Optochemical elucidation of a critical role of the incomplete spindle assembly checkpoint in zebrafish development.

Early animal embryos must balance the efficiency with the accuracy of mitotic control. However, the extent of mitotic errors that can be safely endured at different stages of development is unclear. In this study, using a recently developed photoswitchable CENP-E inhibitor, we introduce transient mitotic errors at various developmental windows and systematically address their organismal effects. Upon CENP-E inhibition in the pre-gastrula period, embryos suffer gradual aggravation of developmental defects as the duration of the inhibition extends. Conversely, embryos tolerate several hours of consecutive CENP-E inhibition in the gastrula period, frequently achieving full development. Live imaging reveals that chromosome misalignment caused by CENP-E inhibition results in a modest mitotic delay in the gastrula, but not in the early pre-gastrula period, suggesting the gradual functionalization of the spindle assembly checkpoint (SAC) at this stage. This mitotic delay helps alleviate, though not perfectly resolve, polar chromosome misalignment before anaphase onset. Importantly, pharmacological suppression of SAC renders gastrula embryos inviable upon CENP-E inhibition. Therefore, despite its leaky nature, the embryonic SAC contributes to partial mitotic error correction, which proves essential to manage consecutive mitotic perturbations. Our results demonstrate the power of optochemical approaches in understanding the robust control of dynamic processes in development.