Figure 1: Tissue shape defines the direction of tension.

(a, top) Schematic representation of a gastrulating Drosophila embryo; the prospective mesoderm is depicted in green. Anisotropic epithelial tension (red arrows) accompanies ventral furrow formation. Tension is predominantly directed along the a-p axis, whereas there is more movement or tissue flow (blue arrows) in the d-v direction towards the ventral midline. (a, bottom) The VF domain (green) consists of a ventral rectangle defined by the expression pattern of Twist and Snail. (b, top) Depletion of Spn27A activity in the early embryo increases the number of cells adopting the ventral fate, marked by Snail expression (green), around the embryo circumference. (b, bottom) Fat2 depletion in the female ovary inhibits oocyte elongation and leads to the formation of round eggs. (c) Initial recoil velocity was measured along a-p after vertical line incision (yellow line) and along d-v after horizontal line incision. Images are a representative example from live embryos before (0 s) and 3 s after line ablation performed in Ctr-RNAi embryos expressing sqh::GFP (myosin, green). (d) Box-and-whisker plot of initial recoil velocity (v0) along a-p and d-v, measured after line ablation performed in Ctr-RNAi (n=15 vertical incisions and n=9 horizontal incisions), Spn27A-RNAi (n=24 vertical incisions and n=9 horizontal incisions) and Fat2-RNAi (n=19 vertical incisions and n=11 horizontal incisions) embryos. ***P<0.001, n.s., not significant, t-test. (e) Images from live embryos injected with arm double-stranded RNA (arm-RNAi) to lower junctional complexes (top panels) in Ctr-RNAi, Spn27A-RNAi and Fat2-RNAi embryos. The arm-RNAi resulted in tissue tearing and separation of the myosin meshwork (green). Yellow lines highlight the direction of the tears. The angles between the tears and the a-p axis were quantified (lower panels). Angle repartitions are significantly different in Spn27A-RNAi (n=26 tears, 3 embryos) and Fat2-RNAi (n=22 tears, 3 embryos) compared with Ctr-RNAi embryos (n=13 tears, 3 embryos). **P<0.01, ***P<0.001, t-test. Scale bars, 40 μm (a,e) and 10 μm (c). All images of en face views show the ventral side of the embryos, and anterior is left and posterior is right.