Control of tissue fluidity by optogenetic manipulation of actomyosin*

Epithelial tissue sheets can be shaped into complex forms through series of stretching, folding and flowing events, driven by myosin-generated forces. However, it is not well understood how different patterns of tension can influence tissue mechanical properties and cell behaviors that contribute to rapid tissue deformations. Here we use optogenetic tools to manipulate actomyosin contractility in the germband epithelium, which exhibits rapid flow during Drosophila body axis elongation. The ability to flexibly induce changes in myosin contractility allows us to analyze the effects of distinct myosin patterns on cell rearrangements, tissue tensions, and tissue mechanical properties. We find that either optogenetic activation or deactivation of actomyosin at the apical surface of the tissue disrupts tissue tension anisotropy and cell packings, leading to fewer cell rearrangements and reduced tissue-level flow. These results directly link the distribution of myosin II to tissue tension and cell packings and suggest that actomyosin influences not only the anisotropic forces that drive tissue flow but also the mechanical properties of the tissue, leading to a complex relationship between cell-generated forces and tissue fluidity.