Stalling of cilia in hydin mutants. (a and b) Top views of ependymal cilia were analyzed frame-by-frame and the positions of selected cilia were marked by circles in different colors. The time between frames was 5 ms. The color of the circles was changed to black when a cilium changed direction. Wild-type cilia (a) smoothly transitioned from forward to backward movement, whereas mutant cilia (b) frequently stalled at the turnaround points as indicated by the accumulation of marks. The duration (in milliseconds) of stalling is indicated. Bars, 5 μm. (c–g) Line scans through the plane of beat from individual wild-type (c and e) and mutant (d, f, and g) ependymal cilia observed in the top view. Stalling of mutant cilia frequently resulted in plateaus at the peaks and valleys of the line scans (arrowheads). (h) Frame-by-frame line tracings of a complete beat cycle of an ependymal cilium from a hy3/hy3 animal. 36 consecutive frames are shown and some are numbered. The time between lines corresponds to 5 ms and the arrows indicate the direction of movement of the cilium. Note the extended rest times at the turnaround points (first and third sets of tracings).