Rectification of planar orientation angle switches behavior and replenishes contractile junctions.

In the early Drosophila embryo, germband elongation is driven by oriented cell intercalation through t1 transitions, where vertical (dorsal-ventral aligned) interfaces contract and then resolve into new horizontal (anterior-posterior aligned) interfaces. Here, we show that contractile events produce a continuous "rectification" of cell interfaces, in which interfaces systematically rotate toward more vertical orientations. As interfaces rotate, their behavior transitions from elongating to contractile regimes, indicating that the planar polarized identities of cell-cell interfaces are continuously re-interpreted in time depending on their orientation angle. Rotating interfaces acquire higher levels of Myosin II motor proteins as they become more vertical, while disruptions to the contractile molecular machinery reduce the rates of rotation. Through this angle rectification, the available pool of contractile interfaces is continuously replenished, as new interfaces acquire a contractile identity through rotation. Thus, individual cells acquire additional interfaces that are capable of undergoing t1 transitions, allowing cells to participate in multiple staggered rounds of intercalation events.