Spermiogenesis and sperm ultrastructure in Cylindrostoma fingalianum (Platyhelminthes, Prolecithophora) with notes on a protozoan (kinetoplastid) symbiont closely associated with allosperm

Abstract

Summary The only positive synapomorphy uniting members of the platyhelminth turbellarian taxon, Prolecithophora, is a peculiar membranous system present in much of the shaft of the sperm. This investigation followed the development of this membrane system during spermiogenesis in Cylindrostoma fingalianum and its appearance in the mature sperm. Formation begins with invaginations of the early spermatid shaft outer cell membrane. The invaginations enlarge and the membrane becomes increasingly folded. They appear to be subsequently supplemented with membranous material supplied by an extensive vacuolar system originating from the Golgi apparatus in the main cytoplasmic mass of the spermatid. Throughout development, and in mature sperm, the membranous system forms a part of the outer cell membrane and is also intimately associated with the elongate mitochondrion that lies between the membranous column and the nucleus. The nucleus is highly lobed and the sperm cell lacks flagella or axonemes or the dense bodies found in most other turbellarian taxonomic divisions. A kinetoplastid protozoon present in considerable numbers amongst allosperm is also described.