Spermatological characters of the spathebothriidean tapeworm Didymobothrium rudolphii (Monticelli, 1890).

Spermiogenesis and the spermatozoon ultrastructure of the adult Didymobothrium rudolphii (Spathebothriidea) have been examined using transmission electron microscopy for the first time. The dense material is present in the apical region of the differentiation zone in the early stage of spermiogenesis, similarly as in other basal tapeworms examined to date. The orthogonal development of the two flagella is followed by a flagellar rotation and their proximodistal fusion with the median cytoplasmic process (MCP). The two pairs of electron-dense attachment zones in the MCP mark the lines where the proximodistal fusion of MCP with two axonemes takes place. D. rudolphii exhibits polymorphism of the intercentriolar body during spermiogenesis. The mature spermatozoon possesses the two axonemes of 9 + "1" trepaxonematan pattern, nucleus, cortical microtubules (CMs), and electron-dense granules. The anterior extremity of the gamete lacks a crested body and exhibits a centriole surrounded by a semiarc of electron-dense tubular structures. The two parallel rows of the CMs have been found in the proximal part of the two-axoneme region of the spermatozoon for the first time in the Eucestoda. The posterior extremity of the gamete exhibits pattern of the disorganized axoneme. The ultrastructural features of the sperm/spermiogenesis support a view about the close relationships of the Spathebothriidea and Diphyllobothriidea and the basal position of the Spathebothriidea within the Eucestoda.