Whole genome sequences reveal adaption of Snow sheep to subarctic climates and complex introgression across wild sheep species. Whole genome sequences of Snow sheep to investigate genetic diversity, introgression, and adaptation to subarctic environments

To predict species responses to anthropogenic disturbances and climate change, it is reasonable to use key model species with high sensitivity to such factors. Snow sheep could be a good candidate to this end; being the only large herbivores adapted to the cold and alpine habitats of Northeastern Siberia, it also plays a crucial role in the ecosystem. Despite having extensive geographical distribution among all Ovine species, it is also one of the least studied. Here, we sequenced and analyzed six genomes of Snow sheep in combination with 32 available from all other wild sheep species to infer key aspects of their evolutionary history and unveil the genetic basis of their adaptation to sub-arctic environments. Despite their large census population size, Snow sheep genomes showed remarkably low heterozygosities, which could reflect the effect of isolation and historical bottlenecks that we inferred. We also found multiple instances of introgression involving Snow sheep with Argali and Dall sheep, suggesting that these species might have been more widespread during the Pleistocene than at present. Indeed, our results showed that during the last interglacials, these wild sheep apparently experienced severe population reductions. Further, introgressed segments covered genes which were associated with immunity, adipogenesis and morphology related traits, representing potential targets of adaptive introgression. We also identified several genes with traces of selection in Snow sheep. Their functions included mitochondrial functions and thermogenesis associated with adipose tissue, suggesting physiological mechanisms that contribute to the adaptation of Snow sheep to sub-arctic environments.