Fig. 4. Genetic control of R8 differentiation. (A) Loss-of-function senseless (sens) clones do not express Ato in stage 4 R8 precursors in most cases and these precursors display proper space and phase relationships. Expression of Ato in stage 1-3 is unaffected. (B) Loss of rough (ro) function prevents resolution of the stage 3 R8 equivalence group into single R8 precursors and two or three R8 precursors express Ato during stage 4 in many ommatidia. Phase relationships and spacing are not affected. Expression of Ato in stage 1 is expanded. (C) Removal of both sens and ro function results in a phenotype that is very similar to ro. (D) Relationships controlling R8 differentiation. sens-mediated repression of ro, a repressor of R8 differentiation, is crucial for R8 differentiation. See text for details.