Differential expression of two gonadotropin (GTH) β subunit genes during ovarian maturation induced by repeated injection of salmon GTH in the Japanese eel Anguilla japonica

Abstract

Repeated injection of salmon gonadotropin (sGTH) can induce ovarian maturation in the female Japanese eel Anguilla japonica. The participation of endogenous GTH in this process remains unclear. The steady-state mRNA levels of pituitary GTH Iβ and GTH IIβ subunits in individual fish was investigated using a real-time quantitative reverse transcription-polymerase chain reaction assay based on the TaqMan™ fluorogenic detection system. Pituitary GTH Iβ mRNA levels were high when the fish were immature, particularly before being injected with sGTH, but levels decreased with the progression of ovarian development and remained very low from the late vitellogenic stage until postovulation. In contrast, mRNA levels of GTH IIβ were very low until the previtellogenic stage after injection with sGTH, and increased markedly in accordance with ovarian maturation and ovulation. These results suggest that sGTH injection inhibits GTH I synthesis, whereas it induces ovarian maturation and stimulates GTH II synthesis. Thus, the differential expression of GTH Iβ and GTH IIβ genes during artificial maturation suggests that the two GTH are synthesized sequentially and have separate functional roles relating to reproductive events in the Japanese eel.