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 II alpha 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 allelewas significantly associated with susceptibility to SGIV infection (P < 0.05) and the EPCO-DAA*0104, EPCO-DAA* 0601, EPCO-DAA*1101, and EPCO-DAA*1201 alleles were significantly associated with resistance to SGIV (P < 0.05 for each). To confirm these correlations, an additional independent challenge experiment was performed in the Wenchang population of the orange-spotted grouper. The frequency distribution showed that the EPCO-DAA*0101 allele was significantly more frequent in the Wenchang HS (WC-HS) group than in the WC-HR group (P < 0.05), whereas the EPCO-DAA*1101 allele was significantly more frequent in the WC-HR group than in the WC-HS group (P < 0.05), consistent with the first challenge. However, the frequency distributions of the EPCO-DAA*0104, EPCO-DAA*0601, and EPCO-DAA*1201 alleles did not differ significantly between the WC-HR and WC-HS groups (P > 0.05). These results indicate that the EPCO-DAA*0101 allele confers susceptibility to SGIV infection, whereas the EPCO-DAA*1101 allele confers resistance to it. These disease-resistance- related MHC markers could be useful for the molecular-marker-assisted selective breeding of the orange-spotted grouper. (C) 2016 Elsevier B.V. All rights reserved.