and Enhancer of Split (E(spl)) loci. Dl and Ser encode inductive signalling at the wing margin require the Su(H) signal t t s s t t

has been established, Notch signalling represses atonal we also show that particular Enhancer-of-split bHLH genesmembrane-bound ligands of Notch. Su(H) acts as an intracellular transducer of the signal from the membrane to the nucleus. E(spl) is a nuclear target in the relay of the signal(Artavanis-Tsakonas et al., 1995). E(spl) is a complex locus, including seven genes that encode closely related transcription factors bearing a basic Helix-Loop-Helix (bHLH) motif (m8, m7, m5, m3, m b, m g and m d), and two genes that encode non-bHLH proteins (m4 and groucho; Delidakis and Artavanis-Tsakonas, 1992; Knust et al., 1992; Schrons et al., 1992) (Fig. 1). In many cases Notch signalling is thought to delay cell differentiation, keeping cells competent to respond to later inductive signals (Coffman et al., 1993; Fortini et al., 1993). Lateral specification, exemplified by embryonic neuroblast commitment, is an example. During lateral specification an transducer, but at the wing margin target genes that ac positively to promote cell fates are activated, in addition to the repressors of the E(spl) gene complex (E(spl)-C) (de Celis e al., 1996a,b; Kim et al., 1996). Although the E(spl)-C i expressed at the wing margin, little function has yet been found for this expression (de Celis et al., 1996a,b). Here we investigate the signal transducers and target gene involved in Notch signalling in the eye imaginal disc. The adul eye of the fly contains 800 ommatidia (unit eyes) arranged in a two-dimensional lattice. Each ommatidium includes eigh photoreceptor neurons and twelve accessory cells arranged in a stereotypical pattern, and is founded by a single R8 photoreceptor cell, which induces most of the other cell type (Wolff and Ready, 1993; Tio and Moses, 1997). Expression o the proneural gene atonal (ato) is autonomously required foINTRODUCTION A signalling pathway involved in many cell fate decisions is centred around the transmembrane receptor encoded by the gene Notch (N). Several elements in the Notch signalling pathway have been identified (Artavanis-Tsakonas et al., 1995). In Drosophila these are encoded by, among others