Spermiogenesis in Mimagoniates barberi (Teleostei : Ostariophysi : Characidae), an oviparous, internally fertilizing fish

The ultrastructure of spermiogenesis of Mimagoniates barberi (Characidae), an internally fertilizing oviparous species is described. Spermiogenesis in volves conspicuous changes of the topography of the cellular constituents a nd modifications of their shape. The round nucleus elongates to become a ve ry long flat plate. The long axis of the nucleus establishes the polarity o f the spermatid and spermatozoon. The nucleus does not rotate and, as a con sequence, the centrioles are located at the proximal tip of the spermatozoo n and the flagellum runs parallel to the long axis of the nucleus. Mitochon dria and most of the cytoplasm are shifted to the pole opposite the centrio les. The proximal portion of the flagellum is located in a long cytoplasmic canal which shortens during spermiogenesis and is free for most of its len gth in the mature spermatozoon flagellum. In the cytoplasm of elongated spe rmatids, close to the nuclear membrane, numerous microtubules appear which run from the centriolar pole to the mitochondrial pole forming a microtubul ar manchette. The microtubules persist in mature spermatozoa. It is conclud ed that the highly specialized structure of the spermatozoon of M, barberi is the result of only few developmental modifications of spermiogenesis.