Fig. 8. A8-specific regulation of hh and en at stages 12 and 13. (A) Schematic representation of the `delay experiment' used to detect de novo En expression in the anterior compartment of A8. En production requires a single step (1) but that of ß-Gal requires three steps (1-3). The en introns, the small size of which does not induce a significant delay in En production, have not been represented. (B) Wild-type expression of En in abdominal segments at stage 11, shown here for A6-A8. In order to align the segments with those of a stage 13 embryo that has completed germ-band retraction, the magnified view shown in B comes from a stage 11 embryo oriented with anterior towards the left. (C) Stage 13 en-Gal4/UAS-lacZ embryo stained in blue for En and brown for ß-gal. En and ß-gal are co-expressed in A6 and A7. In A8, co-expression only occurs in cells lying in the posterior part of the stigmatophore (white arrow), while anterior stigmatophore cells express En but not yet ß-gal (black arrowhead). Cells located ventrally in A8 do not express En, but ß-gal is still detected because of the transcriptional delay and ß-gal stability (white arrowhead, out of focus). (D-F) En is required for stigmatophore development. Cuticle of an en^E embryo (D) shows loss of stigmatophores, while filzkörper-like structures are still present (black arrows). en^E embryos bearing the sal-Gal4 and UAS-en transgenes display developed stigmatophores (E, white bracket) comparable in size with those observed in embryos containing the two transgenes but in an otherwise wild-type context (F, white bracket). (G-I) hh is expressed in an A8-specific pattern from stage 12 onwards (G). In A8, but not in more anterior segments (arrow in A7), hh (blue) is expressed in cells (arrow in A8) that border the En segmental stripe (brown). This expression is lost (arrow) in AbdB mutants (H) and still occurs in en mutants (I).