Abstract

Canalization of developmental processes ensures the reproducibility and robustness of embryogenesis within each species. In its extreme form, found in ascidians, early embryonic cell lineages are invariant between embryos within and between species, despite rapid genomic divergence. To resolve this paradox, we used live light-sheet imaging and developed automated single-cell segmentation and tracking procedures to quantify individual embryonic cell behaviors. This quantitative approach revealed that individual cell lineages, cell geometries, positions and even contacts are highly reproducible between embryos. This extreme reproducibility may be linked to the control of fate specification by local cell inductions. While in vertebrates the outcome of cell inductions is usually controlled by the concentration of diffusible extracellular ligands, ascidian cell inductions appear to be controlled by the area of contacts between signalling and responding cells, rather than by differential concentrations of ligands. We propose that the duality between genetic and geometric control of inductions contributes to the counterintuitive inverse correlation between geometric and genetic variability during embryogenesis.