Decreasing β-heavy-spectrin size is sufficient to aggravate the rupture of cyk-1(RNAi) rings. (A) Schematic showing the size and domain organization of full-length SMA-1, SMA-1ΔPH, and SMA-1Δ11SR. (B) Individual ring closure profiles in ABa/p cells. (C) Instantaneous ring perimeter change (10 s interval) versus ring perimeter in ABa/p cells throughout constriction. 11 contractile rings were analyzed per condition. (D) Gap size of ruptures or small discontinuities observed in 12 rings for each condition, measured as illustrated in Fig. 7 D. Conditions are colored as in B and C. (E) Ring constriction rate (mean ±95% CI). Conditions are colored as in B and C. n is the number of contractile rings analyzed (n = 11 in control, n = 11 in sma-1(ru18), n = 12 in cyk-1(RNAi), n = 12 in cyk-1(RNAi);sma-1ΔPH, n = 12 in cyk-1(RNAi);sma-1Δ11SR, and n = 12 in cyk-1(RNAi);sma-1(ru18)). Statistical significance was determined using unpaired one-way ANOVA followed by Bonferroni’s multiple comparison test. ****P < 0.0001; ns, not significant, P ≥ 0.05. (F) Images from time-lapse series of cyk-1(RNAi);sma-1ΔPH and cyk-1(RNAi);sma-1Δ11SR ABa rings labeled with NMY-2::GFP. Regressions in cyk-1(RNAi);sma-1ΔPH rings occur mostly due to small discontinuities (orange arrows), while regressions in cyk-1(RNAi);sma-1Δ11SR rings are mostly due to large ruptures (blue arrows). (G) Images of a cyk-1(RNAi);sma-1Δ11SR ABp ring in an embryo expressing NMY-2::GFP. The ring ruptures consecutively at the same site (blue arrows). After rupture, ring repair is accompanied by myosin accumulation at the repair site (asterisks). Green arrow points at fiber detachment from the ring before a new rupture occurs. Numbers correspond to time in seconds. Scale bars in F and G, 5 µm.