THREAT DOWN! Gene flow from polar bears into brown bears

Bears, perennial toppers of the ThreatDown list on The Colbert Report (copyright Comedy Central)

What do you get when you cross a male polar bear with a female brown bear? Trick question: Nothing, because it doesn’t really happen!*

But seriously, folks. A new Molecular Ecology study from James Cahill, Ed Green, and Beth Shapiro compared the genomes of 38 bears in an attempt to resolve the patterns of gene flow between brown and polar bears in the ABC islands of Alaska, the Alaskan mainland, and Europe.
Their past work, based on relatively few brown bear genomes (and only one from the mainland, Denali), assumed that the mainland brown bear genome was free from polar bear introgression. The new analysis, however, which included more mainland brown bear genomes, revealed that their previous findings weren’t quite on point: there is a lot of polar bear DNA in the genomes of mainland brown bears.

“Our new results indicate that this assumption was incorrect – the Denali [mainland] brown bear is not free of polar bear ancestry.”

In fact, they were way off:

“the Denali bear has the greatest polar bear ancestry [of all mainland brown bears sequenced]: at least 5.38% of the autosomes and 1.04% of the X chromosome derives from polar bear.”

On the other hand, while any given brown bear genome can have up to 8.8% polar bear DNA (at least in the ABC islands), the polar bear genome lacks any discernible brown bear DNA. The authors hypothesize that this:

“reflect[s] an ecological barrier to admixed individuals surviving as polar bears, where any introduction of brown bear DNA into polar bears may be strongly deleterious.”

They speculate that there may be strong selection selection against F1 hybrids in polar bear habitats. Specifically, they hypothesize that because polar bear-brown bear hybrids have darker fur, they may not be quite as stealthy:

“Hybrid bears with patches or darker pelage, or darker shades would be more visible to the seals and therefore less successful hunters”

So how does this limited amount of gene flow affect the global warming poster-child?

It seems unlikely therefore that hybridization or the paucity of genetic diversity among polar bears represents the principle threat to the long-term survival of polar bears. Rather, the rapid rate of recent climate change and consequent disappearance of their habitat (Stirling & Derocher 2012) remain the most proximate and serious threats to polar bears.

Cahill JA, Stirling I, Kistler L, Salamzade R, Ersmark E, Fulton TL, Stiller M, Green RE & Shapiro B (2014) Genomic evidence of geographically widespread effect of gene flow from polar bears into brown bears. Mol. Ecol. DOI: 10.1111/mec.13038
Cahill JA, Green RE, Fulton TL, Stiller M, Jay F, Ovsyanikov N, Salamzade R, St John J, Stirling I, Slatkin M & Shapiro B (2013) Genomic evidence for island population conversion resolves conflicting theories of polar bear evolution. PLoS Genet. 9, e1003345. DOI: 10.1371/journal.pgen.1003345

  • At least on the ABC Islands of Alaska. And I’m talking about F1 offspring (no back-crosses!).
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