GENESIS OF THE APYRENE PARASPERM IN THE APPLE SNAIL POMACEA CANALICULATA (GASTROPODA: AMPULLARIIDAE): AN ULTRASTRUCTURAL STUDY

The apyrene paraspermatogenesis in the freshwater gastropod Pomacea canaliculata has been studied with electron microscopy. Mature apyrene parasperm result from a cytodifferentiation process without maturation division. The atypical condition is recognized early. Paraspermatogonia are characterized by voluminous nuclei with irregular clusters of heterochromatin, numerous dilated cisternae of the rough endoplasmic reticulum and small electron-dense granules spread through the cytoplasm. As this process advances, the nuclei become lobed and chromatin degenerates. The remnant chromatin condenses to form dense bodies which are finally excreted from the cells. At the cytoplasmic level a centriolar multiplication with the consequent flagellogenesis takes place. The axonemal microtubules run along the entire length of the cell and emerge from the posterior end forming a tuft of three or more free flagella. The rough endoplasmic reticulum and the Golgi complex are involved in the production of secretory granules, some of which are later released by exocytosis. At the end of paraspermatogenesis mature apyrene parasperm are fusiform-shaped anuclear ciliated cells.