Default values of parameters.

All the units reported in the table are arbitrary. Parameter values were obtained by manual fitting to get best agreement with experimental observations. The values of the parameters of the two modules of the model were fitted separately. For the first module (model of ERK activity), fitting was performed manually to reproduce the experimental data obtained in ephrin inhibited embryos (S1B and S1C Fig), to set the values of the following parameters: Vs, K1, Ks, Kb, Kerk, Kd, [FGF]. Fitting was then performed on the experimental data obtained in wild type embryos (Fig 2A and 2B) to set the values of the remaining parameters: Vrg, K2, Krg, Ke, [ephrin]. Finally, to fix the values of the parameters in the model of Otx expression (kMM3, KMM3, vMM4, KMM4, kMM1, KMM1, vMM2, KMM2, vb, vo, Ka, Ki, k) fitting was done on Fig 3D.