A re-evaluation of sperm ultrastructure in the emu, Dromaius novaehollandiae

Existing reports on sperm structure in the emu do not adequately illustrate or describe all the salient ultrastructural features necessary for a meaningful comparison of normal and abnormal sperm in this species. As sperm morphology forms an important parameter in determining semen quality, and in view of the proposed role of artificial insemination in the farming of ratites, this article re-evaluates and complements the existing data on the topic, provides a fully illustrated description of emu sperm ultrastructure, and documents some unreported morphologic features. Conventional transmission and scanning electron microscopy and high resolution scanning electron microscopy were used to describe the ultrastructure of sperm harvested from the distal deferent duct of sexually mature birds slaughtered during the breeding season. In addition to broadly confirming the basic ultrastructural characteristics previously described for emu sperm, this study revealed a number of unreported morphologic features. These included distinct differences in surface properties between the acrosome and nucleus, the presence of a thread-like appendage near the base of the nucleus, variable positioning of the annulus relative to structures located at the midpiece-principal piece junction and regional differentiation of the principal piece. Although the emu displayed similar basic morphologic features to sperm of other ratites and the tinamou, marked structural peculiarities were obvious, notably the lack of an endonuclear canal and a perforatorium and the presence of significantly more mitochondria in the midpiece coupled with an absence of intermitochondrial cement. Although the broad morphologic features of emu sperm would appear to add credence to the general view that the ratites, together with the tinamous, form a monophyletic group at the base of the avian phylogenetic tree, it is also clear that emu sperm are distinctly different from those of the ostrich, rhea, and tinamou which together share morphologic affinities. This observation may lend some support to the alternate view that the Australasian ratites represent a separate clade that developed independently from flightless ancestors.