Wb acts as an upstream cue for midgut polarity. (a–d) Stage 15 wild-type (a and b) and wb (c and d) embryos. Dashed boxes in a and c show the areas enlarged in b and d. In wb mutants, the strong LanA localization to the endoderm/mesoderm border seen in wild type (a and b) is lost (c and d). (d) Many LanA punctae are found inside midgut cells in wb mutants. (e–g) In shg mutants, midgut cells form a disorganized-looking monolayer of columnar or wedge-shaped cells, with βPS integrins localized to the apical and basal sides of the cells (f2). In contrast, Baz (f1) is not localized to the apical side of the cells, but shows low levels of diffuse staining throughout. (g) Plots of the average fluorescence intensity (represented as mean gray value) of Baz and βPS in stage 15 midgut cells mutant for shg measured along the apical (A) to basal (B) axis of a cell (represented in f by red line). Each n represents the average of 10 cells measured in one embryo (black dotted lines). n = 6 per condition; the mean for each condition is plotted in red. The apical peaks of Baz are lost in wb mutants, and the levels of both apical and basal peaks of βPS are reduced to just under half of wild type. White dashed line in e indicates the apical side of the midgut, and the yellow line, the basal. Red box in e depicts the area of the midgut epithelium shown in f. Confocal images are oriented with the anterior to the left and posterior to the right. Scale bars, 20 µm (a–d) and 10 µm (e and f).