Ultrastructure of sperm and spermiogenesis in the monocotylid monogeneans Monocotyle helicophallus and Calicotyle australiensis (Platyhelminthes).

Spermiogenesis in Monocotyle helicophallus involves formation of free flagella which rotate to lie parallel before fusing. There is no median cytoplasmic process. A small number of microtubules (usually 2-4) is associated with the axonemes in the zone of differentiation. Nucleus and a fused mitochondrion migrate alongside the axonemes and one basal body moves distally. A coil of nucleus, originally in the cytophore, also moves out along the shaft. The mature spermatozoan has only a single axoneme for a short distance at each end, with two associated microtubules at the proximal end, as well as one axoneme, nucleus and mitochondrion throughout most of its length (the nucleus being enlarged and roughly coiled in one region), and a short region of two overlapping axonemes. We interpret our findings as two axonemes arranged almost end to end, one extending from the proximal end to some point in the mid-region, where the second axoneme begins and continues to the distal end of the sperm. Spermatozoa of Calicotyle australiensis develop from a zone of differentiation which has two basal bodies and a complete ring of cortical microtubules. Two initially free axonemes fuse with each other and there is no median cytoplasmic process. Spermatids form within parallel canals in the cytophore, by backward movement of the zone of differentiation. Prior to detachment, an electron-dense spiral end-piece forms around and proximal to the basal body region. Sixty-four spermatids are present in each isogenic group. With the study of spermiogenesis in more species of Monogenea Monopisthocotylea, it is apparent that the previously designated sperm patterns 2 and 3 are not distinct and should be combined and re-defined. Species previously designated as having sperm patterns 2 and 3 can all be accommodated by the description "two normal axonemes or one normal and one shortened, altered or displaced axoneme, and none, one or a few cortical microtubules remaining in a region of the sperm derived from the zone of differentiation in which a few or a complete ring of microtubules was present".