Changes in cell shape involve reorganization and contraction of the actin-myosin cytoskeleton. During C. elegans embryogenesis, both cytokinesis and elongation use Rho-mediated shape changes. Elongation is where lateral epidermal cells (seam cells) change shape from cube-like to cylindrical, which is transmitted to the ventral and dorsal epidermal cells, transforming the ovoid embryo into the long, thin worm. In the seam cells, actin filaments become highly organized and are shortened by myosin contraction via the Rho kinase LET-502. Myosin phosphatase (MEL-11) inactivates myosin in the dorsal and ventral cells and is sequestered to cell boundaries in the seam cells, which limits its interaction with myosin. LET-502 is likely regulated by RHO-1, but the lack of
rho-1 alleles precluded studying its role in elongation. Using a deletion allele generated by the Knockout Consortium, we found that
rho-1 is required for elongation and other morphogenetic events, including ventral enclosure.
Cytokinesis is the final stage of cell division and requires the Rho-mediated formation and ingression of an actin-myosin ring. The same genes regulate both elongation and cytokinesis, suggesting they use analogous shape change mechanisms. In other eukaryotes, anillin is required for cytokinesis by coordinating actin and myosin filaments. It can also bind to RhoA, scaffolding the upstream regulator to actin and myosin.
ani-1 (anillin in C. elegans) is required for polar body extrusion during meiosis and asymmetric furrow ingression during cytokinesis. Since elongation and cytokinesis may use similar shape change mechanisms, we hypothesize
ani-1 functions in elongation. Using
ani-1 RNAi, we observed phenotypes consistent with elongation defects. We also performed zygotic-specific
ani-1 RNAi (by rescuing maternal RNAi) and observed elongation defects, suggesting this phenotype does not arise from earlier defects in meiosis or cytokinesis. We found genetic interactions between
ani-1 and genes that function in elongation using zygotic alleles for
rho-1 and
mlc-4. Furthermore, using GFP:ANI-1 (gift from A. S. Maddox), we observed non-cytokinetic localization patterns for ANI-1 during embryogenesis. These findings may be the first evidence to support developmental-specific functions for anillin outside cytokinesis in any organism.