PCH-2 is required for the spindle checkpoint in C. elegans. (A, top) Schematic of the distal section of the C. elegans germline. (A, bottom) Activation of the spindle checkpoint by shifting zyg-1^ts worms to the nonpermissive temperature increases the mitotic index (number of phosph-H3S10–positive nuclei). Representative images of control and zyg-1^ts germlines stained with antibodies recognizing phospho-H3S10 and phospho-SUN-1-S8 as well as with DAPI. Bar, 20 µm. (B) Mutation of pch-2 does not affect the mitotic index of germline mitotic nuclei unless zyg-1 is inactivated (zyg-1^ts). (C, top) Schematic and image of the two-cell C. elegans embryo. Bar, 10 µm. (C, bottom) Selected frames from wild-type, pch-2, zyg-1^RNAi, and pch-2;zyg-1^RNAi movies are shown, denoting NEBD and OCC. Arrow indicates persistent membrane blebs between AB and P₁. Bar, 5 µm. (D) Mutation of pch-2 has no effect on mitotic timing during an unperturbed mitosis but reduces mitotic timing to wild-type timing when zyg-1 is knocked down by RNAi (zyg-1^RNAi). Error bars in all graphs represent SEM. **, P < 0.0001.