During cytokinesis, constriction of a cortical contractile ring generates a furrow that partitions one cell into two. The contractile ring contains three filament systems: actin, bipolar myosin II filaments, and septins, GTP-binding hetero-oligomers that polymerize to form a membrane-associated lattice. The contractile ring also contains a potential filament crosslinker, Anillin, that binds all three filament types. Here, we show that the contractile ring possesses an intrinsic symmetry-breaking mechanism that promotes asymmetric furrowing. Asymmetric ingression requires Anillin and the septins, which promote the coalescence of components on one side of the contractile ring, but is insensitive to a 10-fold reduction in myosin II levels. When asymmetry is disrupted, cytokinesis becomes sensitive to partial inhibition of contractility. Thus, asymmetric furrow ingression, a prevalent but previously unexplored feature of cell division in metazoans, is generated by the action of two conserved furrow components and serves a mechanical function that makes cytokinesis robust.
