Figure 4.2
Categories of genes involved in body segmentation and segment individuation.
a. Protein levels (curves or bars) detected or inferred [3605] for a few members of each class in the segmentation gene hierarchy (axis, gap, pair rule, segment polarity) or (below) expression patterns of key homeotic genes at the time when their activity is instrumental in patterning [3248]. Gene abbreviations: abd-A (abdominal-A), Abd-B (Abdominal-B), Antp (Antennapedia), bic (bicoid), cad (caudal), en (engrailed), eve (even skipped), ftz (fushi tarazu), gt (giant), hb (hunchback), kni (knirps), Kr (Krüppel), Ubx (Ultrabithorax), wg (wingless). Embryos at right depict realistic Bic, Kni, and Ftz protein distributions [105, 661, 1102, 1103, 4055]. The Cad gradient arises via Bic repression of cad mRNA translation [3116]. Vertical lines show a few other interactions: (1) boundaries of eve stripe 2 (arrowheads) are defined by gt and Kr [4764]; (2) even-numbered en stripes (arrowheads) emerge due to a trailing gradient of Eve that turns OFF odd-skipped (not shown) in front cells of each ftz stripe (thus activating en by repressing its repressor) [877, 1324, 2681, 2682]. See [1605] for a comparable chain of negative regulators and [2002] for Deformed, where the control is more combinatorial than hierarchical.

b. Embryos make 14 parasegments and 15 segments, with an en stripe (shaded) at the front or rear of each, respectively. Panel a is adapted from [162, 1416, 1946, 3605], with eve stripe 2 and en stripe 4 data from [4764] and [1324]; b follows [3843] (A9 stands for A9-A10).

N.B.: This illustration is inadequate as a wiring diagram, because its simplicity omits so many key genes [47, 380, 1962, 3248], spatial details (e.g., uneven levels within homeotic gene domains [682, 2717]), and dynamic modulations [242, 1790, 2181, 3770, 4834]. Nevertheless, at least it conveys a feeling for the flow of control. See [2869, 4893] for evolutionary context.