MHC II α polymorphism of Nile tilapia, Oreochromis niloticus, and its association with the susceptibility to Gyrodactylus cichlidarum (Monogenea) infection

Nile tilapia, Oreochromis niloticus, is among the most preeminent aquaculture species in the world. In aquaculture systems, this species is vulnerable to various pathogens including Gyrodactylus cichlidarum (Monogenea). The major histocompatibility complex (MHC) genes, a highly polymorphic gene superfamily, have vital roles in adaptive immunity and are closely associated with disease resistance. In the present study, the relationship between MHC II ? polymorphism of Oreochromis niloticus and the susceptibility to G. cichlidarum infection were analyzed. A total of 50 alleles of MHC II ? were identified, which demonstrated a high level of polymorphism representing a variability of 89.6% amino acid sites. In the peptide-binding region (PBR) and entire region, the values of dN to dS were 1.155 and 1.180, respectively. Infection experiments revealed that hosts with moderate polymorphism of MHC II ? (2 to 3 alleles) had relatively higher resistance to G. cichlidarum infection. In addition, the alleles Orni-DAA*0201 and Orni-DAA*0701 were found to be significantly associated with susceptibility (P < 0.05), while Orni-DAA*3601 was highly associated with resistance (P < 0.05). The results suggested that the diversity of MHC II ? allele was associated with the susceptibility of tilapia to this monogenean, which would contribute to the study on MHC diversity maintenance and molecular-assisted selection of O. niloticus to enhance disease resistance. Of course, small scale of productive tests is required to further confirm these results.