Partial depletion of CYK-1 primarily affects F-actin levels in the contractile ring and leads to ring structural defects. (A) Mean fluorescence in whole embryos, measured as illustrated. Statistical significance was determined using a two-tailed, unpaired Student’s t test. ns, not significant, P ≥ 0.05. n is the number of embryos analyzed (n = 10 in control and n = 7 in cyk-1(RNAi)). (B) Images of control and cyk-1(RNAi) four-cell embryos co-expressing ARX-7::GFP and NMY-2::mKate2. A time point during ring constriction in ABa/p cells is shown. (C) Images from a time-lapse series of a cyk-1(RNAi) ring with overlaid lines that were manually traced to determine the ring perimeter (in green in bottom row) in cases of continuous or discontinuous rings (large rupture: blue arrow; small discontinuity: orange arrow). (C′) Individual ring closure profiles in ABa/p cells, measured as illustrated in C. (D) Images from a time lapse series of a cyk-1(RNAi) ring marked by GFP::ANI-1. A large rupture is indicated by a blue arrow, small discontinuities are indicated by orange arrows, and a fiber that detaches from the ring and later re-integrates is indicated by green arrows. (E)cyk-1(RNAi) ring with a detached fiber whose ends are integrated in the ring (green arrows). The ring section underlying the fiber bridge breaks (yellow arrowheads), but the ring does not rupture; see Video 2. (F)cyk-1(RNAi) ring with fibers that detach from the ring (green arrows) and later re-integrate (pink arrows) at different stages of constriction. Numbers on stills correspond to time in seconds in D–F. Scale bar in B, 10 µm; and in C–F, 5 µm.