Fig. 7. Model for ato repression by Bar in the developing eye disc. Ato expression within and anterior to the morphogenetic furrow (MF) is resolved into single R8 founder cells behind the furrow. The founder cells initiate the R8 cell differentiation and form photoreceptor clusters by sequentially recruiting other photoreceptors. The nuclei of photoreceptors are apically localized (orange region). Hh secreted from photoreceptor cells activates Ato expression via Ci^FL (gray region). Bar homeodomain proteins are expressed in basal undifferentiated cells behind the furrow (green region) and repress ato transcription, establishing the complementary anteroposterior pattern of Ato and Bar zones.