PHOTORECEPTOR DIFFERENTIATION IN DROSOPHI LA - TRANSDUCTION AND INTERPRETATION OF THE RTK SIGNALING PATHWAY

Differentiation of the R7 photore-ceptor in the drosophila retina depe nds upon activation of the tyrosine kinase receptor (RTK) Sevenless (S ev). In the absence of Sev or of its mediators, the R7 precursor adopt s a non-neural fate, whereas ectopic activation of the Sev cascade tra nsforms non-neural cells into R7. Genetic and molecular lar studies of these transformations have allowed the dissection of a neuralizing si gnal initiated by a RTK. These studies reveal complex interactions bet ween the Ras/ Raf/MAPKK/MAPK pathway and other cytoplasmic factors, an d show how the Sev signal controls the balance between Yan and Pointed PP, two antagonistic ETS transcription factors. Specification of the o ther photoreceptors does not depend upon Sev, but requires specific tr anscription factors including Rough and Seven-up. Nevertheless, mediat ors of Sev are involved in the differentiation of most other retinal c ells where they respond to another RTK, the Drosophila EGF receptor De r. To adopt their diverse fates, retinal cells must therefore interpre t in distinctive ways the same RTK signal. The choice of their fate co uld be conditioned by variations in the intensity of the RTK signal, b ut also by their own developmental age at the time they receive it.