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

The apyrene paraspermatogenesis in the freshwater gastropodPomaceacanaliculata has been studied with electronmicroscopy. Matureapyrene parasperm result from a cytodifferentiationprocesswithout maturation division. The atypical condition isrecognizedearly. Paraspermatogonia are characterized by voluminousnucleiwith irregular clusters of heterochromatin, numerous dilatedcisternaeof the rough endoplasmic reticulum and small electron-densegranulesspread through the cytoplasm. As this process advances, thenucleibecome lobed and chromatin degenerates. The remnant chromatincondensesto form dense bodies which are finally excreted from thecells.At the cytoplasmic level a centriolar multiplication with theconsequentflagellogenesis takes place. The axonemal microtubules runalongthe entire length of the cell and emerge from the posteriorendforming a tuft of three or more free flagella. The roughendoplasmicreticulum and the Golgi complex are involved inthe production ofsecretory granules, some of which are laterreleased by exocytosis. Atthe end of paraspermatogenesis matureapyrene parasperm arefusiform-shaped anuclear ciliated cells. (Received 20 January 2000; accepted 18 July 2000)