CYTOKINESIS IN SPIROGYRA - INTEGRATION OF CLEAVAGE AND CELL-PLATE FORMATION

In Spirogyra, cytokinesis is initiated by a cleavage furrow. When the furrow contacts the elongating interzonal fibers of the telophase spin dle, a small phragmoplast appears at the zone of contact, inside of wh ich a cell-plate appears to complete cytokinesis. The use of two seque ntial mechanisms for achieving cytokinesis may be an intermediate, and possibly a transitional, state in the evolution of the phragmoplast f ound in higher plants. We tested the relative contribution of the two types of cytokinetic mechanisms by observing the effects of certain dr ugs on living cells, recorded in time-lapse. The antimicrotubule drug oryzalin causes the spindle to break down rapidly. When applied during early to midcytokinesis, the interzonal spindle collapses onto the cl eavage furrow, which continues to grow inward. However, cytokinesis is not complete; examination of such cells 8-12 h later, after the nucle i separate, reveals a small aperture in the center of the cross-wall. This result shows that the phragmoplast/cell-plate is essential for co mplete cytokinesis, which apparently cannot be accomplished by cleavag e alone, and that inward growth of the cleavage furrow is not dependen t on microtubules. The antiactin drug cytochalasin D stops cleavage qu ite rapidly, and the eventual consequences of treatment depend on the stage reached in cleavage. If the cytochalasin is applied early in cyt okinesis, cell division is unable to proceed further. Between telophas e nuclei the cell does generate a large mass of cytoplasm that contain s typical proliferating phragmoplast fibers; cell-plate formation is i nitiated in this mass but is never able to proceed further, and no coa lescence of material into a cross-wall has been recorded. If cytochala sin is applied later, after the cleavage furrow has contacted the inte rzonal spindle fibers, cleavage stops but now cell-plate formation pro ceeds and cytokinesis is completed, albeit slowly. This result demonst rates that an interaction between the cleavage furrow and the forming phragmoplast is necessary for the latter to operate normally. These re sults support our suggestion that Spirogyra illustrates an intermediat e stage in the evolution of the phragmoplast, since both cleavage and cell-plate formation are required for cytokinesis. (This suggestion do es not require Spirogyra to be on the line of evolution leading to hig her plants; rather, Spirogyra appears to be undergoing an evolutionary process similar to what might have occurred in their progenitors,) Fu rthermore, interaction of the cleavage furrow with the forming cell-pl ate/phragmoplast is required for cross-wall completion. These results may have significance in considering mechanisms by which higher plants locate their cell-plates properly, an ability associated with the evo lution and possible function(s) of the preprophase band of microtubule s.