Spermiogenesis without preceding meiosis in the hybrid medaka betweenOryzias latipes andO. curvinotus

Abstract

In the hybrid medaka between Oryzias latipes and O. curvinotus, the testicular structure, including an efferent duct, seminiferous tubules, and testicular cysts, was similar to that of the parent fishes, indicating that gonadal development during embryogenesis proceeds normally. The hybrid fish had spermatogonia, spermatocytes, and spermatid-like cells in the cyst. Sperm-like cells with a condensed nucleus and elongated flagellum were released into an efferent duct. However, measurement of DNA contents in spermatogenic cells demonstrated that no haploid cells were produced in the hybrid testis. Cytological observations revealed that the meiotic cell cycle is arrested before reaching metaphase I. The absence of protamine mRNA, of which expression starts from the later phase of meiosis, confirms the abnormality of meiosis in the hybrid testes. Direct evidence for the absence of meiotic division in the hybrid medaka was obtained from a cell culture study of primary spermatocytes. One spermatocyte isolated from the parent medaka produced 4 spermatozoa after meiotic division, whereas one spermatocyte isolated from the hybrid differentiated into one sperm-like cell without meiotic division on a similar time schedule to that of the parent medaka. These results indicate that despite the absence of normal meiosis, spermiogenesis proceeds on schedule, suggesting that spermiogenesis is intrinsically independent of the preceding meiosis. J. Exp. Zool. 279:102–112, 1997.© 1997 Wiley-Liss, Inc.