EVOLUTION OF THE SPERMATOZOON IN AUSTRALASIAN RODENTS

The head of the spermatozoon in eutherian mammals contains a nucleus, acrosomal cap and cytoskeleton. It is generally spatulate, paddle-shap ed or pear-shaped, but in most murid rodents it is hook-shaped with th e anterior region of the nucleus surrounded by an elaborate acrosome a nd an extension of the subacrosomal cytoskeleton as a perforatorium. T his type of spermatozoon is present in Australasian Rattus, together w ith several other New Guinean genera. However, in most Australasian hy dromyine rodents a far greater complexity of structural organisation o f the sperm head has evolved in which two further elaborate processes extend from its upper concave surface. These processes contain a huge extension of the cytoskeleton within which filamentous actin is presen t. By contrast, the form of the sperm head in a few species of Pseudom ys, Notomys and Solomys is highly divergent and is either truncated, s patulate or pear-shaped. The evolutionary trends of change in sperm he ad shape are discussed and it is suggested that the falciform sperm he ad with the two extra processes in most of the hydromyine rodents is o ne of the most morphologically complex sperm head types to have evolve d in eutherian mammals; it contains a far more extensive development o f the cytoskeleton than that of any other mammalian spermatozoon.