Ventral cuticle phenotypes of segment polarity mutants. (a) and (b) show the wild‐type pattern. (a) Nuclei are stained in light blue, showing that each row of denticles corresponds to one row of cells (dark blue staining shows the wingless (wg)‐expressing cells). The polarity, shape, and polarity of denticles (best seen in (b)) is reproducible from segment to segment (two segments are shown). Denticles are typically numbered 1–6, from anterior (left) to posterior (right). In wg (c), and hedgehog (hh) (d) mutants, a lawn of denticle forms. Note however that some form of segmental repeat remains. In patched (ptc) mutants (e), the posterior portion of each denticle belt is deleted and replaced by an anterior portion in mirror image. Thus ‘row 1’ denticles form at both the anterior (normal) and posterior (ectopic) edges of each denticle belt. Mutants of naked (nkd) (f) are characterized by a variable loss of denticles, especially in alternate segments. Anterior is left.

Expression of wingless (wg) and hedgehog (hh) through various stages of development (as revealed by in situ hybridization with an RNA probe). Left column shows side views while the right column shows ventral views. Note that, as the germ band extends (stages 8–11), it folds over itself. This process is reversed during stage 12. The stripes of wg and hh expression encircle the germ band at early stages (a, b). Subsequently, each wg stripe becomes interrupted at a lateral position (see stage 11 in C). Where wg expression continues, especially at the ventral side, the interface between wg and hh expression is maintained throughout embryogenesis.

The wingless/hedgehog (wg/hh) loop. wg is expressed at the anterior of each parasegment boundary (PS) while hh, and engrailed (en) are expressed at the posterior. Signalling by Wingless maintains the expression of Engrailed, a transcription factor required for hh expression. Hedgehog in turn signals to sustain wg expression. Hedgehog does not signal on hh‐expressing cells because Engrailed represses the expression of cubitus interruptus (ci), which encodes a key component of the Hedgehog signalling pathway. Hedgehog does not activate wg expression at the posterior (towards the right) because, there, cells do not express Sloppy‐paired, a cofactor needed for Hedgehog signalling to activate wg expression. Within the wg expression domain and at the anterior (left), Wingless signalling does not activate en expression because Sloppy‐paired represses en expression.

From Wingless and Hedgehog to the ventral denticle pattern. An array of actin bundles anticipate denticle belts. (a) A stage 13 embryo stained with phalloidin in red to highlight actin bundles. Cell membranes are stained in green. (b) rhomboid (rho) is expressed in prospective rows 2–4 as evidenced with a rhomboid–lac Z enhancer trap. A serrate–lac Z transgene shows that serrate (ser)‐expressing cells give rise to denticles 5 and 6 (c). (d) Summary of the correspondence between denticle type and gene expression. (e, f) Wingless and Hedgehog originate from the parasegment boundary and regulate the expression of rho and ser. Briefly, both Wingless and Hedgehog signalling repress ser expression. Hence ser is expressed where neither signal reaches. Regulation of rho expression is more complicated. It is activated by both Hedgehog and Serrate and repressed by Wingless. Repression by Wingless signalling ensures that rho is only expressed between the stripes of hedgehog (hh) and ser expression.