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. Although feralization does not fully reverse domestication traits, the selective pressures and evolutionary mechanisms driving it 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. We studied genomic signatures of selection in semi-domesticated, feral and wild reindeer populations in Norway. Analysis of population structure confirmed historical records indicating the feral population formed a monophyletic clade that originated from neighbouring semi-domesticated reindeer. However, we also found evidence for excess allele-sharing between wild and feral reindeer, indicating hybridization between these lineages. Signatures of selection were identified using a top-down approach (orthologues of domestication candidate genes) and a bottom-up approach (FST and extended haplotypes). We found that candidate regions under selection have higher rates of excess allele-sharing between wild and feral populations. This provides the first evidence of reversed selection pressures during feralization in reindeer and reveals a link between selection and hybridization. Together, our findings suggest that hybridization likely promoted adaptive introgression during feralization. We discuss the implications of our findings for conserving the genetic integrity of endangered wild reindeer populations.