THE FORCE FOR POLEWARD CHROMOSOME MOTION IN HAEMANTHUS CELLS ACTS ALONG THE LENGTH OF THE CHROMOSOME DURING METAPHASE BUT ONLY AT THE KINETOCHORE DURING ANAPHASE

The force for poleward chromosome motion during mitosis is thought to act, in all higher organisms, exclusively through the kinetochore, We have used time-lapse, video-enhanced, differential interference contra st light microscopy to determine the behavior of kinetochore-free ''ac entric'' chromosome fragments and ''monocentric'' chromosomes containi ng one kinetochore, created at various stages of mitosis in living hig her plant (Haemanthus) cells by laser microsurgery. Acentric fragments and monocentric chromosomes generated during spindle formation and me taphase both moved towards the closest spindle pole at a rate (similar to 1.0 mu m/min) similar to the poleward motion of anaphase chromosom es. This poleward transport of chromosome fragments ceased near the on set of anaphase and was replaced, near midanaphase, by another force t hat now transported the fragments to the spindle equator at 1.5-2.0 mu m/min. These fragments then remained near the spindle midzone until p hragmoplast development, at which time they were again transported ran domly poleward but now at similar to 3 mu m/min. This behavior of acen tric chromosome fragments on anastral plant spindles differs from that reported for the astral spindles of vertebrate cells, and demonstrate s that in forming plant spindles, a force for poleward chromosome moti on is generated independent of the kinetochore, The data further sugge st that the three stages of non-kinetochore chromosome transport we ob served are all mediated by the spindle microtubules. Finally, our find ings reveal that there are fundamental differences between the transpo rt properties of forming mitotic spindles in plants and vertebrates.