INTRAMITOTIC CONTROLS IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE - THE EFFECT OF CELL-SIZE ON SPINDLE LENGTH AND THE TIMING OF MITOTIC EVENTS

We have used a new cinemicroscopy technique in combination with antitubulin immunofluorescence microscopy to investigate the timing of mitotic events in cells of the fission yeast Schizosaccharomyces pombe having lengths at division between 7 and 60 μm. Wild-type fission yeast cells divide at a length of 14 μm. Separation of daughter nuclei (anaphase B) proceeds at a rate of 1.6 ± 0.2 μm min-1, until the spindle extends the length of the cell. Coincident with spindle depolymerization, the nuclei reverse direction and take up positions that will become the center of the two daughter cells. This post-mitotic nuclear migration occurs at a rate of 1.4 ± 0.5 μm-1. In cells in which the wee1+ gene is overexpressed fivefold and that have an average length at mitosis of 28 μm, the rate of nuclear separation was only slightly reduced but, as spindles in these cells measure 20-22 μm, the duration of anaphase B was extended by ~40%. By contrast, in the mutant wee1.50, which divides at 7 μm, both the rate and duration of anaphase B were indistinguishable from wild type. Nuclei reach the ends of these cells earlier but remain there until a point corresponding to the time of postmitotic nuclear migration in wild type. Thus, the events of mitosis can be extended but not abbreviated. These results are discussed in terms of a mitotic termination control that monitors many different events, one of which is spindle elongation.