Complex admixture preceded and followed the extinction of wisent in the wild

Retracing complex population processes that precede extreme population bottlenecks is often impossible based solely on data from living individuals. Such knowledge, however, can be crucial both to elucidate evolutionary histories and to inform appropriate conservation measures. The wisent (Bison bonasus), Europe's largest terrestrial mammal, represents a species that is exemplary for such a population history. Wisent went extinct in the wild within the last century, largely as a result of anthropogenic factors, but were restored by captive breeding starting from a small group of individuals. We investigate patterns of admixture occurring before and after this extinction using low-coverage genomic data from modern individuals, as well as from four historical samples representing two of the original founding lowland wisent and two individuals of the now extinct Caucasian wisent subspecies. Phylogenetic analysis of genomic blocks returns lowland and Caucasian wisent as reciprocally monophyletic across slightly more than half the genomic alignment. Thus, almost half of the complete genomic alignment contradicts the species tree, which is sufficient for whole genome averages to fail to recover the true population history. We also find evidence of admixture between lowland and Caucasian wisent and also, preceding wisent extinction in the wild, between wisent and the cattle/aurochs lineage. In a modern individual that is descended from both lowland wisent and the last surviving Caucasian wisent bull, we are able to not only detect a significant component of Caucasian wisent ancestry, but also to accurately map likely admixed genomic blocks along each chromosome. Overall, our study shows that the evolution of wisent has been substantially more complex than previously thought. We also establish wisent as an exemplary taxon for the study of admixture in wild populations.