Variations of Gonadotropin Subunits mRNA Levels at Different Stages of Ovarian Development in Masu Salmon, Oncorhynchus masou

The variations of gene expression and pituitary contents of GTH subunits during the ovarian development of masu salmon, Oncorhynchus masou, were investigated. The pituitary GTHs contents was measured by radioimmunoassays (RIAs) using purified GTH subunits and their antibodies. Pituitary contents of GTH $I\beta$ gradually increased from April through August, and reached the maximum in October. On the other hand, pituitary contents of GTH $II\beta$ remained low until August, but they rapidly increased in October. Total RNAs were prepared from pooled pituitaries and the GTH subunits mRNA in pituitaries was quantified by Northern blot hybridization using masu salmon cDNA probes for each GTH subunit. GTH $I\beta$ mRNA level increased with the progression of ovarian maturity. However, GTH $II\beta$ mRNA was detected only at a more advanced stage, and were extremly high at ovulation. A high levels for GTH a mRNA was detected only at ovulation stage. The synchronous increase in pituitary contents and mRNA levels suggested that ovarian maturity in masu salmon was regulated by both GTH I and GTH II.