Practice question

Primary and secondary vulval fates are graded by LIN-3/EGF morphogen secreted from anchor cell. LIN-3 forms diffusion gradient highest at P6.p overlying AC, intermediate at P5.p and P7.p, low at distant P3.p, P4.p, P8.p. High concentration surpasses threshold activating LET-23 EGFR to MAPK to LIN-1 inhibition, driving primary genes egl-17. Lower LIN-3 combined with lateral LIN-12 Notch activation specifies secondary fate expressing secondary markers. Experimental expression of LIN-3 under heat-shock promoter induces ectopic primary cells proportional to dose, demonstrating morphogen interpretation. Therefore LIN-3 gradient patterns 3-2-1-2-3 array in competent group.