Elucidating the role of unzipped in the visual system of Drosophila

The Drosophila eye is composed of 800 ommatidial units, each containing eight photoreceptors (PR). The outer PRs R1-R6 are responsible for motion detection, while the inner PRs R7 and R8 and are responsible for colour vision. The outer PRs express Rhodopsin1 (Rh), while the inner PRs express Rh3-Rh6. The pale subtype is formed when the R7 cells express Rh3 and the coupled R8 cells express Rh5, while in the yellow subtype R7 cells express Rh4 and the coupled R8 cells express Rh6. This final state of differentiation is reached after a sequential recruitment process of PRs at the 3rd instar larval stage. At the initial stage, the PR cells start differentiating and their axons target to their temporal layers. At the terminal stage the inner and outer PRs are distinguished from each other, each PR expresses a specicific Rh molecule, and the axons target to their final layers in the optic lobe. The aim of this study was to identify new molecules involved in inner PR differentiation, mainly focusing on three genes expressed in R8 cells that have been identified in a previously performed enhancer-trap screen. One of these genes, unzipped was chosen for further analysis because of its function as cell adhesion molecule (CAM). By immunostainings the expression of Uzip was revealed to be in R8 cells and a subgroup of glial cells at the 3rd instar larval stage, R8 cells and glial cells at the 48 hour pupa stage, and glial cells in the adult stage where some of them project to the medulla. Immunostainings showed that Uzip does not have a direct effect on PR formation and glia expression at the 3rd instar larvae and pupae stages; or on subtype specification at the adult stage. For axonal targeting both the downregulation of Uzip in the target area and the misexpression in neighboring PRs and glia, which are not in the target region, lead to an increase in axonal misprojection phenotype. According to our suggested model, Uzip expressing R8 cells follow the adhesive signal from the Uzip expressing glial cells at the target region of projection. This study identifies Unzipped and describes its expression and involvement in axonal targeting the visual system.