Do astral microtubules play a role in metaphase chromosome positioning?

From several recent studies on monopolar spindles, it is now clear that a phase analogous to metaphase in bipolar spindles exists in the monopolar spindle, denying the validity of the favored model for metaphase which is based on the balance between two oppositely directed poleward forces acting on the unsplit kinetochores. Faced with this new fact, several investigators have proposed new models for metaphase plate formation which work for both the monopolar and the bipolar spindles. Since astral microtubules are thought to play important roles in certain models, we have investigated the role of astral microtubules in maintaining chromosomes at the metaphase position by using monopolar spindles induced in sea urchin embryo cells. When monopolar spindles were exposed to 1 microM nocodazole, a microtubule depolymerizing agent, most of the astral microtubules were rapidly depolymerized while the kinetochore fibers appeared to be little affected. Chromosomes were locked into the metaphase position for as long as 2 min, indicating that the metaphase chromosome position in sea urchin monopolar spindles can be maintained with little or no astral microtubules. By the effect of the antimitotic drug, kinetochore fibers slowly shortened and chromosomes which were attached to the far end of kinetochore fibers moved toward the pole. On the other hand, the chromosome position rapidly shifted farther away from the pole when monopolar spindles were treated with taxol or D2O. These results indicate that the metaphase chromosome position can be altered by affecting microtubule dynamics, particularly that of the kinetochore fiber microtubules as suggested by the results of the nocodazole experiments.