Genesis of the apyrene parasperm in the apple snail Pomacea canaliculata (Gastropoda : Ampullariidae): An ultrastructural study

The apyrene paraspermatogenesis in the freshwater gastropod Pomacea canalic ulata has been studied with electron microscopy. Mature apyrene parasperm r esult from a cytodifferentiation process without maturation division. The a typical condition is recognized early. Paraspermatogonia are characterized by voluminous nuclei with irregular clusters of heterochromatin, numerous d ilated cisternae of the rough endoplasmic reticulum and small electron-dens e granules spread through the cytoplasm. As this process advances, the nucl ei become lobed and chromatin degenerates. The remnant chromatin condenses to form dense bodies which are finally excreted from the cells. At the cyto plasmic level a centriolar multiplication with the consequent flagellogenes is takes place. The axonemal microtubules run along the entire length of th e cell and emerge from the posterior end forming a tuft of three or more fr ee flagella. The rough endoplasmic reticulum and the Golgi complex are invo lved in the production of secretory granules, some of which are later relea sed by exocytosis. Al the end of paraspermatogenesis mature a pyrene parasp erm are fusiform-shaped anuclear ciliated cells.