The little Pygmy hog (Porcula salvania), a big piece in resolving the fascinating history of the complex speciation of the Suidae

Wild boar (Sus scrofa) colonized mainland Eurasia and North Africa, most likely from East Asia during the Plio-Pleistocene (2–1 Mya). Extensive fossil evidence from Eurasia suggests that this expansion resulted in the replacement of multiple suids species. Recent reports, based on genome-wide information, however, suggested that wild boar did not replace the species it encountered, but instead exchanged genetic materials with them through admixture. The incorporation of these lineages into the gene-pool of wild boar has been hypothesized to be one of the keys to its successful expansion. The pygmy hog (Porcula salvania) is the only suid species, in mainland Eurasia, known to have outlived this wave of expansion. This highly endangered species thus provides a unique opportunity to test the hypothesis that wild boar interbred with local suid species as it expanded across Eurasia. Analyses of 6 pygmy hog genomes, combined with additional suid genomes, indicate that despite large phylogenetic divergence (~2 My), wild boar and pygmy hog did indeed interbreed as the former expanded across Eurasia. Moreover, it sheds further light on a genealogical discordance observed on the X chromosome of pigs, which is caused by post-speciation admixture. The donor of this admixture pertains to a now-extinct species with a deep phylogenetic placement between pygmy hog and sub-Saharan suids. This suggests that, the rapid spread of S. scrofa across the old world might have been facilitated through inter-specific admixture. Overall, our analysis demonstrates the importance of inter-specific admixture during rapid range expansion.