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

Abstract

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.

Regulation of vertebrate eye development by Rx genes

“…While dachshund is critical for invertebrate eye formation (Mardon et al, 1994), it does not seem to have an important role in vertebrate eye formation (Davis et al, 2001). And last, but not least, vertebrate eye formation is Rx dependent while invertebrate eye development is Rx independent (Davis et al, 2003). We believe that the differential dependence of eye formation on Rx in insects and vertebrates reflects different evolutionarily origin of these two types of eyes.…”

Genetic control of retinal specification and determination in Drosophila

“…However, it has also become apparent that there is evolutionary divergence and not all genes and pathways are functionally conserved across phylogeny. A prime example is the discovery that the Drosophila ortholog of the vertebrate retinal homeobox gene Rx, drx, is not required for eye development (Davis et al, 2003). Mouse knockouts of the Rx gene, however, develop with no eyes and Rx functions upstream of Pax6 during vertebrate eye specification (Mathers et al, 1997).…”

Historical perspective on the development and evolution of eyes and photoreceptors

“…We are extending these studies to the later developmental stages and we plan to compare the transcriptome of Drosophila with that of the mouse. Besides the highly conserved transcription factor genes, there are also important differences between mammals and insects; for example, the homeobox gene Rx is essential for vertebrate eye development (Mathers et al, 1997) and is expressed in rod and cone ciliary photoreceptors, whereas in Drosophila it is expressed in the brain only, but not in the rhabdomeric photoreceptors of the eye (Eggert et al, 1998;Davis et al, 2003).…”