Fig. 3. Establishing contractile polarity: Rho. (A-G) Illustrations of C. elegans embryos (E) depleted of Rho (RHO-1), (F) depleted of RhoGEF ECT-2, and (G) depleted of RhoGAP CYK-4, in comparison with (D) embryos lacking a contractile acto-myosin network and mutants of (B) posterior and (C) anterior PAR proteins. Embryo anterior, or the meiotic pole in D-G, is to the left. Rho and the acto-myosin network (pink) are required for contractile polarity. PAR polarity does not determine contractile polarity, although anterior PAR proteins such as PAR-6 modify contractility, and thus the respective sizes of the anterior and posterior domains are altered in par-3 mutants (C). (B) Contractile polarity can dictate anterior PAR polarity during polarity establishment in par-2 mutants, but thereafter PAR-2 is essential for maintaining anterior PAR polarity (not depicted). The acto-myosin network (pink) is indicated from a surface view of the embryo cortex; PAR-6 (red) is shown in the embryo mid-plane. (H-J) Diagrams of acto-myosin contractility regulation by Rho signaling. Arrows indicate positive regulation, although the precise molecular components involved have not been determined. (H) Without spatial or temporal regulation of Rho activity, `contractility' and `no contractility' compete. (I) In the anterior cortical domain, only RhoGEF ECT-2 is present, promoting contractility. (J) In the posterior cortical domain, only RhoGAP CYK-4 is present, eliminating contractility.