Fig. 7. Model for polarization of the C. elegans zygote. (A) Graphic depiction of the establishment and maintenance phases during polarization of the zygote. Proteins that localize to the anterior are in blue (PAR-3, PAR-6, PKC-3 on the cortex, MEX-5 and MEX-6 in the cytoplasm). Proteins that localize to the posterior are in pink (PAR-2 and PAR-1 on the cortex, PIE-1 in the cytoplasm). During meiosis, all are uniformly distributed throughout the zygote (purple color). Circles: pronuclei, Black lines: microtubules. (B) Sequential repression model [modified from Kemphues (Kemphues, 2000)]. Lines with bars depict antagonistic interactions, whereas lines with arrows depict positive interactions. In this model, we show that PAR-1 is restricted to the posterior by the anterior PARs. This hypothesis is supported by the observation that, in late one-cell embryos, PAR-1 is present throughout the cortex in par-3 mutants and par-3;par-2 double mutants (Etemad-Moghadam et al., 1995; Boyd et al., 1996). Further support for this model awaits analysis of PAR-1 dynamics in live embryos.