Assessing rates and patterns of hybridization between wild boar and domestic pig in Europe

Introgressive hybridization between wild boar and domestic pig has unpredictable evolutionary
consequences and has the potential to alter reproductive, immunological and behavioural traits,
influencing individual fitness and possibly increasing species' invasiveness.
Though of great interest, reconstructing patterns of gene flow between wild and domestic form of Sus
scrofa is arduous. In fact, hybridization patterns may differ greatly with respect to time (ancient vs. recent),
source (intentional in captivity vs. unintentional in the wild), spread (extensive vs. occasional) and
directionality (i.e. involved sexes of the two forms). This is likely to produce a puzzling scenario, where close
populations may show contrasting signals of genetic introgression. Whether and how much this can impact
the dynamics of local populations remains unproven. Diagnosing hybrids is the first step to address the
issue. However, despite the availability of multiple molecular markers, with different inheritance systems
and undergoing different selective pressures, the analysis on the extent and impact of introgressive
hybridization in Europe remains challenging.
Here we show the outcome of the application of different genetic marker systems and the potentiality of
newly developed markers across a range of European wild boar populations.
Maternally inherited mitochondrial DNA alleles are by large shared between wild and domestic stocks, and
the most reliable indicator of introgression is commonly considered the presence of East Asian haplotypes,
which are found at a very low frequency in European wild boar. Variation at the MC1R and NR6A1 nuclear
genes - influencing coat colour and number of vertebrae, respectively, and under strong artificial selection
during domestication - revealed higher levels of a bidirectional gene flow. Unlike previously used Ychromosome
markers, which seem to suffer low variation, new polymorphic Y-linked microsatellites
represent a promising tool to disclose the paternal contribution to gene flow, but should be tested on a
larger sample of domestic and wild Sus scrofa to identify actual diagnostic alleles.