Non-coding accelerated regions underlie polar bear arctic adaptation

Polar bears ( Ursus maritimus) are an ideal system to understand selective pressures as populations adapt to new environments. Their ecological niche shifted radically from a generalist temperate omnivore to a specialist arctic carnivore, subsisting primarily on seal fat, making polar bears a unique evolutionary model for arctic adaptation. Prior studies of positive selection on protein coding sequences have identified selection on genes involved in cholesterol metabolism. However, selection on non-coding gene regulatory elements, the most common target of positive selection, has not been explored. Here we identified 790 accelerated regions as non-coding targets of positive selection in polar bears. Using two ancient polar bear samples, we were also able to distinguish between recent adaptations and adaptations that were already present in ancient samples. We found significant enrichment for selection on critical gene regulatory systems in polar bears including insulin resistance, inflammation and lipid metabolism, fatty acid degradation, adipogenesis, and pigmentation. Globally, our findings show that selection is acting on pathways that represent important adaptations to the arctic environment such as growth, development, and immunity. This data presents an exciting and novel opportunity to identify the metabolic innovations that allow polar bears to tolerate an extremely high-fat diet, substantially improving knowledge of how species adapt to new climate niches.