar bears and brown bears into overlapping habitats.

"We found that brown bears and polar bears, which are hybridizing today in the wild, have been hybridizing opportunistically throughout the last 100,000 years and probably longer," Shapiro said in a statement.

As such, those involved in the study say it may be beneficial to think about strategies for protecting both polar bears as well as hybrids between these bears and brown bears.

The researchers also noted that additional information will come from looking not only at the bears' mitochondrial sequences, but also at their nuclear genomes, which offer clues about both maternal and paternal ancestry.

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