Figure 5

Parametric boost versus neutrality: a mechanistic view.

The figure illustrates the generic consequences of introducing B-C interaction leading to indirect positive feedback. Middle panels show the stripe (or lack of it) in the pure IFFMs and in the corresponding IFFM reinforced with the mutual-interaction. The dotted lines in the stripes represent attractor-switching, while the solid lines follow the attractor's movement. The Gray vertical bands qualitatively indicate the three regions, Low-High-Low, constituting the stripe in gene B. The yellow-background square panels are (C, B) phase plots generically corresponding to the three regions. There, the steady states (stable, S or unstable, U) are the intersections of nullcline curves (where one variable does not change in time – notation in the upper left corner). The black star indicates the initial condition close to the origin. The red arrows in these phase plots show that the nullclines move only horizontally or vertically in response to the morphogen gradient. When creating mutual inhibition, even a weak B-C inhibition allows single-stripe formation through attractor-switching process. When creating mutual activation, any value of B-C activation sharpens the stripe that already exists and allows attractor-switching in the posterior border. The thin line passing through the unstable state shows the separatrix, delimiting the basins of attraction for the two stable states.