Fig. 5. Comparison between Drosophila R8 and SOP development. R8 and SOP development can be divided into four roughly analogous stages: patterning of neural fields, establishment of zones of neural competency (proneural clusters), selection of the presumptive R8/SOP and R8/SOP-mediated recruitment. R8 patterning begins with ubiquitous expression of Ato (light-blue circles) in all retinal cells. The R8 competency zone is determined when ubiquitous Ato is reshaped into clusters of about 10 cells (intermediate groups) via the action of endogenously activated factors. Lateral inhibition represses Ato in all members of the intermediate group except for the selected presumptive R8 (dark blue circles). The differentiating R8 cell recruits neighboring cells, which do not express Ato, to become non-R8 photoreceptors (red circles). These non-R8 photoreceptors have a different function, morphology and axonal projection from R8 cells. As eye development occurs progressively, all stages of R8 specification are visualized simultaneously. By contrast, the future region of an SOP is established by exogenous prepatterning genes (hexagon). All cells within this prepatterned region form a competency zone (proneural cluster) of 15-20 cells and express proneural genes (light-blue circles). Selection occurs when the future SOP expresses proneural genes at a high level and, by lateral inhibition, prevents this enhanced expression in the surrounding cells (dark-blue circles). SOP-mediated recruitment does not occur during external sensory organ (es) development but does occur during chordotonal organ (ch) development. However, the SOP of a chordotonal organ is only sufficient to recruit proneural-expressing cells from within the existing proneural cluster to become SOPs (dark-blue circles). These SOPs then produce equivalent chordotonal organs.