FIGURE 4 

Morphogen gradient interpretation. (A) The morphogen gradient elicits a signal (S, in blue) to which a cell (orange circle) responds. The signal induces (blue arrows) the expression of targets X, Y and Z. These targets have different sensitivity (denoted by open squared boxes) to the same signal S. X is weakly sensitive to S, Y is mildly sensitive and Z is very sensitive. (B) Binary response of target genes X, Y and Z to signal S. Low levels of S activate only Z, medium levels activate both Z and Y, and high levels activate all targets. (C) The signal induces the expression of targets X, Y and Z, which here have the same sensitivity but interact with each other. An example of plausible cell-autonomous interactions is depicted, in which Z represses both Y and X, and Y represses X (repression is shown by curves with line-end; arrows indicate induction). In this case, to elicit different responses along a gradient, different sensitivities are not required, but could also be participating. As X is repressed by Y and Z, the overall signal it perceives is smaller than the signal Y perceives, which, in turn, is smaller than the signal Z perceives. Thus, the binary response of the target genes to different values of signal S is also shown in (B). (D) X, Y and Z binary response (lines) to a graded signal (S, blue triangle) along a field of cells (orange circles). Three different spatial regions and fates are induced, which are characterized by those genes that are expressed. Expression inside cells is denoted by a coloured rectangle (violet for X, red for Y and orange for Z).