BACKGROUND: A novel transient structure was observed in the spermatids of three ratite species using transmission
RESULTS: The structure first appeared at the circular manchette stage of sperm development, was most prominent
during the longitudinal manchette phase and disappeared abruptly prior to spermiation. It was composed of
regularly-spaced finger-like projections which were closely associated with the outer nuclear membrane, giving the
nucleus a cogwheel-like appearance. The projections were approximately 30 nm long and 14 nm wide. Although a
similar structure has been described in certain lizard and crocodile species, this is the first report of a similar
structure in the developing spermatids of birds.
CONCLUSIONS: The potential value of non-traditional characters, such as spermiogenesis and sperm ultrastructure, as
phylogenetic markers has recently been advocated. The morphologically unique structure found in ratite
spermatids provides additional evidence of a possible phylogenetic link between the reptiles and birds. It also
endorses the basal positioning of the ratites as a monophyletic group within the avian phylogenetic tree.