Drosophila Aop imposes a delay on E(spl)-mediated repression of Ato during R8 specification.

Drosophila retinal patterning requires the expression of Atonal (Ato) through coordinated regulation of 5’ and 3’ enhancer modules. ato-3’ directs initial expression of Ato which then directs autoregulation via 5’-ato. Notch (N) signaling also regulates 5’-ato, first enhancing Ato expression and later repressing Ato by inducing E(spl) bHLHs. N signaling balances these opposing functions by directing its obligate nuclear transcription factor, Suppressor of Hairless (Su(H)), only in repressing 5’-ato. In this study, we reveal a novel and more nuanced role for Su(H) in its regulation of 5’-ato. During retinal patterning, Su(H) is required for the expression Anterior open (Aop), which, in turn, promotes 5’-ato activity. We demonstrate that Aop is induced early in retinal patterning via N pathway activity, wherein Aop is required cell-autonomously for robust Ato expression during photoreceptor specification. In aop mutants, expression from both ato enhancers is perturbed, suggesting that Aop promotes the Ato autoregulation through maintenance of ato-3’ activity. Clonal analysis indicates that Aop indirectly opposes E(spl)-mediated repression of Ato. In the absence of both Aop and E(spl), Ato expression is restored and the founding ommatidial photoreceptors, R8s, are specified. These findings suggest that N signaling, through a potentially conserved relationship with Aop, imposes a delay on ato repression, thus permitting autoregulation and retinogenesis.