Algae. 1989; 4(2): 87-96.
Spatial and Temporal Organization of Microtubules in the Cyst Formation of Acetabularia calyculus Quoy et Gaimard: Visual Observation on the Segregation of Cytoplasmic units
Ikuko Shihira-Ishikawa
Department of Biology, Tokyo Gakugei University
ABSTRACT
A single, large nucleus in the rhizoid of Acetabularia calyculus in vegetative stage is divided into thousand of daughter nuclei immediately after meiosis, all of which immigrate into a newly formed reproductive organ (cap). The cytoplasm in the cap begins to differentiate soon after the immigration of nuclei and in 24 hours it transforms itself into the form of a cluster of spores (cysts). It had been an unsolved question as to why cytoplasm could be divided equally into small pieces, each of which contains a single nucleus without fail. In this study, the mechanism for the partition of cytoplasm and cyst formation was revealed using indirect immunofluorescence. The daughter nuclei settled equidistantly in a thin layer of cytoplasm underneath the surface of each cap-ray (sporangium). Microtubule bundles then extended radially from each nucleus and turned counter-cockwise whenever they collided with the bundles extending from any other neighbouring nuclei. Eventually turning action of these bundles formed a ring delimiting some cytoplasmic area with a single nucleus in its center. This ring then shrank in size with the help of actin and at the same time ring was pulled towards the center of the cap-ray causing the cytoplasm to push outward and form a sphere containing a single nucleus. Plasma membrane was then produced at the surface of each cytoplasmic sphere and cyst formation was completed.
Keywords : Acetabularia, coenocytic algae, cyst formation, microtubule.