FEEDBACK-CONTROL OF THE METAPHASE-ANAPHASE TRANSITION IN SEA-URCHIN ZYGOTES - ROLE OF MALORIENTED CHROMOSOMES

To help ensure the fidelity of chromosome transmission during mitosis, sea urchin zygotes have feedback control mechanisms for the metaphase -anaphase transition that monitor the assembly of spindle microtubules and the complete absence of proper chromosome attachment to the spind le. The way in which these feedback controls work has not been known. In this study we directly test the proposal that these controls operat e by maloriented chromosomes producing a diffusible inhibitor of the m etaphase-anaphase transition. We show that zygotes having 50% of their chromosomes (similar to 20) unattached or mono-oriented initiate anap hase at the same time as the controls, a time that is well within the maximum period these zygotes will spend in mitosis. In vivo observatio ns of the unattached maternal chromosomes indicate that they are funct ionally within the sphere of influence of the molecular events that ca use chromosome disjunction in the spindle. Although the unattached chr omosomes disjoin (anaphase onset without chromosome movement) several minutes after spindle anaphase onset, their disjunction is correlated with the time of spindle anaphase onset, not the time their nucleus br eaks down. This suggests that the molecular events that trigger chromo some disjunction originate in the central spindle and propagate outwar d. Our results show that the mechanisms for the feedback control of th e metaphase-anaphase transition in sea urchin zygotes do not involve a diffusible inhibitor produced by maloriented chromosomes. Even though the feedback controls for the metaphase-anaphase transition may detec t the complete absence of properly attached chromosomes, they are inse nsitive to unattached or mono-oriented chromosomes as long as some chr omosomes are properly attached to the spindle.