Mechanical dissection of ring tension and cortical relaxation throughout ring constriction. (A) Kymograph of the cut ring region showing the snapping event and straightening of the severed ends (a). The time interval between frames 3 and 4 is 2.6 s; other frames are 1 s apart. The overlay (b) shows frames immediately before (red) and after (cyan) the cut. (B) Schematic defining initial ring perimeter before the cut, arc length, and gap size. (C) Gap size measured during the first 10 s after the cut (n = 24). Cut rings were divided into three groups with initial perimeter averages of 35.8 µm (group I), 23.6 µm (group II), and 14.6 µm (group III). (D) Maximal gap size plotted against the initial perimeter for control rings cut once (black circles, n = 69) or twice, as in Fig. 1 C (first cut in purple, second cut in green; n = 6). Maximum gap size and initial perimeter are proportional (R2 = 0.81 and r = 0.90; P < 0.0001). Maximum gap size for rings of mlc-4–depleted embryos are in gray. (E–G) Ring recoil (Δl; E), strain of the ring (ε; F), and the difference between gap size and Δl reflecting ring curvature change (G) plotted against the initial perimeter for control (blue, orange, and green circles, color coded as in C; n = 22) or mlc-4–depleted rings (gray circles; n = 20). In G, a clear proportionality is observed (R2 = 0.84 and r = 0.92; P < 0.0001). (H) As in A, but for a cut ring of an embryo partially depleted of mlc-4. The time interval between frames 3 and 4 is 3.92 s; other frames are 2.32 s apart. (I) Schematic illustrating how the net tension results from the sum of static and dynamic components and increases for decreasing perimeters. (J) Rate of change in arc length during the first 8 s after cut against the initial perimeter (left). Significant values are in red and nonsignificant in gray. Error bars represent the 95% CI. An inset of the red points is shown (right). Bars, 5 µm.
