Changes in centriole growth and dwarfism in reindeer: the secrets of Santa's little helper revealed by circumpolar whole-genome sequencing

Reduction of body size is a recurrent evolutionary process that has been often described in response to insular environments or extreme climates. A suitable species to study the underlying mechanisms of such adaptations is reindeer, which contain multiple isolated ecotypes exhibiting extreme dwarfism. By performing whole-genome sequencing of reindeer from their entire circumpolar distribution, we first trace their historic expansion from Europe to North America and Asia. Our findings delineate independent origins for known dwarf ecotypes on Svalbard and the Canadian Arctic Archipelago. Using newly created, high-quality reference genomes of wild Norwegian (Rangifer tarandus tarandus) and Svalbard (R. t. platyrhynchus) reindeer, we identify a reservoir of variants within genes involved in centriole elongation as strong candidate to explain the dwarfism phenotype. Furthermore, the variants distribution supports independent emergence of dwarfism across reindeer ecotypes, suggesting that genetic variants within these genes persist to enable recurring adaptation to extreme conditions.