Evolution of MHC class II SLA-DRB1 locus in the Croatian wild boar (Sus scrofa) implies duplication and weak signals of positive selection.

The wild boar is an ancestor of the domestic pig and an important game species with the widest geographical range of all ungulates. Although a large amount of data are available on major histocompatibility complex (MHC) variability in domestic pigs, only a few studies have been performed on wild boars. Due to their crucial role in appropriate immune responses and extreme polymorphism, MHC genes represent some of the best candidates for studying the processes of adaptive evolution. Here, we present the results on the variability and evolution of the entire MHC class II SLA-DRB1 locus exon 2 in 133 wild boars from Croatia. Using direct sequencing and cloning methods, we identified 20 SLA-DRB1 alleles, including eight new variants, with notable divergence. In some individuals, we documented functional locus duplication, and SLA-DRB1*04:10 was identified as the allele involved in the duplication. The expression of a duplicated locus was confirmed by cloning and sequencing cDNA-derived amplicons. Based on individual genotypes, we were able to assume that alleles SLA-DRB1*04:10 and SLA-DRB1*06:07 are linked as an allelic combination that co-evolves as a two-locus haplotype. Our investigation of evolutionary processes at the SLA-DRB1 locus confirmed the role of intralocus recombination in generating allelic variability, whereas tests of positive selection based on the dN/dS (non-synonymous/synonymous substitution rate ratio) test revealed atypically weak and ambiguous signals.