Identification of new mutations affecting photoreceptor sub-type specification. D. Pistillo¹, F. Pichaud², C. Desplan¹. 1) Dept Biol, New York Univ, New York, NY; 2) MRC Laboratory for Molecular and Cell Biology, UCL, London, UK.

   The eye of Drosophila is composed of approximately 800 units, the ommatidia, each consisting of eight photoreceptor cells (R1 to R8), as well as cone and pigment cells. In each ommatidium the outer photoreceptors R1 to R6 surround the two inner R7 and R8. The inner photoreceptors R7 and R8 belong to two classes depending on their photopigment content: they either express coordinately UV-sensitive Rhodopsin 3 (Rh3) in R7 and blue-sensitive Rhodopsin 5 (Rh5) in R8 (pale ommatidia) or UV-sensitive Rhodopsin 4 (Rh4) in R7 and green-sensitive Rhodopsin 6 (Rh6) in R8 (yellow ommatidia). These two sub-types of ommatidia are distributed stochastically in the retina with a conserved ratio of 70% yellow and 30% pale. Proper coordination between R7 and R8 occurs through a directional R7 to R8 signal: once a cell becomes a pale R7, it expresses rh3 and represses the default expression of rh4. It then instructs an otherwise yellow R8 to express rh5. In order to identify new genes affecting subtype specification, an EMS mutagenesis screen has been carried out and several mutants affecting the pale/yellow ratio have been isolated. We focused our attention on four of these mutants: in #11, rh3 is expressed in 55% of R7 (instead of the wt 30%), in #19 and #37, rh3 is expressed in less than 20% of R7, while a mutation named daltonian does not affect R7 opsin expression, but shows almost no rh5 expression in R8. We are currently trying to identify the genes affected in these mutants using deletion and recombination mapping, as well as further characterizing the mutant phenotypes. We are also performing another screen using the transposable element piggyBac (which exhibits little specificity of insertion) as an insertional mutagen. The transposase required to mobilise piggyBac does not act on P elements and piggyBac can thus be used as an agent to mutagenize FRT chromosomes. Preliminary results from this screening will also be presented.