Cortical centralspindlin and G alpha have parallel roles in furrow initiation in early C. elegans embryos

Evidence from various systems suggests that either asters or the midzone of the mitotic spindle are the predominant determinants of cleavage plane position. Disrupting spindle midzone formation in the one-cell Caenorhabditis elegans embryo, such as by using mutants of the centralspindlin component ZEN-4, prevents completion of cytokinesis but does not inhibit furrowing. However, furrowing is inhibited by the simultaneous depletion of ZEN-4 with either PAR-2 or G alpha, which are required for asymmetric divisions. Through studies of other genes required for the presence of an intact spindle midzone containing microtubule bundles, we found that furrowing failed in the absence of PAR-2 or G alpha only when centralspindlin was absent from the furrow. We also found spindle length or microtubule distribution did not correlate with furrow initiation. We propose that centralspindlin acts redundantly with G alpha to regulate furrow initiation.