Tension on chromosomes increases the number of kinetochore microtubules but only within limits

When chromosomes attach properly to a mitotic spindle, their kinetochores g enerate force in opposite directions, creating tension. Tension is presumed to increase kinetochore microtubule number, but there has been no direct e vidence this is true. We micromanipulated grasshopper spermatocyte chromoso mes to test this assumption and found that tension does indeed affect the n umber of kinetochore microtubules. Releasing tension at kinetochores causes a drop to less than half the original number of kinetochore microtubules. Restoring tension onto these depleted kinetochores restores the microtubule s to their original number. However, the effects of tension are limited. Pr ometaphase kinetochores, when under normal tension from mitotic forces, hav e about half as many microtubules as they will in late metaphase. We impose d a tension force of 6x10(-5) dynes, three times the normal tension, on pro metaphase kinetochores. The elevated tension did not drive kinetochore micr otubule number above normal prometaphase values. Tension probably increases the number of kinetochore microtubules by slowing their turnover rate. The limited effect of tension at prometaphase kinetochores suggests that they have fewer microtubule binding sites than at late metaphase, The relatively few sites available in prometaphase may be the decisive sites whose bindin g of microtubules regulates the dynamics of transient kinetochore constitue nts, including checkpoint components.