Genetic polymorphism of MHC class IIα alleles and its association with resistance/susceptibility to viral nervous necrosis virus (VNNV) in golden pompano (Trachinotus ovatus)

The major histocompatibility complex (MHC) plays an important role in the vertebrate immune system by binding and presenting foreign peptides to T cell receptors. MHC genes encode cellular proteins that regulate the acquired immune response of vertebrates and affect the development of disease in animals. In the present study, we obtained 36 different full-length open reading frame sequences of Trov-MHC II alpha from 20 adult golden pompano individuals. High levels of polymorphisms were detected among the 36 sequences. An excess of non-synonymous substitutions (d(N)) relative to synonymous substitutions (d(s)) was presented in the peptide-binding regions (PBRs), which suggested that the positive balancing selection driven by pathogens may maintain and promote the high diversity of the PBR domain. Bayesian inference revealed evidence for positive selection of specific codons in Trov-MHC II alpha, and most of the positive selection sites were identified in the alpha 1 domain. Furthermore, 26 exon 2 sequences of Trov-MHC II alpha were identified from 70 individuals and exhibited high levels of diversity. Six alleles were selected to explore the correlations between the alleles and resistance/susceptibility to viral nervous necrosis virus (VNNV) in golden pompano. The frequency distribution showed that the Trov-DAA*6304 allele in golden pompano was associated with resistance to VNNV, while the Trov-DAA*7701 allele was associated with susceptibility to VNNV. These results confirmed the association between alleles of MHC class II alpha and resistance to viral infection in golden pompano and they may provide valuable strategies to enhance the species resistance to disease caused by viral infections and to help in marker-assisted selection breeding programmes for golden pompano.