Changes in the levels of gonadotropin subunit mRNAs in the pituitary of pre-spawning chum salmon

We only have little information on expression of gonadotropin (GTH) subunit genes during spawning migration in salmonids. Changes in the levels of mRN As for GTH subunits (GTH alpha 2, I beta and II beta) were therefore analyz ed in the pituitaries of chum salmon (Oncorhynchus keta) during the final s tages of spawning migration to the Ishikari river. The fish were caught at Atsuta, a fisherman's village facing the Ishikari bay, and at Chitose, a tr ibutary of the Ishikari river, in 1993 and 1994. The former is referred to as seawater (SW) fish, and the latter as freshwater (FW) fish. The levels o f GTH subunit mRNAs in the pituitaries were determined by a quantitative do t blot analysis, using single-stranded sense DNA as the standard. The sense DNAs have the same nucleic acid sequences of mRNAs. The level of GTH alpha 2 mRNA in the FW males was higher than that in the SW ones. Similar tenden cy was seen in the females. No significant changes were observed in the lev els of GTH I beta mRNA in both the males and females. Whereas, the level of GTH II beta mRNA in the FW fish was higher than that in the SW fish regard less of sexes in 1994. Although not statistically significant in the males, similar tendency was seen in the 1993 fish. The present study thus showed that the level of GTH II beta mRNA was increased concomitantly with that of GTH alpha 2 mRNA during the final stages of spawning migration.