Response of the transcritical bifurcation switch module. The response of the module to different (A) gradient signals S 1,S 2 and S 3 and (B) localized signals S 1 and S 2 for the non-diffusible and highly diffusible cases is shown. (A) Non-diffusible case: S1- is above the switch threshold (i.e. it is small enough for a non-zero steady state output: see text) in the entire domain (solid line), S2 is partially above and below the switching threshold (solid line with circles) and S3 is below the switch threshold in the entire region (solid line with dots at zero), Diffusible case: S1 (dashed line), S2 (solid line with x markers) and S3 (solid line with plus signs at zero). (B) Non-diffusible case: S1 is above the switch threshold in the entire domain (solid line), S2 is partially above and below the switching threshold (solid line with x markers), Diffusible case: S1 (solid line with circles) and S2 (dashed line). For the non-diffusible case, in the spatial regions where the signal value crosses the ”switch” threshold, the response exhibits a switch. This can be seen for S2 in (A) and in (B), where a part of the concentration profile shows non-zero spatial variation, and the other part is at zero or switched off. These plots clearly illustrate the presence of a spatial switch. Note that for S3 in (A) the profiles are at zero steady state in the entire domain. In the highly diffusible case, the spatial concentration profile of X∗has weaker spatial variation and the switching nature of the response is not observed here.