LET-99 is required for spatial asymmetry of cortical pulling forces during spindle positioning. (A) Schematic of centrosome fragmentation analysis in a WT embryo. Gray lines extending to the cortex indicate astral MTs linking centrosome fragments with cortical force domains. Arrows show the direction of fragment travel. (B) Tracking data from individual centrosomes in representative embryos. (C) Complete dataset from centrosome fragmentation at anaphase. Tracking velocities (y axis) are plotted against the direction of fragment travel (x axis). Posterior centrosome data points are dark blue; anterior, light blue. Colored bars show the extent of LET-99 and GPR-1/2 cortical domains. (D) Velocity data binned by direction of travel, and averaged for cortical domains. Error bars show SEM; lines between columns highlight statistically significant differences between lateral–posterior and absolute posterior mean velocities in wild type (P = 0.05). The following comparisons were also significant. WT (anaphase): 0–25 versus 75–100, 0–25 versus 50–75. WT (metaphase): 0–25 versus 50–75.