Fig. 1. Planar polarity in the Drosophila eye. All images are oriented anterior to the left and dorsal down. (A) An SEM of a Drosophila compound eye. The blue and magenta boxes indicate the regions of the eye examined in equatorial and polar sections, respectively, throughout the paper. (B) Dorsal (green arrows) and ventral (red arrows) ommatidia shown in apical (left) and basal sections of the eye. The gray structures are the rhabdomeres of the indicated photoreceptor cells. In apical sections, the central rhabdomere belongs to the R7 cell, whereas in basal sections, the R8 rhabdomere is visible instead. (C) A tangential section of the eye showing that the eye is divided into dorsal and ventral fields. The equator is indicated by the blue line. (D) A schematic of the section shown in C. In this, and all subsequent figures, the orientation of ommatidia is indicated by arrows drawn from the rhabdomere of R1 to that of R3. Green arrows indicate ommatidia of dorsal chirality and orientation, whereas red arrows designate ventral type ommatidia. (E) A diagram of the process of R3/R4 cell fate specification and ommatidial rotation. When clusters first emerge behind the morphogenetic furrow (1), the R3/R4 precursors (beige) are not in contact. Following cell-cell contact between the R3 and the R4 precursors (2), the two cells adopt distinct fates, with the cell located closer to the equator becoming R3 (red), while the other becomes R4 (green). The clusters then rotate 90° (3 and 4) to establish the final pattern of polarity. R1, R6, and R7 are not included in the diagram for simplicity. (F) A diagram of the proposed model for the control of Fz signaling and R3/R4 cell fate specification by Fj, Ds and Ft. Diffusible signaling proteins drive graded transcription of Fj and Ds. The resulting Ds and Fj protein gradients then regulate the action of Ft, which is uniformly expressed, to generate graded Ft function across the eye. The resulting difference in Ft between the neighboring R3 and R4 cells biases their Fz competition, and ensures that the equatorial cell assumes the elevated Fz signaling state and thus becomes R3.