| Summary: | The gonochoristic feature with environmental sex determination that occurs during the yellow stage in the eel provides an interesting model to investigate the mechanisms of gonadal development. We previously studied various sex-related genes during gonadal sex differentiation in Japanese eels. In the present study, the members of transforming growth factor beta (TGF-β) superfamily were investigated. Transcript levels of anti-Müllerian hormone, its receptor, gonadal soma-derived factor (<i>amh</i>, <i>amhr2</i>, and <i>gsdf</i>, respectively) measured by real-time polymerase chain reaction (qPCR) showed a strong sexual dimorphism. Transcripts were dominantly expressed in the testis, and their levels significantly increased with testicular differentiation. In contrast, the expressions of <i>amh</i>, <i>amhr2,</i> and <i>gsdf</i> transcripts were low in the ovary of E2-feminized female eels. In situ hybridization detected <i>gsdf</i> (but not <i>amh</i>) transcript signals in undifferentiated gonads. <i>amh</i> and <i>gsdf</i> signals were localized to Sertoli cells and had increased significantly with testicular differentiation. Weak <i>gsdf</i> and no <i>amh</i> signals were detected in early ovaries of E2-feminized female eels. Transcript levels of <i>amh</i> and <i>gsdf</i> (not <i>amhr2</i>) decreased during human chorionic gonadotropin (HCG)-induced spermatogenesis in males. This study suggests that <i>amh</i>, <i>amhr2,</i> and especially <i>gsdf</i> might be involved in the gene pathway regulating testicular differentiation of Japanese eels.
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