[
International Worm Meeting,
2015]
Development is highly robust, resulting in the same outcome despite strong genetic, environmental or intrinsic variability. How such robustness is achieved remains a fundamental open question. A prime example of robust development is C. elegans vulva induction, where a spatially graded LIN-3/EGF signal from the Anchor Cell (AC) induces an invariant 3deg-3deg-2deg-1deg-2deg-3deg fate pattern in the vulva precursor cells (VPCs), P(4-8).p. It is usually thought that the P6.p cell assumes 1deg fate because it is the closest VPC to the AC. However, we found that at the start of vulva induction the position of the AC with respect to the VPCs is in fact highly variable, with the AC positioned equidistant to the P5.p and P6.p cells in a significant fraction of animals. Yet, despite this initial variability induction always resulted in the same final VPC fate pattern, with the AC directly adjacent to the 1deg fate cell. To understand how this was achieved, we quantified 1deg fate induction as a function of AC position, both in wild-type animals and in vulva induction mutants, using single molecule FISH to measure LIN-3/EGF-induced gene expression with single molecule resolution. We found that if the AC was positioned equidistant to two VPCs, then both VPCs received equal levels of the EGF signal. However, subsequent EGF-induced Notch signaling between these VPCs then restricted the response to EGF and 1deg fate randomly to a single VPC. Subsequently, this 1deg fate cell moved towards the AC in an EGF-dependent manner, thereby completely correcting for the initial AC misplacement. Our experiments, combined with mathematical modeling, showed that both lateral Notch signaling and VPC movement towards the AC were required simultaneously to reach the desired VPC fate pattern for all initial AC positions. In particular, in the absence of Notch signaling induction often resulted in animals with two induced VPCs, clustered close to the AC. Together, our results show that vulva induction is canalized, meaning that the underlying signaling network is optimized to reach the same final cell fate pattern despite large variability in the initial conditions.