Practice question

EMS blastomere default state without P2 signal is mesodermal MS fate. This is determined by nuclear POP-1 levels. In anterior MS daughter, high nuclear POP-1 binds and represses end-1 promoter together with histone deacetylases, allowing tbx-35 for pharynx and muscle. Low POP-1 permits endoderm. POP-1 repression defines default program: when Wnt signaling absent, both daughters maintain high POP-1 and become MS-like. SKN-1 provides competence for both MS and E, but fate choice gated by POP-1 differential. Therefore POP-1 concentration acts as binary switch establishing MS as ground state, repressed variant requiring Wnt input for E divergence.