ULTRAVIOLET MICROBEAM IRRADIATIONS OF EPITHELIAL AND SPERMATOCYTE SPINDLES SUGGEST THAT FORCES ACT ON THE KINETOCHORE FIBER AND ARE NOT GENERATED BY ITS DISASSEMBLY

Ultraviolet (UV) microbeam irradiations of crane-fly spermatocyte and newt epithelial spindles severed kinetochore fibres (KT-fibres), creat ing areas of reduced birefringence (ARBs): the remnant KT-fibre consis ts of two ''stubs,'' a pole-stub attached to the pole and a KT-stub at tached to the kinetochore. KT-stubs remained visible but pole-stubs so on became undetectable [Forer et al., 1996]. At metaphase, in both cel l types the KT-stub often changed orientation immediately after irradi ation and its tip steadily moved poleward. In spermatocytes, the chrom osome attached to the KT-stub remained at the equator as the KT-stub e longated. In epithelial cells, the KT-stub sometimes elongated as the associated chromosome remained at the equator; other times the associa ted chromosome moved poleward together with the KT-stub, albeit only a short distance toward the pole. When an ARB was generated at anaphase , chromosome(s) with a KT-stub often continued to move poleward. In sp ermatocytes, this movement was accompanied by steady elongation of the KT-stub. In epithelial cells, chromosomes accelerated polewards after irradiation until the KT-stubs reached the pole, after which chromoso me movement returned to normal speeds. In some epithelial cells fine b irefringent fibres by chance were present along one edge of ARBs; thes e remnant fibres buckled and broke as the KT-stub and chromosome moved polewards. Similarly, KT-stubs that moved into pole stubs (or astral fibres) caused the pole stubs (or astral fibres) to bend sharply from the point of impact. Our results contradict models of chromosome movem ent that postulate that force is generated by the kinetochore disassem bling the KT-fibre. Instead, these results suggest that poleward direc ted forces act on the KT-fibre and the KT-stub and suggest that contin uity of microtubules between kinetochore and pole is not obligatory fo r achieving anaphase motion to the pole. (C) 1997 Wiley-Liss, Inc.