Fig. 1. C. elegans vulval cell fate specification. (A) Six vulval precursor cells (VPCs) are competent to adopt vulval fates in response to cell signaling events, but only three VPCs (P5.p, P6.p and P7.p) normally do so (Moghal and Sternberg, 2003). Loss-of-function mutations in the Ras pathway cause fewer than three VPCs to adopt vulval fates, whereas gain-of-function mutations in the Ras pathway cause greater than three VPCs to adopt vulval fates. (B) Ras and Wnt pathways cooperate to specify vulval cell fates (Gleason et al., 2002). The Wnt pathway behaves genetically downstream of the Ras pathway as the pry-1 Muv phenotype is not suppressed by Ras pathway mutations (Gleason et al., 2002) but the activated MPK-1 phenotype is partly suppressed by bar-1 mutations (Eisenmann et al., 1998). Only pathway components relevant to this paper are depicted.