Duality of Gonadotropin in a Primitive Teleost, Japanese Eel (Anguilla japonica)

Abstract

The duality of gonadotropin (GTH) is well-established in relatively modern teleosts. In primitive teleosts such as eel and catfish, however, only a single GTH (GTH-II) has been isolated and characterized. Therefore, the objective of this study was to clarify the duality of GTH, particularly the presence of GTH-I in primitive teleosts. We attempted to obtain a cDNA encoding the β subunit of GTH-I from Japanese eel,Anguilla japonica,as a representative primitive teleost species. Rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) was used to amplify GTH-Iβ cDNA prepared from immature male Japanese eel pituitaries, and the obtained PCR products were subcloned and sequenced. A degenerate PCR primer was designed based on a highly conserved region between teleost GTH-Iβ and mammalian FSHβ. DNA sequence analysis of the cloned PCR products confirmed the presence of the predicted complete coding region as well as the 5′ and 3′ untranslated regions. The deduced amino acid sequence from these clones showed high homology to goldfish GTH-Iβ (60%), whereas the identity between Japanese eel GTH-Iβ and GTH-IIβ was lower (42%). Phylogenetic analysis confirmed that Japanese eel GTH-Iβ belonged to the teleost GTH-Iβ group. These results provide a definitive proof of the presence of two types of GTHs (GTH-I and GTH-II) in Japanese eel, as has been shown in other teleosts. The duality of GTHs is applicable for teleosts in general. Northern blot analysis showed the transcripts of Japanese eel GTH-Iβ and GTH-IIβ to be 1200 and 750 bases, respectively. GTH-Iβ gene was expressed in immature fish, while GTH-IIβ gene was expressed in spermiating males and ovulated females, suggesting that two GTHs are differentially expressed at different sexual stages and may play separate roles in the reproductive process in Japanese eel.