The Existence of Phycoplast

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Cytokinesis in green algae occurs via a diverse range of mechanisms, including cleavage furrows in some algae and cell plates in others. Some green algae of the class Charophyceae use phragmoplasts similar to those in embryophytes to organize and guide the growing cell plate. In these algae, the microtubules of the telophase spindle give rise to the phragmoplast and are oriented perpendicular to the plane of cell division and the forming cell plate. The growth of the cell plate eventually disrupts the telophase spindle.

In the Chlorophyceae, the most common form of cell division occurs via a phycoplast. In these algae, the spindle collapses and a new system of microtubules forms that is oriented in parallel to the plane of cell division. This phycoplast can be observed in algae undergoing cytokinesis via cleavage furrow as well as algae utilizing a cell plate. The phycoplast may play a role in assuring that the plane of cell division will pass between the two daughter nuclei. Typically, these algae undergo "closed" mitosis where the nuclear envelope persists throughout mitosis.

Phycoplast-mediated cytokinesis in the primitive green algal flagellate,Carteria crucifera, has been examined by electron microscopy. The key developmental foci during cell division are mobile centriole-MTOCs which control mitotic spindle formation, the establishment of the plane of cytokinesis, the initiation of the cytokinetic furrow, the formation of the phycoplast and the formation of morphogenetic microtubular arrays. The cytokinetic cleavage mechanism entails an ingressive furrowing closely associated with a prolific network of internuclear endoplasmic reticulum.

Dictyosome activity is limited to the cleavage initiation zone and is responsible for the production of wall precursor-containing vesicles. Dictyosome materials do not contribute directly to the growing furrow edge. Potassium antimonate staining patterns reveal the cytokinetic ER as a storage/control site for calcium during cytokinesis. Discussion of possible models concerning this cytokinetic mechanism is presented.

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