[Comparative fine structure of the cestode spermatozoon. Relationship to phylogenesis]

Abstract

The comparison of the ultrastructure of spermatozoa in Cestodes from seven different orders shows that they have the same general pattern of organization. The main common features are following: (a) a long thread-like body; (b) an elongated nucleus; (c) cortical microtubules underlying plasma membrane; (d) the absence of mitochondria and of a typical acrosome. The mode of spermatozoon differentiation indicates an inversion of the polarity. The number of axonemes of the 9 + "1" pattern, characteristic of the platyhelminthes, allow us to distinguish two types of spermatozoa among cestodes: --The "two-axonemes"-type is considered as primitive as it has also been found in the free-living platyhelminths. This type has been observed in the Pseudophyllidea, Proteocephalidea, Tetrarhynchidea and Tetraphyllidea (Onchobothriidae). --The "one-axoneme"-type has been found in the Cyclophyllidea, Caryophyllaeidea, Diphyllidea and Tetraphyllidea (Phyllobothriidae). It has been proved that the one axoneme type of the spermatozoon, observed in Tetraphyllidea, Phyllobothriidae arises from the two axoneme type by early abortion of the second axoneme. In all the tetraphyllidean species studied so far, we found an helicoidal body forming so-called "crested body" or "spiral crest" at the proximal end of the spermatozoon, which is opposite to the nuclear part. This division of the cestodes into two groups, according to the number of axonemes in the spermatozoon is compared with the phylogenetic diagrammes of Cestoda proposed by Freeman (1973) and Euzet (1974). As result, two hypothesis can be forwarded to explain the relationships between the Tetraphyllidea, Proteocephalidea and Cyclophyllidea. The importance of data on the cestode spermatozoa as a criterion in analysis of cestode phylogenesis is drawn and discussed.