CELL-DIVISION IN THE GENUS KOLIELLA (CHAROPHYCEAE) WITH EMPHASIS ON THE CYCLIC BEHAVIOR OF CLEAVAGE-ASSOCIATED AND CORTICAL MICROTUBULES

The ultrastructure of both vegetative cells and division mechanisms in the weakly filamentous genus Koliella HINDAK 1963 were studied using transmission electron microscopy (TEM) and scanning electron microscop y (SEM). Interphase cells of Koliella are characterized by a system of annular cortical microtubules which are arranged in a scalariform pat tern. Mitosis and cytokinesis in Koliella strongly resemble these occu rring in the related genera Stichococcus, Raphidonema and Klebsormidiu m. However, some salient differences have been found which are presuma bly the effect of the use of the rapid freeze-fixation and freeze-subs titution procedures for processing the Koliella cells for TEM. These d ifferences include: (1) a nucleus-encircling band of 2-4 microtubules which is present from the very first beginning of mitosis until the cl eavage furrow is accomplished (thus already existing during preprophas e, while the plane of cytokinesis seems to be determined by it); (2) a close spatial association of lined-up electron dense vesicles with th e initial cleavage furrow which develops centrally in between the nucl eus-encircling microtubules; (3) a semi-closed spindle; (4) a less pro minent collection of interzonal vacuolar vesicles between the separati ng chromosomes during late anaphase and telophase; (5) the intimate as sociation of the conspicuous peroxisome/mitochondrion-complex with the chloroplast-facing flank of the mitotic spindle throughout mitosis. C entrioles were not detected, neither in interphase or in mitotic cells . It is hypothesized that - on the basis of its preprophase appearance and its assumed role in marking the plane of cytokinesis - the nucleu s-encircling band of microtubules in Koliella may represent an early p recursor of the preprophase band (PPB) which occurs in the green alga Mougeotia and, in a more advanced state, in higher plants. The systema tic and phylogenetic position of Koliella among the green algae is dis cussed.