Many developmental processes produce stable phenotypes despite perturbations (internal noise, environmental variations). Such a robustness may also result in a decreased sensitivity to genetic alterations, thus potentially allowing the evolution of developmental processes in the absence of change in the final phenotype.In the case of C. elegans vulva development, the final output (the cell pattern) is invariant within and among strains. Out of 6 competent cells (P(3-8).p), only three cells normally adopt a vulval cell fate: a central fate (P6.p) and lateral fates (P5.p and P7.p). Buffering mechanisms and partial redundancies within and between the three molecular pathways involved (LET-60/Ras, LIN-12/Notch and BAR-1/b-Catenin) appear to guarantee a low degree of error (see contribution by Braendle and Felix).To unravel potential cryptic genetic differences among six wild C. elegans isolates (JU258, CB4856, PB303, PB306, PS2025 and AB1), we sensitized the system using mutations <
sym10>introduced by repeated backcrosses from N2<
sym10> that affect the activity of the three pathways. We find that the effect of a given mutation varies significantly between backgrounds.For the LET-60 pathway, the mean number of induced cells ranges from 4,4 in N2 to 5,4 in PS2025 (n>50) with
let-60(
n1046gf) and with
let-23(
sy1rf), from 0,31 in PB303 to 0,76 in PS2025 (n>40). For the LIN-12 pathway,
lin-12(
n676n930rf) is suppressed or close to suppression in all strains except N2 (47% Vulvaless animals at 20C, n>60), which correlates with the effect of
sup-17(
n316rf) (62% Vulvaless in N2, less than 20% in PB303 and CB4856 (n>50). For the BAR-1 pathway, which is involved in establishing the competence group at the L2 stage and in the induction at the L3 stage,
bar-1(
ga80rf) is suppressed in JU258 (65% WT) compared to the other strains (2-12% WT, n>40).Overall, these variations in sensitivity to the same mutation suggest that the vulva patterning network has evolved in the absence of any changes in the final phenotype. We now aim to get a functional understanding of these evolved differences by studying the relative activities of the signaling pathways in different wild genetic backgrounds.