Major histocompatibility II beta diversity and peptide-binding groove properties associated with red sea bream iridovirus resistance

Major histocompatibility complex (MHC) II genes are key factors in presenting antigens derived from extracellular pathogens to CD4(+) T cells. These genes are known to be associated with disease resistance in livestock, including cultured fish species. Genome-wide linkage analysis of RSIV-resistant red sea bream has revealed that MHC II beta is linked to the RSIV-resistant trait and was suggested as a probable candidate gene responsible for the resistant trait. In the present study, diversity of the MHC II beta gene in wild and farmed red sea bream populations was evaluated by analysis of an intronic minisatellite locus, PmaMHC2b. Haplotypes of the beta I domain of MHC II beta were identified by cloning, and linkage of minisatellite polymorphisms with haplotype was confirmed in farmed populations. Large numbers of alleles were observed in the wild population (30), while the domesticated populations showed significantly lower values (13). Thirteen haplotypes of MHC II beta were observed; most haplotypes possessed the unique PmaMHC2b allele, but some were present in more than one PmaMHC2b allele. Each haplotype possessed a distinct amino acid sequence with no synonymous mutations. Among 16 amino acid positions involved in peptide-binding grooves, four were monomorphic and 12 polymorphic. Several positions differed in the RSIV-resistant and RSIV-susceptible phenotypes: five in hydrophilic/hydrophobic character and five in polarity. These results provide insight applicable to broodstock management and contribute to understanding RSIV resistance.