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Fig. 1. Wingless is required for proper development of Slouch-expressing founder cells (FCs). (A) Schematic showing the temporal progression of Slouch-positive FC development from early (left) to late (right) stages of embryogenesis. Brackets indicate position of the Wg-expressing epidermal cells. Slouch is first expressed in a single progenitor cell during early stage 11 (5 hours AEL) of embryonic development; this cell divides to give rise to two FCs (purple), which together form cI located posterior to Slouch-expressing CNS cells (black). During late stage 11 (6 hours AEL), two additional Slouch-positive progenitors appear and divide sequentially to form four FCs that make up cII (green). At stage 12, a single progenitor arises dorsally and divides to give rise to cIII (red). Slouch expression is maintained in a subset of these founder cells that give rise, in the final muscle pattern, to muscle VT1 (25 from cI), VA2 (27 from cII) and DT1 (18 from III). In the figure, all muscles formed from these three clusters, as well as the ventral adult progenitor that comes from cII, are shown in color. Remaining gray muscles arise from non-Slouch-expressing FCs. (B-E) Ventral views of stage 11 embryos are shown; black arrows mark the midline, black arrowheads mark cI, white arrowheads mark cII. Abdominal segments 5-8 are shown; posterior is right. (B) High magnification of a wild-type embryo. cII is in line with the Slouch-expressing CNS cells, while cI is posterior to cII. (C) High magnification of a wg^CX4 embryo. All mesodermal Slouch expression is absent, indicating that Wg is necessary for the formation of these clusters. Slouch-expressing cells in the CNS remain. (D) High magnification of a wg^CX4, twiGAL4>UASarm^S10, wg^IG22 embryo shows normal patterning of all Slouch-expressing FCs. The ability of activated Armadillo to support normal patterning of both cI and cII suggests that the Wg pathway is sufficient to pattern these mesodermal clusters. (E) High magnification of a CyO, wglacZ embryo. Wg-expressing cells (brown, white arrowheads) and Slouch-expressing cells (black) are shown. The ectodermal Wg stripe specifically overlaid Slouch cII.

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