Figure 1.

Mitotic spindle movements in the C. elegans zygote. (A) Schematic diagram of a single-celled C. elegans embryo showing cortical pulling by cytoplasmic dynein during pronuclear centration and rotation. The nuclei move toward the anterior (left) so that the spindle assembles in the center of the embryo. (B) Schematic diagram of a single-celled C. elegans embryo showing cortical pulling by dynein during anaphase. The spindle moves to the posterior (right) so that cytokinesis generates two cells of different sizes. The squares highlight a dynein molecule that pulls toward the posterior before spindle displacement, then pulls toward the anterior after spindle displacement. (C) Schematic drawing of cytoplasmic pulling that contributes to centering the pronuclei. (D) Illustration of a spindle that was centered at metaphase but in which both poles moved all the way to the posterior end of the embryo. This occurs in zyg-8 mutants (Gönczy et al., 2001), cls-1,2 (RNAi) embryos (Espiritu et al., 2012), and zyg-9(ts) mutants shifted to a nonpermissive temperature at metaphase (Bellanger et al., 2007), possibly because astral microtubules are too short to reach force generators that would pull toward the anterior.