MHC class IIA polymorphisms and their association with resistance–susceptibility to Streptococcus agalactiae in Nile tilapia, Oreochromis niloticus

The association between major histocompatibility complex (MHC) class IIA polymorphisms and the severity of infection by Streptococcus agalactiae was investigated using 40 susceptible and 40 resistant individuals of Nile tilapia Oreochromis niloticus. Twenty‐five alleles were identified from 80 individuals, which belong to 22 major allele types. High polymorphism of mhcIIa gene and at least two loci were discovered in O. niloticus. In peptide‐binding region (PBR) and non‐PBR, the ratio of nonsynonymous substitution (dN) to synonymous substitution (dS) was 1.294 (>1) and 1.240 (>1), suggesting that the loci are evolving under positive balancing selection. Association analysis showed that the allele, orni‐daa*0501, was significantly associated with resistance to S. agalactiae, while the alleles, orni‐daa*1101, orni‐daa*1301, orni‐daa*1401 and orni‐daa*1201, were associated with susceptibility to S. agalactiae. To confirm these correlations, another independent challenge experiment was performed in the Huizhou population of the O. niloticus. The frequency distribution showed that the orni‐daa*1101 allele was significantly more frequent in the Huizhou‐Susceptible group (HZ‐SG) than in the Huizhou‐Resistant group (HZ‐RG) (P