Substantial hybridisation between wild boars (Sus scrofa scrofa) and East Balkan pigs (Sus scrofa f. domestica) in natural environment as a result of semi-wild rearing in Bulgaria

The East Balkan pig (EBP) is a traditional domestic pig breed in Bulgaria managed in semi-wild conditions and well-adapted to the continental climate and rearing on pastures. From the genetical and historical point of view it is important to preserve this ancient breed. However, over the last several years, a dramatic decline of EBP herds has been observed. Moreover, introgression between EBP and wild boar in Bulgaria (WBB) is very likely to have occurred. In this study we used a set of 10 microsatellites and the polymerase chain reaction-restriction fragment length polymorphism method on melanocortin-1 receptor (MC1R) polymorphisms to study the degree of hybridisation between WBB, EBP, and a commercial pig breed (CPB). MC1R results identified WBB-EBP hybrids and the analysis of the microsatellite data with a Bayesian assignment approach and the Discriminant Analysis of Principal Components revealed a low genetic differentiation between WBB and EBP and a high amount of introgression of WBB into EBP. A mentionable introgression of CPB into EBP was also revealed. It is apparent that the traditional rearing system of EBP, which is hundreds of years old, has led to a permanent hybridisation between WBB and EBP. In our opinion, the preservation of the semi-wild rearing system is a prerequisite for the conservation of this old, indigenous pig breed and its genetic composition, as the semi-wild rearing system allows the continuous introgression with WBB. Moreover, the introgression of commercial breeds into EBP or WBB should be prevented. Due to the bidirectional gene flow these hybridisation events would have negative consequences (i.e. loss of viability and adaptation) for the wild boars as well as for the indigenous pig breed.