In the Drosophila eye, loss of Hh signalling blocks the initiation of photoreceptor morphogenesis, but how this comes about is poorly understood. Using loss- and gain-of-function genetics, Graeme Mardon and colleagues have now discovered that the principal role of Hh signalling in Drosophilaeye development is to alleviate the repression of two key genes that function in a retinal determination network – dpp and eya. Hh signalling does this, they propose on , by blocking the cleavage of the active form of Cubitus interruptus, a Hh pathway component,into its repressive form, which represses dpp. Once dppexpression is established, it acts with another gene, ey, to initiate eya expression, in a vital step towards establishing the retinal determination network. Importantly, Hh acts here not as a classical morphogen but as a binary switch that initiates photoreceptor differentiation.
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