A hierarchy [above, left] of transcription factors encoded by twin of eyeless (toy), eyeless (ey), eyes absent (eya), sine oculis (so), dachshund (dac) and eyegone (eyg), acting during embryonic development, controls the specification of the eye disc primordium.

Initiation of photoreceptor differentiation in the third instar is driven by hedgehog (hh) [red region] and decapentaplegic (dpp) expressed at the posterior margin, and inhibited by wingless (wg) at the dorsal and ventral margins[green regions]. Initiation occurs at the dorsoventral midline because Notch (N) is activated there by a boundary of fringe (fng) expression [blue] that controls N response to its ligands Delta (Dl) and Serrate (Ser). fng is inhibited in the dorsal half of the eye disc by the iroquois genes araucan (ara), caupolican (caup), and mirror (mrr) [yellow].