Figure 1. Development of the eye imaginal disc during the larval third instar. (a) A cobalt sulfide stain shows the morphogenetic furrow (MF) and developing ommatidial clusters (inset). (b) In the morphogenetic furrow, high levels of phosphorylated MAP kinase in the proneural clusters (red; overlap with green is yellow) indicate Ras-pathway activity in these cells; Atonal expression (green) narrows from these proneural groups to single R8 cells, the first differentiated cells in the eye disc. (c) Specific transcription factors are necessary to produce the other photoreceptor fates (Boss, a transmembrane protein, marks R8); ommatidia rotate as they mature. Boss, bride-of-sevenless; ro, rough; svp, seven-up; pros, prospero.

Figure 2. Pupal eye development. (a) Schematic of apical eye development beginning with two late larval ommatidia (right). The arrows indicate the rotation of each ommatidium with respect to the equator. The middle two panels depict apical development of ommatidia through the pupal stage. Following cone cell differentiation, two 1° pigment cells enwrap the four cone cells and these are surrounded by undifferentiated cells. Many of these surrounding cells are removed by PCD, resulting in the hexagonal lattice shown, consisting of three bristle cells, three 3° pigment cells and six 2° pigment cells. The left panel is a schematic of two adult ommatidia, one on each side of the equator. This figure shows the photoreceptor rhabdomeres as they would be seen below the surface of the eye. Note the different orientation of the photoreceptor trapezoids with respect to the equator, a difference not visible at the apical surface (see middle panels). (b) An apical view of a late-pupal retina visualized with an antibody to the membrane-localized protein, Armadillo. (c) A basal view of a late-pupal retina visualized as in (b). Secondary cells have reorganized to form a flower pattern emanating from each axon exit port (aep) directly below each ommatidium. Abbreviations: cc, cone cell; 1°, primary pigment cell; 2°, secondary pigment cell; 3°, tertiary pigment cell; br or b, bristle group; aep, axon exit port.

Figure 3. Schematic of adult ommatidium.