Figure 5.

Binary encoding of the phase of the oscillation by bistability. Creation of high (A) and low (B) values of the segmentation marker E in two cells by the coupled effects of oscillatory and bistable dynamics. The network corresponds to Figure 4D and the colors and scalings are identical. While the morphogen G is high, E is high and oscillates in response to the clock variable R. As time passes, G decreases and at a given moment (black arrow), it can no longer significantly activate gene E. The cell fate is determined by the concentration of E at this particular moment relative to a threshold E⁰ (shown by a dashed line). E⁰ is the (unstable) fixed point (for G=R=0) that separates protein concentrations E>E⁰ converging to the high state of E expression, from smaller values that end in the low state of E expression. In (A), E is high enough at the arrowed time so that G and R can disappear while leaving E>E⁰. In (B), the concentration of E at the arrowed time is under the threshold E⁰.