A Novel Type of Spermiogenesis in the Nile Catfish Chrysichthys auratus  (Siluriformes: Bagridae) in Egypt, with Description of Spermatozoon Ultrastructure

Spermiogenesis and spermatozoon ultrastructure in the Nile catfish Chrysichthys auratus are described using transmission electron microscopy. Spermiogenesis involves some unique peculiarities such as: the development of the centriolar complex and the initial segment of the flagellum in a position directly perpendicular to the basal pole of the nucleus, as a result of absence of nuclear rotation; lack of a cytoplasmic canal during differentiation of the spermatids into spermatozoa; the base of the basal body is not traversed by the basal plate; a basal foot anchors the basal body to the nucleus; and the presence of numerous vesicles around the midpiece and base of the flagellum. In addition, spermiogenesis includes some common features such as: chromatin compaction; formation of a medial shallow nuclear fossa; and elimination of excess cytoplasm. The mature spermatozoon has an elongate conical-shaped head with no acrosome or acrosomal vesicle, a long midpiece with numerous vesicles that continue backwards around the base of the flagellum and a long tail or flagellum, which has no lateral fins or a membranous compartment. The mitochondria lie close to the nucleus basal pole and surround the initial segment of the axoneme and are separated from the flagellum by the inner mitochondrial envelope due to disappearance of the cytoplasmic canal. The flagellum has the classical axoneme structure of a 9+2 microtubular pattern. On the basis of the peculiar features mentioned above, it is concluded that spermiogenesis in this Nile catfish is a synapomorphic type derived from types I and II spermiogenesis, which are common among teleosts. Accordingly, this type could be considered as a novel type of spermiogenesis and could be termed as "type III".