Phylogeny of parasitic Platyhelminthes: a critical study of synapomorphies proposed on the basis of the ultrastructure of spermiogenesis and spermatozoa

Abstract

Recent cladistic phylogenies of the Platyhelminthes, especially the parasitic species, use for synapomorphies several characters of the spermatozoa, together with other characters. This paper reinvestigates synapomorphies based on sperm characteristics. The outgroup usually chosen for such studies on the parasitic Platyhelminthes (Cercomeria) is the "Dalyelliida" or "Dalyellioida," generally considered a paraphyletic group, but sperm structure in this group shows great variation. In this paper, the Typhloplanoida, which have a plesiomorphic structure (two free axonemes), are considered the outgroup. One of the synapomorphies recently proposed for the Neodermata is "spermatozoon with axonemes incorporated in the cytoplasm." This definition introduces convergences with several other groups of nonparasitic Platyhelminthes. A more precise definition of spermiogenesis, based on a detailed description, is proposed in this paper and avoids homoplasies: "proximodistal fusion of the axonemes." This implies the presence of dorsal and ventral microtubules in the nuclear region of the spermatozoon. This synapomorphy characterizes the Cercomeridea but not the Neodermata (= Udonellidea + Cercomeridea). The character "absence of dense granules in the spermatozoon" should be considered a synapomorphy for the Neodermata and has almost no homoplasies. For the Temnocephalidea, the spiral pattern of the peripheral microtubule row is proposed as a synapomorphy. This character is present in Didymorchis; however, Didymorchis lacks tentacles, which are present in the other genera, and thus the ultrastructural synapomorphy is in conflict with a recently proposed morphological synapomorphy for this group: the presence of tentacles. No synapomorphy of the spermatozoon can be proposed for the Monogenea, but the presence of lateral microtubules lining the axonemes is proposed as a synapomorphy for the polyopisthocotylean Monogenea, and the absence of dorsal and lateral microtubules is proposed as a synapomorphy for the monopisthocotylean Monogenea (this is a reversal of the synapomorphy for the Cercomeridea defined above). The cyclophyllidean Eucestoda show a twisting of the peripheral microtubules of the spermatozoon, which is proposed as a synapomorphy.