Gonadotropins and Their Receptors in the Hermaphroditic Ricefield Eel Monopterus albus: cDNA Cloning and mRNA Expression.

Abstract

The ricefield eel is a protogynous hermaphroditic teleost fish that changes sex from functional female, through an intersexual stage, to the functional male phase during its life cycle, and a good model organism to study sex determination and differentiation of vertebrates. Previous studies suggested that HPG axis may play important roles in the initiation of sex change in ricefield eel; however, the gonadotropins and their cognate receptors remains to be elucidated in this fish. By using reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE), cDNAs encoding Cgalpha, Lhbeta, Fshbeta, Fsh receptor (Fshr), and Lh receptor (Lhr) of ricefield eel were cloned in the present study. Homology analysis showed that ricefield eel Cgalpha, Lhbeta, Fshbeta, Fshr, and Lhr shared highest identities with their counterparts in perciform fish,which conforms to the lineage relationship. A lhr pseudogene was identified from the genome of ricefield eel, and what is interesting is that phylogenetic analysis categorized teleost lhrs into two clusters, and ricefield eel lhr gene and pseudogene are grouped into one of the two clusters, respectively, which suggests that either one of the duplicated lhr genes became a pseudogene during evolution of different teleost lineages. During ovarian development, realtime PCR analysis showed an increasing trend of fshbeta mRNA levels in the pituitary from previtellogenesis to vitellogenesis, but then a decreasing trend toward maturation. As for lhbeta mRNA levels in the pituitary, a continuous increasing trend was observed from previtellogenesis to maturation. The ovarian fshr mRNA levels were significantly increased from previtellogenesis to early vitellogenesis, but then decreased significantly thereafter. However, the ovarian lhr mRNA levels were increased significantly from vitellogenesis to the mature stage. The above results may suggest that Fsh signal pathway is more important for vitellogenesis, whereas Lh signal pathway more important for oocyte maturation in ricefield eel. During sex change from female through an intersexual stage to male, fshr and lhr showed lower expression levels in transitional gonads than either ovary or testis. In the pituitary, fshbeta and lhbeta mRNA levels were significantly lower during the intersexual and male stages than the female stage. However, lhbeta /fshbeta mRNA ratios were increased significantly during sex change, suggesting that the changes in the ratio of LHbeta to FSHbeta may be important for the gonadal differentiation of ricefield eel. The present study provided a foundation for further elucidation of the roles of HPG axis in the gonadal development and sex change of ricefield eel. This research was supported by Natural Science Foundation of China (30970359, 30771651) and National High Technology Research and Development Program of China (863 Program) (No.2008AA09Z406). (poster)