Characteristics of spermiogenesis and mature spermatozoa in Gyratrix hermaphroditus (Plathelminthes, Rhabdocoela, Kalyptorhynchia)

 Some events of spermiogenesis and the submicroscopic anatomy of male gametes in Gyratrix hermaphroditus are described. Special features occurring during the steps of sperm cell maturation are the development of delicate rootlet-like and spike-shaped structures attached to the basal bodies, an anchor apparatus of cilia staying near the tip of the outgrowing spermatids, the differentiation of a single mitochondrial rod, and the processes of compartmentalizing the chromatin resulting in the formation of extranuclear lamellar stacks. Mature spermatozoa are especially characterized by two incorporated axonemes, a string of large mushroom-shaped dense bodies in the anterior section of the cell, and two lateral rows of nuclear lamellar stacks in the median cell segment. Based on the present findings, the process of spermiogenesis and the organization of spermatozoa in the ground pattern of the monophylum Kalyptorhynchia are reconstructed. Apparently, the sister group of the Kalyptorhynchia can be found within the other free-living and symbiotic Rhabdocoela; the hypothesis of a sister group relationship with the Neodermata is not corroborated. Probably, spermatozoal characteristics can contribute to a discrimination between distinct populations of G. hermaphroditus which is proved to be a complex of several sibling species. Accepted: 25 June 1998