Revealing a critical role of the incomplete spindle assembly checkpoint in zebrafish development through an optochemical approach

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 introduced transient mitotic errors at various developmental windows and systematically addressed their organismal effects. Upon CENP-E inhibition in the pre-gastrula period, embryos suffered gradual aggravation of developmental defects as the duration of the inhibition extended. Conversely, embryos tolerated several hours of consecutive CENP-E inhibition in the gastrula period, frequently achieving full development. Live imaging revealed that chromosome misalignment caused by CENP-E inhibition resulted in a modest mitotic delay in the gastrula, but not in the early pre-gastrula period, suggesting the functionalization of the spindle assembly checkpoint (SAC) at this stage. This mitotic delay helped alleviate, though not perfectly resolve, polar chromosome misalignment before anaphase onset. Importantly, pharmacological suppression of SAC rendered gastrula embryos inviable upon CENP-E inhibition. Therefore, despite its leaky nature, the embryonic SAC contributed to partial mitotic error correction, which proved essential to manage consecutive mitotic perturbations. Our results demonstrate the power of optochemical approaches in understanding the robust control of dynamic processes in development.