During vulval development, the inductive LIN-3 EGF signal activates in P6.p the LET-23 EGFR/ RAS/ MAPK pathway, resulting in the specification of the 1 degrees cell fate. P6.p then induces the neighbouring VPCs P5.p and P7.p to adopt the 2 degrees cell fate. This lateral signaling mediated by LIN-12 Notch is linked to the VPC cell-cycle phases (Ambros, 1999) and has two functions: In the G1 phase of the VPC cell cycle, LIN-12 Notch inhibits 1 degrees cell fate specification in P5.p and P7.p. After completion of the S phase, LIN-12 Notch induces the 2 degrees cell fate in P5.p and P7.p. The dual specify MAPK phosphatase LIP-1 (lateral signaling induced phosphatase) mediates the lateral inhibition of the 1 degrees cell fate by LIN-12 Notch, thereby connecting the lateral and inductive signaling pathways (Berset et al. 2001). LIN-12 Notch up-regulates
lip-1 expression during the late G1 phase in P5.p and P7.p where LIP-1 dephosphorylates and thus inhibits the MAPK. In
lip-1(
zh15) single mutants lateral inhibition occurs normally. However, when the EGFR/ RAS/ MAPK pathway is hyperactivated, loss of
lip-1 function causes P5.p and P7.p to adopt the 1 degrees instead of the 2 degrees vulval cell fate, resulting in an Apf (adjacent primary cell fate) phenotype. These observations have indicated that lateral inhibition likely acts at multiple steps of the inductive signaling pathway in a genetically redundant fashion. To find new genes involved in lateral inhibition, we performed an EMS mutagensis screen for mutations that cause a synthetic Apf phenotype in a
lip-1(
zh15) background. After screening 30000 haploid genomes, we identified the
zh34 mutation on LGII as a strong Apf mutant (in addition to another 10 mutants exhibiting a penetrant Muv non-Apf phenotype). The presence of adjacent 1 degrees cells in
zh34;
lip-1(
zh15) double mutants was confirmed using
egl-17::gfp as a 1 degrees fate marker (Burdine et al. 1998). Interestingly, the
zh34 mutation appears to specifically affect the fate of P5.p and P7.p, while the more distal VPCs P3.p, P4.p and P8.p only rarely adopt vulval fates in
zh34;
lip-1(
zh15) animals. Using a combination of SNP-mapping and RNAi analysis we identified a single ORF (F44G4.8) as a candidate gene. F44G4.8 encodes a receptor-like protein tyrosine phosphatase (R-PTP) homologous to human PTP-beta, which inhibits EGFR signaling (Krueger et al. 1990; Haj et al., 2003), and Drosophila Ptp4E (Yang et al. 1991; Tian et al. 1991). Sequence analysis of the F44G4.8 ORF in
zh34 mutants revealed a G to A transition that introduces a stop codon at position 1162, truncating the protein before the catalytic centre of the phosphatase domain. Thus,
zh34 is probably a null allele. We will present the results of our genetic and molecular analysis.