MHC class IIα polymorphisms and their association with resistance/susceptibility to Singapore grouper iridovirus (SGIV) in orange-spotted grouper, Epinephelus coioides

The major histocompatibility complex (MHC) is a crucial component of the immune systems of vertebrate animals and is associated with their resistance/susceptibility to pathogenic diseases. In this study, the association between MHC IIα gene polymorphisms and disease resistance/susceptibility was analyzed in the orange-spotted grouper (Epinephelus coioides), by challenging it with Singapore grouper iridovirus (SGIV). Thirty-seven alleles showing high levels of polymorphism (24.7% mutation sites) were isolated from 80 individuals. In the peptide-binding region, the ratio of nonsynonymous (dN) substitutions to synonymous (dS) substitutions was 1.427 (>1), suggesting that the loci are evolving under positive balancing selection. Eight alleles were selected to estimate their distributions in high-resistance (HR) and high-susceptibility (HS) groups of fish. The EPCO-DAA*0101 allele was significantly associated with susceptibility to SGIV infection (P