This work was the result of a fruitful collaboration with Yuting I. Li and @ehannezo.bsky.social.
Also thanks to the support provided by
@istaustria-iof.bsky.social 🔬 and the Aquatics Facility 🐟 @ISTA
Congratulations to all the authors! Excellent work!! 👏👏

Finally, the N:C gradient and the resulting gradient of cell cycle length lead to an earlier onset of zygotic transcription at the margin, thereby establishing a novel, ‘symmetry-breaking’ function of early embryo geometry in zygotic transcription onset.

… and those with a steeper cell volume gradient due to artificially increased blastodisc curvature show slower propagating waves and decreased synchrony. SM6, SM7

Indeed, altering embryo geometry, and therefore the N:C ratio, perturbs the mitotic waves. Embryos with two 'APs' produce two waves, one from each ectopic AP.

The N:C ratio becomes graded due to the specific geometry of the early embryo, which causes patterned asymmetric cell divisions, producing smaller daughter cells at the blastoderm margin. This causes greater S-phase lengthening in these marginal cells.

Mitotic waves propagate largely independently of cell-cell coupling. In fact, increasing coupling through embryo syncytialization reshapes the wave and causes it to originate at the margin rather than near the AP.

We investigated the spatial patterning of early cell cycles in the zebrafish embryo and discovered that a gradient of nucleocytoplasmic (N:C) ratio within the embryo spatially patterns S-phase length along the animal pole (AP)-margin axis, producing radial mitotic waves near the AP.