Fig. 1. Ds and Fj expression, and the signaling model. In this and subsequent figures anterior is up and proximal is to the left. (A,A') Anti-Ds staining (A, white; A', red) in wing at 5 hours AP. The primordia of longitudinal veins one through five (L1-L5) and the anterior cross vein (ACV) are shown in A' by the absence of anti-DSRF staining (green). (B) fj-lacZ (green) is expressed in the distal wing at 5 hours AP, in a region complementary to the region of high anti-Ds staining (red). (C) Expression levels of Ds and fj-lacZ at 5 hours AP. (D) ds-lacZ expression (red) and anti-Ds staining (white) in the ds⁰⁵¹⁴²/+ wing at 26 hours AP. Top panel is stained with anti-pMad (green) to identify the veins; lower panels show relative levels of Ds at indicated positions. Broad stripes of Ds expression extend out along the center of the wing, forming a proximal to distal gradient. (E) Signaling model. Ds on one cell binds to Ft on the adjacent cells and inhibits its activity; Fj inhibits this inhibition. Ft activity in the central cell is polarized by the higher levels of Ds and the lower levels of Fj on the proximal side. Polarized Ft activity biases the subsequent Fz signaling between cells, leading to the polarized distribution of proteins to the proximal (Pk, Vang) or distal (Dsh, Fz) faces of the cell, and to the formation of hairs on the distal faces.