In Drosophila, most structures of the fly head, including a set of sensory organs (eyes, antennae, palps, ocelli) and the head capsule, derive from a pair of larval eye–antennal discs (Haynie and Bryant, 1986). wingless (wg), the fly homologue of the mammalian Wnt-1 gene, plays essential roles during the development of the Drosophila head capsule and eye primordia by controlling the establishment of the dorsoventral (DV) and anteroposterior (AP) axes of the eye disc (Lee and Treisman, 2001). Thus, wg is expressed early on in dorsal cells of the first and second larval stage (L1 and L2) eye disc, both in the disc proper, which will give rise to the ptilinum and periocellar region of the head capsule, and in the peripodial epithelium, which also contributes to head capsule structures (Haynie and Bryant, 1986; Royet and Finkelstein, 1996; Cavodeassi et al., 1999; Cho et al., 2000; Pichaud and Casares, 2000). The dorsal expression of wg induces the expression of the Iroquois-complex (IroC) genes araucan, caupolican (Cavodeassi et al., 1999), and mirror (Heberlein et al., 1998). The dorsal IroC genes are required to restrict the expression of the glycosyl-transferase encoding gene fringe (fng) to the ventral disc cells. The resulting DV boundary between fng-nonexpressing (D) and fng-expressing (V) cells leads to the localized activation of the Notch signaling pathway along the boundary, which is in turn necessary for the growth of the eye primordium and for the further patterning of the developing retina (Cho and Choi, 1998; Dominguez and de Celis, 1998). During late L2, dorsal wg expression retracts to the anterior margins of the eye disc, and a new smaller domain appears on its anterior ventral margin (Cavodeassi et al., 1999; Ma and Moses, 1995). Removing wg transcription or blocking its signaling pathway in these anterior domains results in an anterior expansion of eye development along the disc's margins (especially along the dorsal one) at the expense of the head capsule, which normally derives from these regions (Ma and Moses, 1995; Treisman and Rubin, 1995). Therefore, the precise regulation of the spatiotemporal expression of wg is essential for the development of the eye disc.
Several genes and signaling pathways have been proposed to control wg expression in the eye disc. First, the GATA-binding transcription factor Pannier (Ramain et al., 1993) has been proposed to positively regulate wg expression in the dorsal eye disc, placing this gene at the top of the genetic cascade that establishes the DV subdivision of the disc (Maurel-Zaffran and Treisman, 2000). Thus, in L3 discs Pnr is coexpressed with wg along the dorsal eye disc margin, and large clones of Pnr− mutant cells lose wg expression (Maurel-Zaffran and Treisman, 2000). Nevertheless, and although Pnr is expressed in the eye disc primordium in late embryos (Maurel-Zaffran and Treisman, 2000), it is not expressed in L1 eye discs (Singh and Choi, 2003), when wg expression is already detected in dorsal cells (Cho et al., 2000). In addition, wg expression domain is larger than Pnr's in the L3 disc: while wg is expressed in the outer dorsal margin and peripodial epithelium (PE, that overlays the main epithelium or disc proper), plus the dorsal region of the disc proper, Pnr is expressed just in the outer margin and PE, abutting the Iro-C domain of expression in the dorsal disc proper (Pichaud and Casares, 2000). These results suggest Pnr might not be the sole dorsal activator of wg expression. The ventral wg expression domain requires the transcription factor homothorax (hth; Pichaud and Casares, 2000), yet how this domain is initiated is still unknown. The decapentaplegic (dpp, a BMP-4 like gene) signaling pathway regulates wg transcription, preventing wg ectopic expression along the posterior eye margin and, thus, avoiding the wg-mediated repression of retinal differentiation. In this way, dpp controls the partition of the disc into eye competent regions, under Dpp influence, and head capsule competent region, under Wg's (Chanut and Heberlein, 1997; Dominguez and Hafen, 1997; Pignoni and Zipursky, 1997).
In this report, we identify a short and conserved eye-specific enhancer sequence located in the 3′ region of the wg gene that recapitulates its dorsal eye disc expression. Nevertheless Pnr is not required for the activity of the eye-specific enhancer or the transcription or translation of wg in the disc proper. Pnr is only required for wg expression in the PE of the eye disc. This role of Pnr in the peripodial epithelium might be a relatively recent acquisition during the evolution of insects. Sequences adjacent to the eye disc enhancer harbor additional wg enhancers for the wing and ventral (leg, antennal, and genital) discs and, together, constitute a disc-specific regulatory region of the wg gene.