Forces acting on the fission yeast anaphase spindle.

The fission yeast mitotic spindle consists of three sets of microtubules: one that extends between the chromosomes and the spindle pole bodies (SPBs); one that extends between the two SPBs forming a region of overlap; and a third, the so-called astral microtubules, that associates laterally with the cytoplasmic face of the SPBs, during anaphase B. The major bundles of the latter can exist with equal probability in two configurations which we have termed parallel and convergent. Mitosis in fission yeast is characterised by an extended anaphase B during which the spindle elongates from 2 mu m (the diameter of the interphase nucleus) to about 14 mu m, spanning the entire length of the cell. Anaphase B spindles viewed by indirect immunofluorescence microscopy frequently appeared bowed but only when the astral microtubules were in the convergent orientation. To investigate the possible significance of this observation, we have examined the situation in the abnormally long spindles that are formed in cells in which cell length has been extended either by overexpression of the weel + gene or by inactivation of the cdc25 + gene. The spindles in these strains were often considerably longer (up to 30 mu m) than in wild type cells but, unlike the latter, did not extend the entire length of the cell. Bowed spindles were again observed but only when the astral microtubules were convergent. We discuss these findings in the context of the astral microtubules either exerting a pulling force on the poles of the anaphase B spindle, or counteracting a pushing force generated by sliding of anti-parallel pole to pole microtubules of the mitotic spindle, or both of the above.