Structure-function analysis of the Drosophila retinal determination protein Dachshund.

Dachshund (Dac) is a highly conserved nuclear protein that is distantly related to the Ski/Sno family of corepressor proteins. In Drosophila, Dac is necessary and sufficient for eye development and, along with Eyeless (Ey), Sine oculis (So), and Eyes absent (Eya), forms the core of the retinal determination (RD) network. In vivo and in vitro experiments suggest that members of the RD network function together in one or more complexes to regulate the expression of downstream targets. For example, Dac and Eya synergize in vivo to induce ectopic eye formation and they physically interact through conserved domains. Dac contains two highly conserved domains, named DD1 and DD2, but no function has been assigned to either of them in an in vivo context. We performed structure-function studies to understand the relationship between the conserved domains of Dac and the rest of the protein and to determine the function of each domain during development. We show that only DD1 is essential for Dac function and while DD2 facilitates DD1, it is not absolutely essential in spite of more than 500 million years of conservation. Moreover, the physical interaction between Eya and DD2 is not required for the genetic synergy between the two proteins. Finally, we show that DD1 also plays a central role for nuclear localization of Dac.

Drosophila retinal homeobox (drx) is not required for establishment of the visual system, but is required for brain and clypeus development.

The possibility that mechanisms of retinal determination may be similar between vertebrates and Drosophila has been supported by the observations that Pax6/eyeless genes are necessary and sufficient for retinal development. These studies suggest that the function of other gene families, operating during early eye development, might also be conserved. One candidate is the retinal homeobox (Rx) family of transcription factors. Vertebrate Rx is expressed in the prospective eye and forebrain and is required for eye morphogenesis, retinal precursor appearance, and normal forebrain development, indicating that it is an essential regulator of early eye and brain formation. Here, we test the hypothesis that Drosophila Rx (drx) is required for adult and larval eye development. We have isolated a drx null allele and demonstrate that the mutant compound eye and larval visual system is not detectably abnormal. However, we find that drx is required for development of a central brain structure, the ellipsoid body, suggesting that Rx function in the brain may be conserved. Finally, we characterize a novel anterior head phenotype and demonstrate that drx is required for clypeus development. Thus, our data suggest that drx may be required for the regulation of genes involved in brain morphogenesis and clypeus precursor development. We propose that differences in insect and vertebrate eye development may be explained by changes in gene regulation and/or the tissue of origin for eye precursor cells.