Science Collected Works - Theses

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Subject: Actin × Subject: Cytokinesis ×

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    Interaction of actin and microtubules during spatially organised cell division in green plants
    McIntosh, Kirsten (University of Melbourne, 2002)
    I investigated cytokinesis in three species of green algae: Spirogyra, Coleochaete scutata and C. nitellarum. Spirogyra initiates cytokinesis with an actin-cleavage furrow. At telophase, the edge of this furrow impinges upon the spindle fibres, transforming them into a small phragmoplast. Cell plate formation completes the cross-wall. These two sequential mechanisms cannot replace each other and a successful interaction between the microtubules of the phragmoplast and the actin of the cleavage furrow is crucial for completion of cytokinesis. Both the radial and circumferential divisions of C. scutata use a phragmoplast during cytokinesis in the apparent absence of an actin system of cleavage. The circumferential division forms a wall ingrowth which appears to be produced by the phragmoplast. Radial cytokinesis involves a phragmoplast and centrifugal expansion of a cell plate. In C. nitellarum, transverse and branch divisions use a phragmoplast which can equally well form a typical cell plate or a wall ingrowth. Neither of these divisions uses an actin cleavage furrow. Mitosis in these green algae was also investigated. High concentrations of anti-actin drugs caused abnormalities during mitosis. In Spirogyra, these abnormalities included an apparent skip from prometaphase to anaphase (i.e., bypassing metaphase) and an abnormal elongation of the spindle. These drugs caused a reversible block at metaphase in C. scutata, and a prolonged prometaphase, and sometimes a prolonged metaphase, in C. nitellarum. Collectively, these observations suggest that actin is involved in mitosis. In addition, in C. nitellarum an anti-microtubule drug caused persistent oscillations of the chromosomes after collapse of the metaphase plate. These continued movements of the chromosomes suggest that they may attach to a second motility system (not using microtubules) during mitosis. Finally, I investigated the curious phenomenon of cytoplasmic rotation in the hair cells of C. scutata and C. nitellarum. An actin-myosin system appears to operate during rotation, and in C. scutata microtubules may also be involved.