Comparative structure and function of marsupial spermatozoa.

Marsupial sperm structure has been the focus of many comparative studies in the last 30 years. Although the basic organization of the marsupial spermatozoon is similar to that of eutherian mammals, spermatozoa from this branch of therian evolution have developed a specific suite of characters which clearly distinguish them from the Eutheria. This review surveys these specializations and examines current knowledge on their respective functions and the forces which shaped their evolution. Nuclear shaping and stability, the asymmetric positioning of the acrosome, and the unusual neck articulation are discussed. Although recent observations have provided evidence of a marsupial equatorial segment and posterior ring, the marsupial equivalent of the eutherian postacrosomal sheath has not been identified. The unusual neck structure of marsupial spermatozoa and the mobile articulation of the connecting piece are discussed in relation to nuclear rotation and the events associated with this process. Increasing flagellar length in some species is associated with extremes in flagellar organization and its effect on sperm motility is discussed.