FORCES ACTING ON THE FISSION YEAST ANAPHASE SPINDLE

The fission yeast mitotic spindle consists of three sets of microtubul es: one that extends between the chromosomes and the spindle pole bodi es (SPBs); one that extends between the two SPBs forming a region of o verlap; and a third, the so-called astral microtubules, that associate s laterally with the cytoplasmic face of the SPBs during anaphase B. T he 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 whi ch the spindle elongates from 2 mu m (the diameter of the interphase n ucleus) to about 14 mu m, spanning the entire length of the cell. Anap hase B spindles viewed by indirect immunofluorescence microscopy frequ ently appeared bowed but only when the astral microtubules were in the convergent orientation. To investigate the possible significance of t his observation, we have examined the situation in the abnormally long spindles that are formed in cells in which cell length has been exten ded either by overexpression of the wee1 + gene or by inactivation of the cdc25 + gene. The spindles in these strains were often considerabl y longer (up to 30 mu m) than in wild type cells but, unlike the latte r, did not extend the entire length of the cell. Bowed spindles were a gain observed but only when the astral microtubules were convergent. W e discuss these findings in the context of the astral microtubules eit her exerting a pulling force on the poles of the anaphase B spindle, o r counteracting a pushing force generated by sliding of anti-parallel pole to pole microtubules of the mitotic spindle, or both of the above . (C) 1996 Wiley-Liss, Inc.