F-actin organization during the cell cycle of Sphacelaria rigidula (Phaeophyceae)

The organization of F-actin in vegetative cells of the brown alga Sphacelar ia rigidula was studied after staining with a modified rhodamine-phalloidin (Rh-Ph) protocol. The interphase vegetative cells display a well-organized F-actin cytoskeleton, consisting of cortical, endoplasmic and perinuclear arrays of actin filaments (AFs). The organization of these AFs changes slig htly during mitosis, while they almost disappear at cytokinesis. The perinu clear AF population becomes more obvious during prophase, especially at the poles. At metaphase, an actin spindle is organized, co-localized with the microtubule spindle, while at anaphase an interzonal AF population appears, which persists at early telophase. At advanced telophase the image changes to a rather diffuse actin meshwork in the mid-area between the daughter nu clei. During post-telophase-early cytokinesis this AF system becomes gradua lly disassembled and a conspicuous actin plate is formed on the cytokinetic plane. This plate becomes more compact with the progress of cytokinesis, a nd seems to overcoat the patches of the forming diaphragm, and finally the daughter plasmalemmata. Treatment with cytochalasin B disturbs mitosis, not allowing the cells to proceed to anaphase, and prevents cytokinesis. These observations show that AFs are ubiquitous cytoskeletal elements of the veg etative cells of brown algae. The probable role of AFs during the cell cycl e and particularly the unique actin configuration involved in cytokinesis a re discussed.