Characterization and functional analysis of hybrid pearl gentian grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀) complement C3 against Cryptocaryon irritans infection

Pearl gentian grouper is a new aquacultural hybrid resulted from breeding of female tiger grouper (Epinephelus fuscogutattus) and male giant grouper (E. lanceolatus). Our preliminary study found that pearl gentian grouper exhibits less susceptible to the primary infection of Cryptocaryon irritans, which is an important parasitic ciliate in marine aquaculture, indicated that pearl gentian grouper might own a strong innate immune system. Complement system play key roles in innate immunity, whether pearl gentian grouper's complement component contribute for the defensing against the C. irritans infection remain unclear. In the present study, we found that C. irritans can be immobilized by untreated serum but not heat-treated serum from pearl gentian grouper, suggested that the heat-labile components in serum are responsible for the immobilization of C. irritans. Moreover, we cloned and characterized the encoding sequence of pearl gentian grouper complement C3 (PGC3), a key component in complement system. We also found that the expression level of PGC3 was increased in infected grouper serum when compared with that of control grouper. Furthermore, the binding of PGC3 on the surface of C. irritans trophonts located on the grouper skin was detected. These data suggested that pearl gentian grouper's complement system indeed play roles in the immune response against the C. irritans infection.