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 < 0.05), which was consistent with the first challenge. However, orni-daa*0501 did not present in HZ-SG and HZ-RG and the distribution frequencies of the orni-daa*1201, orni-daa*1301 and orni-daa*1401 alleles were not significantly more frequent in HZ-SG than in HZ-RG. These results indicate that the orni-daa*1101 allele confers susceptibility to S. agalactia infection. These results suggest that the diversity of exon 2 of mcaIIa alleles could be used to explore the association between disease susceptibility or resistance and the multiformity of mcaIIa and to achieve the molecular-assisted selection of O. niloticus with enhanced disease resistance.