Movie 1.

Mechanical tension is rapidly increased during the initiation phase of the AP boundary. Time-lapse movies showing dorsal views of an abdominal epidermal segment of a pupa 5 min. after formation of the AP boundary (as defined by the first contact between an A and a P cell). Adherens junctions, labelled by E-cadherin-GFP, are depicted before and after ablation of a control AA cell junction or an AP cell junction. Times relative to the laser cut are indicated. Red dots indicate the two ends of the ablated cell junction. Scale bars: 10 μm.