Actomyosin cables position cell cohorts during Drosophila germband retraction by entraining their morphodynamic and mechanical properties

The unfolding and displacement of the germband during Drosophila germband retraction (GBR) accomplish the straightening of the embryonic anterior-posterior axis. The failure of GBR reduces embryonic viability and results in the mispositioning of the embryonic segments and the gastrointestinal tract. Despite its importance, the cellular, molecular and physical mechanisms that govern the unfolding of the germband and ensure the accurate positioning of cell fields within it remain poorly understood. Here, we uncover the requirement of planar polarized, supracellular, tensile actomyosin cables for entraining cellular morphodynamics, cell field positioning and retraction kinematics. Circumferential, non-constricting cables that form during early retraction ensure the coherence of placode-like cell cohorts, pattern medio-lateral gradients in cell shape and sidedness within it, and dampen retraction speed. Linear, constricting cables that power displacement at the end of retraction enable sequential, multi-tissue, collective T1 transitions to reposition medial cell fields to more posterior locations. Together, our results reveal how the spatiotemporally regulated deployment of actomyosin structures, functioning either as barricades or as purse strings, modulate the speed of tissue unfolding and enable cell field positioning by influencing the morphodynamic and mechanical properties of cell cohorts during morphogenesis.