Two-step process for photoreceptor formation in Drosophila

The formation of photoreceptor cells (PRCs) in Drosophila serves as a parad igm for understanding neuronal determination and differentiation. During la rval stages, a precise series of sequential inductive processes leads to th e recruitment of eight distinct PRCs (R1-R8)(1). But, final photoreceptor d ifferentiation, including rhabdomere morphogenesis and opsin expression, is completed four days later, during pupal development(2,3). It is thought th at photoreceptor cell fate is irreversibly established during larval develo pment, when each photoreceptor expresses a particular set of transcriptiona l regulators and sends its projection to different layers of the optic lobe s. Here, we show that the spalt (sal) gene complex(4-7) encodes two transcr iption factors that are required late in pupation for photoreceptor differe ntiation. In the absence of the sal complex, rhabdomere morphology and expr ession of opsin genes in the inner PRCs R7 and R8 are changed to become ide ntical to those of outer R1-R6 PRCs. However, these cells maintain their no rmal projections to the medulla part of the optic lobe, and not to the lami na where outer PRCs project. These data indicate that photoreceptor differe ntiation occurs as a two-step process. First, during larval development, th e photoreceptor neurons become committed and send their axonal projections to their targets in the brain. Second, terminal differentiation is executed during pupal development and the photoreceptors adopt their final cellular properties.