An ultrastructural study of megachiropteran (Mammalia: Chiroptera) spermatozoa: implications for chiropteran phylogeny.

The fine structure of epididymal or electro-ejaculated spermatozoa of Pteropus poliocephalus, P. scapulatus, P. conspiculatus, P. alecto and Syconycteris australis is described. The sperm of all species were found to be very similar. The head is extremely flattened and spatulate; the nucleus is capped by a long acrosome that comprises the proximal half of the head and covers two-thirds of the nucleus area, and a prominent sub-acrosomal space possesses a unique 'anvil'-like shape. Redundant nuclear envelope forms a 'scroll' in a restricted region of the neck, next to the base of the proximal centriole and the most proximal of the mitochondria. The axoneme is atypical for mammals, the central singlets arising distal to the outer doublets. Coarse fibres 1, 5, 6 and 9 are larger than the remainder, a feature shared with the Microchiroptera, most insectivores and the Primates. The large sub-acrosomal space of the megachiropteran sperm is significantly is different from that of the Microchiroptera. We consider that the difference in sperm ultrastructure between the chiropteran sub-orders is not inconsistent with theories of a di-phyletic origin for this group. The use of sperm ultrastructure as a phylogenetic tool is discussed and comparisons with the spermatozoa of other closely related Eutheria are made.