Contractile rings partially depleted of CYK-1 frequently pause and transiently regress during constriction. (A) Schematic illustrating the first divisions in the C. elegans embryo. Contractile rings of ABa/p cells in the four-cell embryo are depicted in green. These cells divide perpendicularly to the imaging plane, which allows the entire circumference of the contractile ring to be imaged throughout constriction. (B and B′) Four-cell embryos (B) and ABa cells (B′) co-expressing LifeAct::GFP and mCherry::PH(PLC1δ1). Two different stages of ring constriction in ABa cells are shown in B. (C) Normalized mean fluorescence intensity of LifeAct::GFP and NMY-2::mCherry (±95% CI) in the peripheral arc of ABa rings. n is the number of contractile rings analyzed (n = 9 in control and n = 11 in cyk-1(RNAi)). Statistical significance was determined using a two-tailed, unpaired Student's t test. ***P < 0.001; ns, not significant, P ≥ 0.05. Schematic on top illustrates how intensities were measured: a line was drawn over the peripheral arc of ABa rings and the mean fluorescence intensity along the line was measured. (D) Ring closure profile in ABa/p cells. Lines represent ring perimeter versus time of individual examples. Time point 0 s refers to nuclear envelope breakdown (NEBD). (E) Mean ring constriction rate ±95% CI. Statistical significance was determined using a two-tailed, unpaired Student’s t test. ****P < 0.0001. In D and E, n is the number of contractile rings analyzed (n = 7 in control and n = 13 in cyk-1(RNAi)). (F) Time projections of time-lapse imaging series for an interval of 150 s, generated as indicated in the schematic. Consecutive images were acquired 10 s apart. Both rings have an initial perimeter of ∼59 µm. The last frames correspond to a perimeter of 15 µm in the control ring and 42 µm in the cyk-1(RNAi) ring. Scale bars in B, B′, and F, 5 µm.