Fig. 7. Schematic representing the Egfr-mediated signalling events during Ato expression (green). In wild type, Egfr signalling is occurring in the IGs. This signalling occurs at the same time as R8 precursor selection within the equivalence group, and the role of Ed is to prevent the Egfr signalling from interfering with this process. In ed mutants, there is no Ed protein and Egfr signalling has a local inductive effect on the cells of the equivalence group resulting in the selection of more than one R8 precursor.