European wild boars and domestic pigs display different polymorphic patterns in the Toll-like receptor (TLR) 1, TLR2, and TLR6 genes

During the last decade, the Toll-like receptors (TLRs) have been extensively studied, and their immense importance in innate immunity is now being unveiled. Here, we report pronounced differences—probably reflecting the domestication process and differences in selective pressure—between wild boars and domestic pigs regarding single nucleotide polymorphisms (SNPs) in TLR genes. The open reading frames of TLR1, TLR2, and TLR6 were sequenced in 25 wild boars, representing three populations, and in 15 unrelated domestic pigs of Hampshire, Landrace, and Large White origin. In total, 20, 27, and 26 SNPs were detected in TLR1, TLR2, and TLR6, respectively. In TLR1 and TLR2, the numbers of SNPs detected were significantly lower (P ≤ 0.05, P ≤ 0.01) in the wild boars than in the domestic pigs. In the wild boars, one major high frequency haplotype was found in all three genes, while the same pattern was exhibited only by TLR2 in the domestic pigs. The relative frequency of non-synonymous (dN) and synonymous (dS) SNPs was lower for the wild boars than for the domestic pigs in all three genes. In addition, differences in diversity between the genes were revealed: the mean heterozygosity at the polymorphic positions was markedly lower in TLR2 than in TLR1 and TLR6. Because of its localization—in proximity of the bound ligand—one of the non-synonymous SNPs detected in TLR6 may represent species-specific function on the protein level. Furthermore, the codon usage pattern in the genes studied deviated from the general codon usage pattern in Sus scrofa.