Anaphase in early C. elegans embryos. (A) In wild-type embryos, interactions between astral microtubules (red) and dynein motors anchored at the cell cortex (gray) generate strong pulling forces that place the mitotic spindle under high tension. Separation of the chromosomes (blue) in these cells is driven mainly by movement of spindle poles (pink) away from one another (i.e., mainly by anaphase “B”). The k-fibers (green) that connect chromosomes to poles shorten only slightly (i.e., anaphase “A” is minimal), perhaps because their shortening is directly inhibited by spindle tension that they carry from the poles to the midzone microtubules (yellow). (B) Knockdown of the dynein adapter GPR-1/2 weakens cortical pulling forces and reduces separation of the poles, presumably because overall spindle tension is reduced. However, k-fiber shortening is accelerated, thereby increasing the contribution of anaphase A and partially compensating for the loss of anaphase B. The acceleration of k-fiber shortening might arise directly from a reduction in k-fiber tension.