Mechanical basis of meiotic metaphase arrest

Control of the metaphase to anaphase transition is a central component of cell-cycle regulation. Arrest at either metaphase I or II before fertilization is a common component of oogenesis in many organisms. In Drosophila melanogaster females, this arrest occurs at meiosis I with the chiasmate bivalents tightly massed at the metaphase plate and the nonexchange chromosomes positioned between the plate and the poles on long tapered spindles. Meiosis resumes only after passage through the oviduct. Thus, metaphase arrest defines an important checkpoint in the meiotic cell cycle. We report here that this arrest results from the balancing of chiasmate bivalents at the metaphase plate. Two meiotic mutations, mei-9b and mei-218a4, both of which greatly reduce the frequency of chiasma formation, bypass the metaphase block and allow stage 14 oocytes to finish both meiotic divisions without arrest. We conclude that metaphase arrest results from the balancing of kinetochore forces due to chiasmata.