Drosophila Eye Development and Photoreceptor Specification

Abstract

Abstract The compound eye of the fruitfly Drosophila melanogaster is composed of 800 unit eyes called ommatidia, which are composed of eight neural photoreceptor cells and 12 supporting cells. All these cells are generated from an epithelial sheet called the eye imaginal disc through sequential specification and differentiation achieved by the spatially and temporally restricted expression of transcription factors and signalling pathways. Many of these factors and signals and their mode of action have been identified in recent years. Here, the authors give a brief overview of the molecular pathways that play major roles in photoreceptor specification and their terminal differentiation. Thus, the fly eye has provided important data to current understanding of developmental signalling processes and the specification of neural cell types, their patterning and structure and thus represents one of the most powerful genetic model systems for studying nervous system development. Key Concepts: The Drosophila eye is a highly preferred model system to dissect molecular interactions and developmental mechanisms of the nervous system. It consists of ∼800 units called ommatidia; each has eight neural (photoreceptors) and 12 non‐neural (cone, pigment and bristle) cells. The Drosophila eye emerges during late larval and pupal stage, from a monolayer epithelium called the eye‐antenna imaginal disc. The morphogenetic furrow is a wave of differentiation sweeping across the eye imaginal disc from posterior to anterior initiating sequential differentiation at each line of cells it passes. The morphogenetic furrow is initiated and pushed by a Hedgehog signal; this short‐range signal induces a long‐range Dpp signal to induce neurogenesis via Atonal, while the Dpp signal is suppressed by a Wingless signal secreted from the anterior margin of the eye imaginal disc. Once the Morphogenetic Furrow passes a point in the eye imaginal disc, it creates a cluster of proneural cells resembling rosettes; a single cell out of this cluster is selected to become the the founder cell, R8. The rest of the photoreceptor cells are specified with the help of R8, using a network of molecular interactions. During the development of the retina, the cellular cytoskeleton and cell–cell junctions are actively reorganised. Although the outer photoreceptors (R1–R6) express dim light‐sensitive rhodopsin Rh1, the inner photoreceptors 7 express either UV‐sensitive rhodopsin Rh3 or rhodopsin Rh4, where the blue‐sensitive rhodopsin Rh5 is always accompanied with Rh6 in photoreceptor 8. Sequential activation of different transcription factors leads to regional or stochastic determination of photoreceptor subtypes.