The control mechanisms of initiation of meiosis in fish spermatogenesis

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Meiosis is indispensable process for ensuring the sexual reproduction. However, detailed information on the regulational mechanisms of its initiation is not clear. Progestin is an important steroid for regulation of final maturation in male and female. In male teleosts, it is known that progestin induces spermiation and maturation. However, the function of progestin in early spermatogenesis including meiosis is not completely known. In this study, we examined the functions of progestin on initiation of meiosis in spermatogenesis of the Japanese eel. 17?, 20? -dihydroxy-4-pregnen-3-one (DHP), a natural progestin in teleost, and its receptor was presented in the testis of early stage of spermatogenesis. Using eel testicular culture systems, DHP induced the DNA synthesis of spermatogonia. 11-Ketotestosterone, which is initiator of spermatogenesis, also stimulated the DNA synthesis of spermatogonia, but this DNA synthesis prevented by anti-DHP antibody treatment at the phase of initiation of meiosis. DHP treatment also induced the expression of meiosis specific marker; DmcI and Spo11. Furthermore, the expression of Spo11 and synaptonemal complexes (specific construct of meiotic prophase) were also detected in some germ cells, which cell type is spermatogonial stem cell in testicular fragments cultured with DHP. In oogenesis, DHP induced meiotic prophase using ovarian culture of common carp which is newly developed for this analysis. We therefore concluded from these results that DHP, a progestin is essential factor for initiation of meiosis in both spermatogenesis and oogenesis.