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Abstract

In all eukaryotic cells, correct segregation of the genetic material during cell division requires proper positioning and alignment of the mitotic spindle with respect to the cell division plane. In the fission yeast Schizosaccharomyces pombe, the spindle is aligned by interphase microtubules during early mitosis [1], and by astral microtubules and cell shape during late mitosis [2]. However, the mechanisms that control spindle alignment in mid-mitosis are not known. Lack of such a control may lead to extensive spindle rotation and misalignment, which in turn results in chromosome mis-segregation if spindle elongation is impaired [1]. We show that the association of mitochondria with the spindle poles reduces spindle rotation. In wild type, spindles with associated mitochondria did not rotate as much as free spindles. In a mutant of the centrosomin-related protein Mto1p, mitochondria were less associated with the spindles, which rotated more than in wild type. We propose that there is a symbiotic relationship between mitochondria and the mitotic spindle, where close association between the two organelles facilitates the positioning of both: while the spindle helps to segregate mitochondria equally among the nascent daughter cells [3], mitochondria decrease spindle rotation and thus promote spindle alignment.