Interphase microtubule bundles use global cell shape to guide spindle alignment in fission yeast

Correct spindle alignment requires a cell to detect and interpret its global geometry and to communicate this information to the mitotic spindle. In the fission yeast, Schizosaccharomyces pombe, the mitotic spindle is aligned with the longitudinal axis of the rod-shaped cell. Here, using wild-type and cell-shape mutants we investigate the mechanism of initial spindle alignment and show that attachment of interphase microtubules to the spindle pole bodies (SPB), the yeast equivalent of the centrosome, is required to align duplicated SPBs, and thus the mitotic spindle, with the long axis of the cell. In the absence of interphase microtubules or attachment between the microtubules and the SPB, newly formed spindles are randomly oriented. We show that the axis of the mitotic spindle correlates with the axis along which the SPB, as a consequence of interphase microtubule dynamics, oscillates just before mitosis. We propose that cell geometry guides cytoplasmic microtubule alignment, which in turn, determines initial spindle alignment, and demonstrate that a failure of the spindle pre-alignment mechanism results in unequal chromosome segregation when spindle length is reduced.