argos Is required for projection of photoreceptor axons during optic lobe development in Drosophila

The Drosophila argos gene encodes a secreted protein with an epidermal growth factor (EGF) motif, which acts as an inhibitor of cell recruitment in the developing eye and wing. Here, we have analyzed the role of argos during optic lobe development. argos expression was observed in the optic lobes throughout the developmental stages. In argos mutants, neuropiles failed to develop normally during embryonic and larval stages, and photoreceptor axons did not project properly into the lamina. Ubiquitous expression of argos, under control of the hsp70 promoter, rescued the defects in optic lobes. We have found that glial cells failed to differentiate in the larval optic lobes of argos mutants. Correspondingly, in loss-of-function repo mutants, whose glial cells also fail to differentiate, photoreceptor axons showed the impaired projection pattern similar to the argos phenotype. These results suggest that glial cells play a role for guidance of photoreceptor axons. The loss-of-function Star mutation (StarX155) dominantly suppressed the defects in the argos optic lobes, suggesting that these two genes act in an antagonistic fashion during optic lobe development.