Impact of past climate warming on genomic diversity and demographic history of collared lemmings across the Eurasian Arctic.

The Arctic climate was warmer than today at the last interglacial and the Holocene thermal optimum. To reveal the impact of past climate-warming events on the demographic history of an Arctic specialist, we examined both mitochondrial and nuclear genomic variation in the collared lemming (Dicrostonyx torquatus, Pallas), a keystone species in tundra communities, across its entire distribution in northern Eurasia. The ancestral phylogenetic position of the West Beringian group and divergence time estimates support the hypothesis of continental range contraction to a single refugial area located in West Beringia during high-magnitude warming of the last interglacial, followed by westward recolonization of northern Eurasia in the last glacial period. The West Beringian group harbors the highest mitogenome diversity and its inferred demography indicates a constantly large effective population size over the Late Pleistocene to Holocene. This suggests that northward forest expansion during recent warming of the Holocene thermal optimum did not affect the gene pool of the collared lemming in West Beringia but reduced genomic diversity and effective population size in all other regions of the Eurasian Arctic. Demographic inference from genomic diversity was corroborated by species distribution modeling showing reduction in species distribution during past climate warming. These conclusions are supported by recent paleoecological evidence suggesting smaller temperature increases and moderate northward forest advances in the extreme northeast of Eurasia during the Late Pleistocene-to-Holocene warming events. This study emphasizes the importance of West Beringia as a potential refugium for cold-adapted Arctic species under ongoing climate warming.