Fig. 1. Salm is expressed in R3 and R4 when planar polarity is established. Anterior is to the left and dorsal is up. (A) Third larval instar eye imaginal disc, stained with anti-ELAV (red) and anti-ß-galactosidase (blue), in which lacZ is expressed under sev-Gal4 control to show the R3/R4 position (lower staining levels are also observed in R1, R6 and R7) and reveal the rotation of the developing ommatidia. Close to the morphogenetic furrow (MF, to the left) the R3/R4 pair is perpendicular to the DV midline, the equator (yellow line). By row six, ommatidia have rotated 45°, in a clockwise or counter-clockwise direction in the dorsal and ventral halves, respectively. In the posterior part of the eye disc, the 90° rotation of the ommatidia is almost complete, and the R3/R4 pair is parallel to the equator. (B) Schematic illustrating ommatidial rotation in the imaginal disc. (C) Tangential section of an adult eye (left) and corresponding schematic drawing (right). The section is at the level of R7. R8 is not visible as it is localized below the R7 plane. Ommatidia in the adult eye are arranged as two opposite chiral forms separated by the equator (yellow line), as a consequence of R3/R4 specification and the following 90° rotation. Ommatidia in the dorsal half are represented with black arrows and in the ventral half with red arrows. (D) Magnification of one dorsal (top) and one ventral (bottom) ommaditium; arrows as in C. Numbers indicate the identities of the photoreceptors. (E) Eye imaginal disc stained for Salm (blue) and svp-lacZ (green). The initiation of Salm expression in R3/R4 precedes svp-lacZ by one row. Posterior to row seven, Salm expression in R3/R4 starts to fade, whereas svp-lacZ continues to be expressed in R3/R4 and also, at lower levels, in R1/R6. (F) Salm (green) expression in R3/R4 precedes the onset of m0.5-lacZ (red) expression in R4 by one to two rows. Scale bars: 10 µm.