Binary cell fate specification during C. elegans embryogenesis driven by reiterated reciprocal asymmetry of TCF POP-1 and its coactivator β-catenin SYS-1

C. elegans embryos exhibit an invariant lineage comprised primarily of a stepwise binary diversification of anterior-posterior (A-P) blastomere identities. This binary cell fate specification requires input from both the Wnt and MAP kinase signaling pathways. The nuclear level of the TCF protein POP-1 is lowered in all posterior cells. We show here that the β-catenin SYS-1 also exhibits reiterated asymmetry throughout multiple A-P divisions and that this asymmetry is reciprocal to that of POP-1. Furthermore, we show that SYS-1 functions as a coactivator for POP-1, and that the SYS-1-to-POP-1 ratio appears critical for both the anterior and posterior cell fates. A high ratio drives posterior cell fates, whereas a low ratio drives anterior cell fates. We show that the SYS-1 and POP-1 asymmetries are regulated independently, each by a subset of genes in the WntIMAP kinase pathways. We propose that two genetic pathways, one increasing SYS-1 and the other decreasing POP-1 levels, robustly elevate the SYS-1-to-POP-1 ratio in the posterior cell, thereby driving A-P differential cell fates.