Demographic Reconstruction of the Western Sheep Expansion from Whole-Genome sequences

Abstract

As one of the earliest livestock, Sheep (Ovis aries) were domesticated in the Fertile Crescent about 12000 - 10000 years ago and have a nearly worldwide distribution today. Most of our knowledge about the timing of their expansions stems from archaeological data but it is unclear how the genetic diversity of modern sheep fits with these dates. We used whole-genome sequencing data of 63 domestic breeds and their wild relatives, the Asiatic mouflon (O. gmelini), to explore the demographic history of sheep. On the global scale, our analysis revealed geographic structuring among breeds with unidirectional recent gene flow from domestics into Asiatic mouflons. We then selected four representative breeds from Spain, Morocco, the UK and Iran to build a comprehensive demographic model of the Western sheep expansion. We inferred a single domestication event around 9000 years ago, slightly later than archaeological evidence suggests which might reflect uncertainties in the generation time used for these estimates. The westward expansion is dated to approximately 5000 years ago, later than the original Neolithic expansion of sheep and approximately matching the Secondary Product Revolution associated with woolly sheep. We see some signals of recent gene flow from an ancestral population into Southern European breeds which could reflect admixture with feral European mouflon. Furthermore, our results indicate that many breeds experienced a reduction of their effective population size during the last centuries, probably associated with modern breed development. Our study provides insights into the complex demographic history of Western Eurasian sheep, highlighting interactions between breeds and their wild counterparts.