Hybridization with wild individuals fuels reindeer feralization

Domestication has profoundly influenced the development of human agriculture, and is characterized by intense artificial selection. Feralization occurs when domestic animals return to the wild. While feralization does not fully reverse domestication traits, the selective pressures and evolutionary mechanisms driving feralization remain poorly understood. In northern Europe and Asia, wild, semi-domesticated, and feral reindeer (Rangifer tarandus) populations coexist, offering a unique opportunity to study feralization. Using population genomics and selection scans, we studied signatures of selection in semi-domesticated, feral, and wild reindeer populations in Norway. Phylogenetic analysis confirmed that the feral population formed a monophyletic clade and originated from neighboring semi-domesticated reindeer. We found evidence for excess allele sharing between wild reindeer and the feral population, indicating hybridization and possibly adaptive introgression. We identified selection signatures in a curated set of "domestication genes", i.e. genes found to be implied in domestication in other mammalian species, between feral and semi-domesticated populations, providing the first evidence of reversed selection pressures during feralization in reindeer. Additionally, our findings offer insights into how feral reindeer may acquire wild genotypes through adaptive introgression. Together, our findings suggest that adaptive introgression followed by selection drives feralization. We discuss the implications of our findings in the context of conserving the genetic integrity of endangered wild reindeer populations.