Prosobranch Parasperm: Sterile Germ Cells that Promote Paternity?
John Buckland-Nicks
Department of Biology, St Francis Xavier University, PO Box 3000, Antigonish, Nova Scotia, Canada, B2G 1C0

In a number of animal groups , including insects, chilopods and prosobranch snails, sperm dimorphism occurs in which sterile parasperm are produced alongside fertile eusperm. This phenomenon has been most extensively studied in insects from the standpoint of paternity assurance through sperm competition. Although genesis and structure of snail parasperm have been researched for over a century, some aspects of paraspermatogenesis are still poorly understood and little is known about the function of snail parasperm or their involvement in sperm competition. Paraspermatogenesis of various snails were examined with light and electron microscopy. Parasperm range from simple flagellated cells typically lacking a nucleus, to complex motile giants that transport hundreds of eusperm. Parasperm generally lack an acrosome, although there are a few exception, although there are a few exceptions, including the Cerithioidea and Campaniloidea. Tonnoidea, such as the ranellid Fusitriton oregonensis, have polymorphic sperm; a carrier parasperm provides transport for a cohort of fertile eusperm, whereas a lancet sperm is independent. Plastic embedded sections of testis of Fusitriton stained for polysaccharides and DNA were correlated to thin sections in the electron microscope. Results show that massive production of glycoprotein bodies during paraspermatogenesis stems from activities of the rough ER and golgi bodies and is largely independent of nuclear degradation. This differs from previous ideas that proposed a direct transfer of material derived from degradation of the nucleus into glycoprotein bodies. Post-translational regulation of gene function must largely account for the complex secretory and morphogenetic events that occur in parasperm after the nucleus in incapacitated. Comparable secretions in some insect parasperm have been shown to act as nuptial gifts, suppress female remating desire, or create a hostile environment for rival ejaculates in the female. A variety of secretion products are released by exocytosis from snail parasperm. For example, in Cerithium, dense granules are secreted into the testicular fluid, whereas in Littorina, a fine granular secretion is released. Lysosomes, identified in lancet parasperm using acridine orange, may be involved in creating a hostile pre-fertilization environment for rival sperm. In this context, lancet parasperm form a sperm plug in the bursa copulatrix that binds eusperm in the lumen. These processes are poorly understood in snails but are central to our understanding of the role of parasperm in reproduction. Progress in this area has been limited by our inability to analyse parasperm in isolation. In this study, the parasperm of Fusitriton were successfully separated for the first time by centrifugation on Percoll gradients. In conclusion, the establishment of multiple paternity and mixing of ejaculates in the female has created selection pressure favouring sperm competition between rival males. Parasperm of snails, like their insect counterparts, may be gamete 'eunuchs' that have evolved to serve their fertile siblings.  

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