The periphery of the fly eye is a good place to study the final stage of positional signalling. Here, Wingless (Wg) signalling induces concentrically arranged cellular specializations, such as the pigment rim, which shields the eye from extraneous light. The pigment cells that form this structure originally surrounded the peripheral ommatidia of the eye, which die during pupal eye development. On , Lim and Tomlinson report that three Snail family transcription factors - Worniu, Snail and Escargot - and the enzyme Notum are Wg signalling targets at the edge of the fly eye. Notum limits the extent of Wg signalling,but Snail gene expression, they report, is required for removing the peripheral ommatidia and for forming the peripheral rim. Because Snail family proteins are expressed only in a subset of ommatidia cells, not in the photoreceptors that die, Lim and Tomlinson propose that a death signal released from the Snail-family-expressing cells directs the death of the photoreceptors.
