Unique features of spermiogenesis in the Musky Rat-kangaroo: reflection of a basal lineage or a distinct fertilization process?

Previous research has found the mature spermatozoon of the Musky Rat-kangaroo to share many characteristics with other macopodoids, some phalangeroids and even the dasyurids. While there have been several studies published on the ultrastructure of the mature marsupial spermatozoon, there are only a few detailed studies on marsupial spermatogenesis. Furthermore, there have been no studies undertaken which combine the staging of the epithelial cell cycle with transmission electron microscopy to describe the ultrastructural changes in the developing spermatozoon during these stages. Such studies have the potential to be used in determining the required time taken for certain components of the spermatozoa to develop. During this study, eight stages of the seminiferous epithelium were observed and the ultrastructure of spermatogenesis was divided into nine phases. Maturational processes in the epididymides are also described. Among the features reported are: the formation of a unique acrosomal granule different from those reported in any other marsupial, the absence of contraction of the nuclear ring, a conspicuous acrosomal compaction process despite the almost 100% coverage of the dorsal nuclear surface and the retention of late spermatids within the seminiferous tubules until the early spermatids have developed to the nuclear protrusion phase.