Spermatozoa and relationships in Palaeognath birds

In this paper the authors describe the ultrastructure of the mature spermatozoon and the spermatid in Struthio camelus and Dromaius novaehollandiae. The first species is characterized by a rod-like perforatorium within an endonuclear canal in the anterior third of the nucleus, while the second is characterized by an extremely reduced completely extranuclear perforatorium. Other differences are in the sperm dimensions, the number of mitochondria and the length of the axonemal accessory fibers. Considering both the present data and previous findings, Palaeognath birds appear to be a peculiar and monophyletic group, characterized by: 1), a conical acrosome surrounding the nucleus; 2), a fibrous sheath around most of the axoneme; and 3), an elongated distal centriole occupying the entire midpiece. Within this group, Tinamiformes seem to be more primitive than Struthioniformes. In the latter order Dromaius is distinctly different from the reduced Struthio and Rhea which are closely related to one another by the presence of a rod-like endonuclear perforatorium.