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We only have little information on expression of gonadotropin (GTH) subunit genes during spawning migration in salmonids. Changes in the levels of mRNAs for GTH subunits (GTH α2, Iβ and IIβ) were therefore analyzed in the pituitaries of chum salmon (Oncorhynchus keta) during the final stages 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 tributary 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 of GTH subunit mRNAs in the pituitaries were determined by a quantitative dot 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 α2 mRNA in the FW males was higher than that in the SW ones. Similar tendency was seen in the females. No significant changes were observed in the levels of GTH Iβ mRNA in both the males and females. Whereas, the level of GTH IIβ mRNA in the FW fish was higher than that in the SW fish regardless 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 IIβ mRNA was increased concomitantly with that of GTH a2 mRNA during the final stages of spawning migration.

Changes in the Levels of Gonadotropin Subunit mRNAs in the Pituitary of Pre-Spawning Chum Salmon