Fig. 5. Domains of expression on the developing Drosophila notum. (Top left) A schematic diagram of the Drosophila notum, indicating the extent of the expression domain of pannier (green), achaete-scute (blue) and the positions of the macrochaetes (red). (A-D) Ag-ASH and (E-H) Ag-pnr expression. The spatial expression of the two genes in the medial half of the notum is identical. Expression of both genes starts in two broad bands, one on either side of the midline. This is best seen in the late fourth instar larva in E, where staining is seen beneath the larval cuticle which has a ridged appearance. The cuticle was removed for all other preparations shown. The two early bands are just becoming visible in A, where the positions of the future acrostichal (AC) and dorsocentral (DC) bristle rows are indicated by dashed lines. Stained areas round the edges of this and other preparations are artefacts due to the folded cut edge of the epithelium. Although the larval epidermis is lost, a number of larval sense organs (LSO, arrows) and larval sensory hairs (LSH, circles) persist and remain just above the imaginal epithelium. They are out of focus in some of the photographs. The larval sensory hair provides a useful positional marker with respect to which the anterior limit of the succeeding triangular and kidney-shaped domains can be measured. The full extent of the triangular domain of staining on the posterior scutum just above the scutellar suture can be seen in F. In this image, and others, some of the acrostichal and dorsocentral bristle precursors can be distinguished. The triangle has already started to shrink from its anterior limit in B and is progressively transformed to the kidney-shaped domain in C, G and H. The crescent-shaped domain on the scutellum can be seen below the LSH in D. (Note some damage to this preparation that distorts the picture of the kidney-shaped domain). A patch of staining on the midline at the anterior edge of the notum is seen in B,C,F,G and H (unlabelled). It is a consistent feature but we have been unable to find a morphological correlate to this domain of expression. The correspondence between the expression domains and the positions of the future bristles is indicated in the schematic drawings below. The acrostichal (AC) and dorsocentral (DC) rows are drawn in green and red, the prescutellar (PST) and scutellar (SC) bristles are shown in blue.

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