1. Pattern formation in the Drosophila visual system
Differentiation of cells within the eye primordium, the eye imaginal disc, is a progressive process that moves across the disc from posterior to anterior. The movement of this developmental wave is organized by several conserved signaling molecules. The Hedgehog (Hh) protein induces differentiation of R8, the first photoreceptor to form in each cluster; R8 then recruits additional photoreceptors by producing Spitz, a ligand for the EGF receptor. We are taking a genetic approach to identify other molecules involved in establishing the early pattern of differentiation in the eye. We have identified several novel components of both the Hh and EGFR pathways. These include Rasp, a transmembrane acyltransferase that adds an essential palmitate modification to the Hh and Spitz proteins; Hyd, a ubiquitin ligase that restricts Hh expression and function; and three genes specifically required in the EGFR pathway. Mago nashi is a component of the exon junction complex, Aveugle is an adaptor protein involved in Raf activation and Myopic is an endosomal protein that may control receptor recycling. We have also found that two transcription factors, Pannier and Chip, restrict the eye field dorsally and ventrally. Finally, we have identified several cytoskeletal proteins. Act up is required for changes in cell shape that control the spread of the Hh protein. Mutations in the myosin binding subunit of myosin light chain phosphatase and in actin capping protein alpha cause loss of photoreceptors from the eye disc epithelium, apparently due to migration of the cell bodies towards the growth cones. We are continuing to screen for additional genes to obtain a complete picture of the molecular requirements for eye development.

2. Developmental signals and general transcriptional regulators  
We are also studying the effects of general transcriptional regulators on the transmission of specific developmental signals. The osa gene encodes a subunit of the Brahma chromatin remodeling complex, the Drosophila homologue of the yeast SWI/SNF complex. Osa is present in only a subset of Brahma complexes; the remaining subset instead contain the BAF180 and BAP170 proteins. We are investigating the differential functions in vivo of these two classes of complexes. Osa plays a role downstream of the Wingless (Wg) signal to repress the expression of Wg target genes. We have generated Baf180 and Bap170 mutants; in combination, these mutations prevent metamorphosis. We are searching for transcriptional targets of the complex containing these subunits. We have also identified mutations in two components of the mediator complex, MED12 and MED13. Unlike core components of the complex, these proteins are not required for cell growth or proliferation. However, mutations in both genes have identical and specific developmental phenotypes, affecting a subset of the target genes of the Hh, Notch and Wg pathways. We have demonstrated physical interactions of MED12 and MED13 with the transcription factors downstream of Notch and Wg; our current model is that they act as specific adaptors to link these transcription factors to the mediator complex.

3. Axon guidance in the visual system
Finally, we are studying the layer-specific targeting of photoreceptor axons. We have found that the protein tyrosine phosphatase LAR and its interacting protein Liprin-a are specifically required for the axon of the UV-sensitive photoreceptor R7 to reach its normal termination layer. We have shown that Liprin-a contributes to LAR function in the R7 growth cone primarily by transmitting the LAR signal rather than by localizing LAR within the membrane. We are also generating mutations in two other Drosophila liprin genes in order to study their functions during nervous system development.