Mitosis and autospore formation in the green algaKirchneriella lunaris

Abstract

Asexual reproduction inKirchneriella lunaris involves autospore formation. After an initial mitosis, the curved cell cleaves to a variable extent, and then the nuclei divide again; finally the cytoplasm is partitioned into four around each nucleus. Rudimentary centrioles appear prior to the first mitosis; centriole complexes then become associated with a developing sheath of extranuclear microtubules at prophase; fenestrae appear at the poles through which both microtubules and centrioles migrate, preceding intranuclear spindle formation. The nucleus meanwhile is enveloped by a perinuclear layer of endoplasmic reticulum which is also interposed between the golgi body and nuclear envelope. Chromosome separation is accompanied by considerable spindle elongation. Finally the reforming nuclear envelope excludes both centriole complex and interzonal spindle apparatus from daughter nuclei. Cleavage is preceded by i) nuclear movement to the cell center, ii) movement of centriole complexes around daughter nuclei until they are opposite one another, and iii) the concurrent formation of a system of transverse microtubules extending across the cell. Other microtubules encircle the cell predicting the cleavage plane. A septum then appears amongst these cytokinetic microtubules, possibly derived from the plasmalemma; it extends across the cell too, through the cleaving peripheral chloroplast. Secondary mitoses follow (as above) during which this septum may be partially resorbed. Finally this septum is reformed, if necessary, and two other septa appear (as above) to quadripartition the cell. Mitotic and cytokinetic structures in this algae are briefly compared with some others.