Geometry-driven asymmetric cell divisions pattern cell cycles and zygotic genome activation in the zebrafish embryo

Early embryo geometry is one of the most invariant species-specific traits, yet its role in ensuring developmental reproducibility and robustness remains underexplored. Here, we show that in zebrafish, the geometry of the fertilized egg - specifically its curvature and volume - serves as a critical initial condition triggering a cascade of events that exert a lasting influence on development. Specifically, it guides asymmetric cell divisions in the blastoderm, generating radial gradients of cell volume and nucleocytoplasmic ratio. These gradients generate mitotic phase waves, with individual cell cycle periods determined largely cell-autonomously by the nucleocytoplasmic ratio. Modeling and perturbation experiments demonstrate that reducing cell autonomy reshapes these waves, emphasizing the instructive role of geometry-derived volume patterns in setting the intrinsic period of the cell cycle oscillator. Remarkably, in addition to organizing cell cycles, early embryo geometry also spatially patterns zygotic genome activation (ZGA) at the midblastula transition, a key step in establishing embryonic autonomy. Disrupting geometry alters the ZGA pattern and causes ectopic germ layer specification, underscoring its developmental significance. Together, our findings reveal a novel symmetry-breaking function of early embryo geometry in coordinating cell cycle and transcriptional patterning, establishing a blueprint for robust embryogenesis.