Phylogenetic relationships between the families Capsalidae and Dionchidae (Platyhelminthes, Monogenea, Monopisthocotylea) indicated by the comparative ultrastructural study of spermiogenesis

Comparative ultrastructural observations were carried out on the spermiogenesis of the capsalid Caballerocotyla manteri Price and the dionchid Dionchus remorae MacCallum. At the beginning of spermiogenesis the zones of differentiation (ZD) jut out in all directions. A large mitochondrion shaped like a perforated bead, and through which the elongated nucleus passes, is found facing each ZD. Later the ZD become parallel and are embedded within the common cytoplasmic mass. Cortical longitudinal microtubules are present in the ZD at the outset of spermiogenesis, but they later disappear. The spermatozoon is long and filiform. It shows two parallel axonemes of the 9 +"1" flatworm pattern, the nucleus and mitochondrion, and no cortical microtubule. The ultrastructure of spermiogenesis and spermatozoon is remarkably similar in the two species studied, as in other capsalids previously described. Two characteristics, the perforated bead shape of the spermatid mitochondrion and the progressive disappearance of the microtubules of the ZD, may be considered as synapomorphies which indicate close phylogenetic relationships between the families Dionchidae and Capsalidae. This interpretation coincides with Llewellyn's (1971) scheme of the evolution of the monogeneans.