Fig. 2. Early expression of scute (sc) and asense (ase) in the wing disc of Calliphora vicina. (A,B,D) expression of sc transcripts, (C,E) expression of ase transcripts, (F) visualisation of 22C10 protein. Enlargements of the thoracic domain of the discs in A-C, indicated by the rectangles, are shown directly below. By 3 hours after pupariation, sc transcripts, visualised by in situ hybridisation (A) are present in two broad longitudinal stripes in the medial half of the prospective notum. These correlate with the positions of the future acrostichal (AC) and dorsocentral (DC) bristle rows, and are interrupted by a mediolateral band at the site of the prospective transverse suture in which expression is absent. In the lateral domain, at the sites of the future intra-alar (IA), notopleural (NP) and supra-alar (SA) bristles, sc is expressed in a series of clusters. By 8 hours (D) to 10 hours (B) after pupariation, sc expression is restricted to the sites of the future bristle organs, where small groups of cells have accumulated higher levels of the transcripts. A clear expression domain is seen in the scutellum; it is separated by an area in which sc is absent that corresponds to the prospective scutellar suture. In D a schematic drawing of the relationship between sc expression at 8 hours after pupariation and the morphology of the future adult thorax (macrochaetes are shown in pink, microchaetes in blue). At this stage the disc remains a highly folded epithelium which later expands longitudinally at the midline, and laterally and ventrally at the lateral edge of the notum. This explains the compact aspect of the acrostichal and scutellar domains, and close proximity of the intra- and supra-alar domains at this stage. ase expression in single cells (C,E), and visualisation of neurons with the 22C10 antibody 10 hours after pupariation (F), show that the early expression of sc prefigures the sites at which bristle precursors are born.