THE PERINUCLEAR MICROTUBULE SYSTEM IN THE GREEN-ALGA ACETABULARIA - ANCHOR OR MOTILITY DEVICE

Migrating secondary nuclei in Acetabularia are tightly associated with actin bundles and possess a comet-like tail composed of microtubules. When secondary nuclei begin to settle in preparation for cyst morphog enesis, the tails expand into radially symmetrical arrays of microtubu les. Concomitantly, nuclei become gradually dissociated from the actin bundles and eventually stop moving, even though the actin bundles rem ain intact and persist through this stage. If, however, the radial per inuclear microtubule arrays are destroyed by inhibitors, the nuclei re associate with the actin bundles and regain their motile activity. Bec ause this movement is sensitive to Cytochalasin D, we propose that act in is required for nuclear movements, whereas microtubules most likely function as a trailing anchor that begins to act as a braking device above a certain threshold in the number and length of perinuclear micr otubules.