Abstract
Nile tilapia, Oreochromis niloticus, is among the most highly recommended species by the FAO for culture, and has become the second most abundantly farmed fish species in the world. In aquaculture systems, however, this species is vulnerable to various pathogens including Gyrodactylus cichlidarum (Monogenea). The major histocompatibility complex (MHC), a highly polymorphic gene superfamily, plays vital roles in immunity and expression of this group of genes is associated with disease resistance. In the present study, the relationship between MHC IIβ polymorphism of O. niloticus and the susceptibility/resistance to G. cichlidarum infection were analyzed. A total of 46 alleles of MHC IIβ were identified, which demonstrated a high level of polymorphism representing a variability of 96.8 % amino acid sites. In the peptide-binding region (PBR) and entire region, the values of dN/dS were 1.094 and 1.027, respectively. Infection experiments revealed that the resistance of O. niloticus to G. cichlidarum infection was positively correlated with MHC IIβ polymorphism. Among alleles, Orni-DAB*1401 was highly associated with resistance (P < 0.05), while Orni-DAB*0901 and Orni-DAB*3501 were significantly associated with susceptibility (P < 0.05). The expressions of immune-related genes (MHC IIβ, IL1β and TLR7) in the resistant individuals (with Orni-DAB*1401) are significantly higher than other individuals (P < 0.05). The results suggested that diversity of the MHC IIβ allele was associated with the susceptibility/resistance of tilapia to this monogenean, which can potentially contribute to the study of MHC diversity maintenance and molecular-assisted selection of O. niloticus to enhance disease resistance.
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