Fig 1. Apterous is required for the characteristic straight shape of the DV boundary and for the local increase in cell bond tension.

(A,B) Wing discs from ap^Gal4 (control) and ap^UGO35/ap^Gal4 mutant larvae expressing GFP (red) in the dorsal compartment stained for E-cadherin (green) to visualize adherens junctions. Scale bars: 10 μm. (C,D) Segmentation of the images shown in (A) and (B). The red line marks the DV boundary. (E) Scheme depicting the measurement of roughness w of a boundary. The shape of the boundary is described by the function h(x) as the orthogonal distance of the boundary from the x axis, connecting the end points of the boundary, for any position x. For two points within the segment length L roughness is given by the average deviation from the mean value, ${(h\left(x\right)-\overline{h})}^2$, where $\overline{h}$ is the average of h(x) within the segment length L. We then average roughness along the boundary for any L indicated. (F) Roughness w of the DV boundary for the segment lengths L for the genotypes indicated in (A,B). Segment length and roughness values are normalized by the average cell bond length in the tissue $\overline{\mathcal{l}}$ = 1.7 μm. Mean and s.e.m. are shown (control, n = 6 wing discs; mutant, n = 7 wing discs). P <0.05 for L = 1–10. (G,H) Change in distance d between the vertices of cell bonds located within the compartments (comp.) or at the DV boundary after ablation (normalized to $\overline{\mathcal{l}}$) as a function of time for wing discs of ap^Gal4 (control) and ap^UGO35/ap^Gal4 mutant larvae. Mean and s.e.m. are shown (control, n = 10 comp., n = 11 DV; mutant, n = 9 comp., n = 6 DV cuts). (I,J) Initial velocity of vertex displacement after ablation of the indicated types of cell bonds for control (I) and ap^UGO35/ap^Gal4 mutant (J) larvae. Mean and s.e.m. are shown (n as in G,H). ** P<0.01; n.s. not significant.