Frog oocytes evade the checkpoint

Shao et al. reveal that Xenopus eggs have no spindle assembly checkpoint (SAC) to prevent them from entering anaphase with misaligned chromosomes.

In most dividing cells, the SAC prevents anaphase onset by keeping the ubiquitin ligase APC/C inactive until all chromosomes are correctly attached to the metaphase spindle. In frog oocytes, however, the APC/C is activated before the first meiotic spindle is assembled, suggesting that the SAC may not regulate anaphase initiation in these cells.

To test this idea directly, Shao et al. developed a way to analyze the state of meiotic frog chromosomes. In oocytes treated with microtubule-depolymerizing drugs such as colcemid, the first meiotic spindle was disrupted, but homologous chromosomes separated into individual pairs of sister chromatids without delay, indicating that the oocytes had progressed into meiosis II as normal. When colcemid was removed, oocytes reassembled their spindles, and, just like control oocytes, they arrested in metaphase II before completing the second meiotic division in response to a fertilization signal.

To confirm their findings, the researchers then analyzed oocytes with monopolar meiotic spindles—an abnormal architecture that triggers the SAC in mitotic cells. Frog eggs with monopolar spindles completed both rounds of meiosis with normal timing, however, supporting the idea that these cells have no SAC. Senior author Johné Liu now wants to investigate what other signals may regulate anaphase onset in Xenopus oocytes.