Transcriptional initiation and mechanically driven self-propagation of a tissue contractile wave during axis elongation
Tissue morphogenesis emerges from coordinated cell shape changes driven by actomyosin contraction1, 2. Spatial patterns of gene expression regionalize and polarize cell behaviours, such as apical constriction in the ventral mesoderm and cell intercalation in the lateral ectoderm of Drosophila3. Thus, tissue dynamics is largely governed genetically. Actomyosin contractile networks drive cell and tissue-level shape changes and can respond to mechanical stimuli 4-9. However how genetic information and mechanical control drive tissue-level morphogenesis is not well understood. Here we report two phases and modalities of Rho1 and non-muscle MyosinII (MyoII) activation in the Drosophila posterior endoderm. First, Rho1/MyoII are induced apically in a spatially restricted primordium region via localized transcription of the GPCR ligand Fog. Second, a tissue-scale travelling wave of Rho1/MyoII activation and cell invagination progresses anteriorly across the dorsal epithelium at a constant speed of 1 cell every 3 minutes. Remarkably, the MyoII wave does not require sustained gene transcription, and is also insensitive to perturbations in the level and pattern of Fog expression. Thus, while fog transcription initiates Rho1/MyoII activation in the primordium, Fog delivery does not govern wave dynamics. Instead, perturbing the mechanical environment of the endoderm impaired MyoII wave dynamics. MyoII inhibition blocked acute Rho1 activation and propagation, suggesting that MyoII contractility provides both local feedback amplification and spatial coupling necessary for wave progression. Finally, we identify a cycle of 3D cell deformations that link MyoII activation and invagination in one row of cells to vitelline membrane attachment, apical spreading, MyoII activation and invagination in the next row, to drive anterior progression of the invagination wave. Thus endoderm morphogenesis emerges from local transcriptional initiation and a mechanically driven travelling cycle of cell contraction and deformation.