Prolonged metaphase II arrest weakens Aurora B/C-dependent error correction in mouse oocytes.

Chromosome segregation during meiosis is highly error-prone in mammalian oocytes. The mechanisms controlling chromosome attachments and the spindle assembly checkpoint (SAC) have been extensively studied in meiosis I, but our knowledge of these mechanisms during meiosis II is rather limited. Although mammalian oocytes arrest in metaphase II for an extended period awaiting fertilization, some misattached chromosomes may persist. This suggests that the mechanism correcting misattachments is not fully functional during the arrest. In this study, we investigated whether low inter-kinetochore tension, which characterizes incorrect attachments, can be detected by Aurora B/C-dependent error correction in meiosis II. We found that low tension, induced by low dose of STLC in early metaphase II, does indeed mediate microtubule detachment by Aurora B/C and, consequently, anaphase II delay through SAC activation. Surprisingly, we also found that, during prolonged metaphase II arrest, Aurora B/C activity is no longer sufficient to detach low-tension attachments, correlating with high accumulation of PP2A at kinetochores. As a result, the SAC is not activated, and sister chromatids segregate in anaphase II without delay even in the presence of low tension. Hence, during the prolonged metaphase II arrest to await fertilization, oocytes become unable to discriminate between correct and incorrect attachments and may allow errors to persist.