The role of Wingless signaling in establishing the anteroposterior and dorsoventral axes of the eye disc

The posteriorly expressed signaling molecules Hedgehog and Decapentaplegic drive photoreceptor differentiation in the Drosophila eye disc, while at th e anterior lateral margins Wingless expression blocks ectopic differentiati on. We show here that mutations in axin prevent photoreceptor differentiati on and lead to tissue overgrowth and that both these effects are due to ect opic activation of the Wingless pathway. In addition, ectopic Wingless sign aling causes posterior cells to take on an anterior identity, reorienting t he direction of morphogenetic furrow progression in neighboring wild-type c ells. We also show that signaling by Decapentaplegic and Hedgehog normally blocks the posterior expression of anterior markers such as Eyeless, Wingle ss signaling is not required to maintain anterior Eyeless expression and in combination with Decapentaplegic signaling can promote its downregulation, suggesting that additional molecules contribute to anterior identity, Alon g the dorsoventral axis of the eye disc, Wingless signaling is sufficient t o promote dorsal expression of the Iroquois gene mirror, even in the absenc e of the upstream factor pannier, However, Wingless signaling does not lead to ventral mirror expression, implying the existence of ventral repressors .