Spermiogenesis and sperm ultrastructure of Valipora mutabilis Linton, 1927 (Cestoda, Cyclophyllidea, Gryporhynchidae).

This is the first ultrastructural study of the spermiogenesis and the mature spermatozoon of a cyclophyllidean cestode of the family Gryporhynchidae. The spermiogenesis of Valipora mutabilis begins with the formation of a differentiation zone delimited by arching membranes and containing two centrioles. One of the centrioles develops an axoneme that grows directly into the cytoplasmic protrusion. The other centriole remains situated in a cytoplasmic bud and is subsequently aborted (type IV of cestode spermiogenesis). The mature spermatozoon of V. mutabilis is a filiform cell, tapered at both extremities and lacks mitochondria. The anterior extremity is characterised by the presence of an apical cone and a single helicoidal crested body. The axoneme is of 9 + '1' trepaxonematan pattern, with a periaxonemal sheath. The cortical microtubules are twisted at an angle of about 45 degrees to the spermatozoon axis. The nucleus is electron dense and spirally coiled around the axoneme. The cytoplasm is electron lucent and contains numerous granules of electron-dense material. In contrast to a recent opinion for close phylogenetic relationships, these ultrastructural data demonstrate the distant position between gryporhynchids and the family Taeniidae. The most similar pattern in the ultrastructure of the spermiogenesis and the mature spermatozoon has been described for dilepidids, some hymenolepidids and some anoplocephalids.