Ultrastructure of sperm and spermatogenesis of Anoplodiscus cirrusspiralis (Platyhelminthes, Monogenea, Monopisthocotylea)

Abstract

The mature spermatozoon of Anoplodiscus cirrusspiralis is long and filiform with a nucleus, mitochondrion and single axoneme pressed tightly together for most of its length. In contrast to most other platyhelminthes which have a solid central unit in the complex central element of the sperm axoneme (Trepaxonemata), the central unit in Anoplodiscus is a hollow cylinder. There are no peripheral microtubules in the sperm, and the arrangement conforms to sperm pattern 4 in the scheme of Justine et al. (1985) for the Monogenea (found in monopisthocotyleans from the orders Dactylogyridea and Tetraonchidea). However, spermatogenesis is distinctly different in Anoplodiscus. A short free axoneme rotates and fuses with the spermatid body, then continues to grow alongside the elongating nucleus and the gradually fusing mitochondria. There is no cytoplasmic outgrowth in the original direction of the axoneme, and the nucleus and mitochondria do not migrate past the basal body of the axoneme, as happens in other mono- pisthocotylean Monogenea. As other evidence suggests that Anoplodiscus is a monogenean, it is concluded that spermiogenesis is aberrant and a case of temporary deviation in ontogeny, not affecting the outcome of mature sperm structure.