Figure 1. 4E-8E stages are a critical period for commitment to the pharynx fate.

Pharyngeal precursors were challenged to adopt endodermal or muscle fates by activating expression of (A–O) HS::end-1, or (P–U) HS::hlh-1, respectively. Embryos challenged at (A–C, G–I, P–R) 2E, (D–F, J–L, S–U) 4E and (M–O) 8E stages are shown. (A,D,G,J,M,P,S) Embryos stained with anti-PHA-4. Midgut cells show weak PHA-4 expression, as expected. Pharyngeal bright PHA-4 stain does not co-localize with gut or muscle markers, and is marked with dotted circles. Embryos stained with (B,E) the early endodermal marker, ELT-2, (H,K,N) the late endodermal marker, 1CB4, and (Q,T) the muscle differentiation marker, paramyosin. (C,F,I,L,O,R,U) merge. (V) Graph shows percentage of embryos heat shock treated at the 2E, 4E, 8E, and 16E stages that exhibit pharyngeal cells. Embryos either bear HS::end-1 (red) or HS::hlh-1 (blue).