Contractile actomyosin networks have been shown to power tissue morphogenesis. Although the basic cellular machinery generating mechanical tension appears largely conserved, tensions propagate in unique ways within each tissue. Here we use the vertebrate eye as a paradigm to investigate how tensions are generated and transmitted during the folding of a neuroepithelial layer. We record membrane pulsatile behavior and actomyosin dynamics during zebrafish optic cup morphogenesis by live imaging. We show that retinal neuroblasts undergo fast oscillations and that myosin condensation correlates with episodic contractions that progressively reduce basal feet area. Interference with lamc1 function impairs basal contractility and optic cup folding. Mapping of tensile forces by laser cutting uncover a developmental window in which local ablations trigger the displacement of the entire tissue. Our work shows that optic cup morphogenesis is driven by a constriction mechanism and indicates that supra-cellular transmission of mechanical tension depends on ECM attachment.DOI: http://dx.doi.org/10.7554/eLife.15797.001

Despite their evolutionary, developmental, and functional importance the origin of vertebrate paired appendages remains uncertain. In mice, a single enhancer termed ZRS is solely responsible for Shh expression in limbs. Here, zebrafish and mouse transgenic assays trace the functional equivalence of ZRS across the gnathostome phylogeny. CRISPR/Cas9-mediated deletion of the medaka-ZRS and enhancer assays reveal the existence of ZRS shadow enhancers in both teleost and human genomes. Deletion of both ZRS and shadow ZRS abolish shh expression and completely truncate pectoral fin formation. Strikingly, deletion of ZRS results in an almost complete ablation of the dorsal fin. This finding indicates that a ZRS-Shh regulatory module is shared by paired and median fins, and that paired fins likely emerged by the co‐option of developmental programs established in the median fins of stem gnathostomes. Shh function was later reinforced in pectoral fin development with the recruitment of shadow enhancers, conferring additional robustness.