Sperm head structure in the hydromyinae (rodentia:Muridae): A further evolutionary development of the subacrosomal space in mammals

This scanning and transmission electron microscopical study has been performed to determine the extent of the development of the subacrosomal space in the sperm head of various members of the subfamily, the Hydromyinae, which is a diverse group of murid rodents occurring in various habitats in Australia. Sperm of all species from the three tribes of Hydromyini, Uromyini, and Conilurini investigated in this study had a head with three hooks although their absolute and relative lengths varied markedly. The top hook was similar in structure to that present in many other murid rodents and had a typical pseudoperforatorium present. The two ventral hooks had nuclear material basally, apical to which a large extension of the subacrosomal space occurred. Studies of testicular sections showed that the ventral hooks were formed late in spermiogenesis largely after condensation of the nucleus during which time electron dense material accumulated within them. No structure homologous to these hooks has been found in sperm from any other mammalian group, but, as the three tribes of Hydromyinae probably diverged about 20 million years ago, the extension of the subacrosomal space presumably evolved prior to this date.