Movie S4.

Aligned active nematic generates opposite unidirectional flows. Passive tracer particles are dragged along the defect lanes in antiparallel directions (Left). Particle trajectories are highlighted in green and blue. Arrowheads indicate the direction of motion. When new defects proliferate (Center), a transversal flow originates, making particles deviate from the original rectilinear trajectories and incorporate into preexisting adjacent lanes (Right), thus reversing their direction.