European levels of wild boar hybridization

After a strong demographic decline before World War II, wild boar (Sus scrofa) populations are expanding and the species is now the second-most abundant ungulate in Europe. This increase raises concerns due to wild boar impact on agriculture and natural ecosystems and as potential vector of disease transmissions. The population expansion is the result of multiple factors, among them, it has been hypothesized that hybridization with the domestic pig (Sus scrofa domesticus) has led to modifications of life history traits, including fertility and litter size. We compared a literature review with results from the analysis of 47 148 Single Nucleotide Polymorphisms (SNP) in wild boar from Europe (292) and the Near East (16), and commercial (44) and local (255) pig breeds, to discern patterns of hybridization across Europe. The literature review showed that, over time and using a combination of genetic markers, wild boar x domestic pig hybridization has been reported in most European countries. Principal Component and maximum likelihood analyses performed on the SNP dataset showed a much more geographically confined distribution of hybridization. Observed values varied greatly for both datasets, ranging from absent to very high frequency of hybrids. However, the difference in geographic distribution of hybrids highlights the importance of large-scale investigations and the need to combine multiple markers (slow and fast evolving) for hybrid detection.