Mps1 and Ipl1/Aurora B Act Sequentially to Correctly Orient Chromosomes on the Meiotic Spindle of Budding Yeast

The conserved kinases Mps1 and Ipl1/Aurora B are critical for enabling chromosomes to attach to microtubules so that partner chromosomes will be segregated correctly from each other, but the precise roles of these kinases have been unclear. We imaged live yeast cells to elucidate the stages of chromosome-microtubule interactions and their regulation by Ipl1 and Mps1 through meiosis I. Ipl1 was found to release kinetochore-microtubule (kMT) associations after meiotic entry, liberating chromosomes to begin homologous pairing. Surprisingly, most chromosome pairs began their spindle interactions with incorrect kMT attachments. Ipl1 released these improper connections, whereas Mps1 triggered the formation of new force-generating microtubule attachments. This microtubule release and reattachment cycle could prevent catastrophic chromosome segregation errors in meiosis. In order to be segregated properly, partnerchromosomesmust attach to microtubules thatemanate from opposite poles of the spindle— a configuration referred to as bioriented. Chro-mosomes become bioriented in amultistep process (fig. S1A) that involves the release and reattach-ment of kMT connections that would lead to chromosome segregation errors (1). A surveil