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-shaped or pear-shaped, but in most murid rodents it is hook-shaped with the anterior region of the nucleus surrounded by an elaborate acrosome and an extension of the subacrosomal cytoskeleton as a perforatorium. This type of spermatozoon is present in Australasian Rattus, together with several other New Guinean genera. However, in most Australasian hydromyine rodents a far greater complexity of structural organisation of 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 present. By contrast, the form of the sperm head in a few species of Pseudomys, Notomys and Solomys is highly divergent and is either truncated, spatulate or pear-shaped. The evolutionary trends of change in sperm head shape are discussed and it is suggested that the falciform sperm head with the two extra processes in most of the hydromyine rodents is one of the most morphologically complex sperm head types to have evolved in eutherian mammals; it contains a far more extensive development of the cytoskeleton than that of any other mammalian spermatozoon.