We investigate how temporal and spatial interactions between multiple intercellular and intracellular factors specify the fate of a single cell in Caenorhabditis elegans. P12, which is a ventral cord neuroectoblast, divides postembryonically to generate neurons and a unique epidermal cell. Three classes of proteins are involved in the specification of P12 fate: the LIN-3/LET-23 epidermal growth factor signaling pathway, a Wnt protein LIN-44 and its candidate receptor LIN-17, and a homeotic gene product EGL-5. We show that LIN-3 is an inductive signal sufficient to promote the P12 fate, and the conserved EGF signaling pathway is utilized for P12 fate specification; egl-5 is a downstream target of the lin-3/let-23 pathway in specifying P12 fate; and LIN-44 and LIN-17 act synergistically with lin-3 in the specification of the P12 fate. The Wnt pathway may function early in development to regulate the competence of the cells to respond to the LIN-3 inductive signal.

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