A unique pattern of F-actin organization supports cytokinesis in vacuolated cells of Macrocystis pyrifera (Phaeophyceae) gametophytes

Summary. The organization of actin filaments and their role in cytokinesis was studied in regenerating protoplasts and thallus cells of gametophytes of the brown alga Macrocystis pyrifera. Before the onset of cytokinesis, a ring of actin filaments appeared on the putative cytokinetic plane just under the plasmalemma. Light and electron microscopy of cytokinetic cells revealed that large vacuoles occupy the space between the daughter nuclei, which very often are eccentrically positioned at the cell cortex. By the progress of cytokinesis, actin filament bundles emanating from the cytokinetic ring tend to form an actin plate that enters cytoplasmic pockets in which the cytokinetic diaphragm develops. The mechanism of this cytokinetic pattern that has not been reported so far for brown algae is discussed. Correspondence and reprints: Department of Botany, Faculty of Biology, University of Athens, Athens 157 84, Greece.