Cytokinesis proceeds asymmetrically in C. elegans with the furrow invaginating first on one side and then shifting to the other side of the cell, report Audhya et al. on page 267. This taking of turns, and the structural mechanism that enforces it, may ensure that the furrow is localized in a single plane.

The asymmetry came to light when the authors studied CAR-1, which was previously identified in an RNAi study as being required for embryonic cytokinesis. Audhya et al. show that CAR-1 helps regulate maternally supplied RNAs in the germline and early embryo.

These RNAs are apparently required to assemble and maintain functional interzonal microtubules, which form between the separating chromosomes after anaphase. To get a picture of the resulting cytokinesis defect, the team expressed a GFP construct that specifically binds to the lipids of the plasma membrane.

In wild-type worms, the furrow invaginated from one side and then, as progress slowed on that side, it started in from the other side. Simultaneous imaging of microtubules and the plasma membrane marker indicated that the primary furrow proceeded until it bumped into the interzonal microtubule bundles. Because constriction continues at a constant rate, when one side encounters an obstacle, the force shifts to the far side of the cell.

In CAR-1–depleted embryos, interzonal microtubule bundles are missing, so there was no longer a structural transition in which the first furrow stops, and the furrow from the opposite side begins to ingress.