The giant unicellular alga Acetabularia enters the reproductive phase of growth when the cap has completed its expansion. The primary nucleus then divides and the resulting secondary nuclei take up fixed equidistant positions in the coenocytic cap cytoplasm. These positions are visible as colourless areas in the otherwise green caps.
Studies on the cytology and ultrastructure of the caps at this stage showed that each secondary nucleus was surrounded by a layer of cytoplasm which was free from chloroplasts and mitochondria, but which contained many microtubules, some of which were closely associated with the nuclear envelope, while others appeared to touch the plasmalemma adjacent to the cell wall. (Microtubules are not a regular feature of Acetabularia cells; they have not been reported in the stems, young caps or cysts, and vegetative growth is not inhibited by colchicine.)
The function of the microtubules was investigated by treatment with colchicine or vinblastine sulphate, both of which depolymerize microtubules in many other systems. Administration of these drugs at 10-3 M and greater concentrations had the following effects: (1) the colourless areas were lost as chloroplasts and mitochondria invaded the cytoplasm around each secondary nucleus; (2) the nuclei began to migrate from their fixed positions; and (3) cyst formation (in which the cytoplasm cleaves into uniformly sized cysts, each containing a single secondary nucleus) either was inhibited, or proceeded abnormally.
It is therefore proposed that a major function of these transitory microtubules is to anchor the secondary nuclei at fixed equidistant positions in the cytoplasm. This function is probably mediated by their association both with the nuclear envelope and cell membrane.