PIPSQUEAK ENCODES A NOVEL NUCLEAR-PROTEIN REQUIRED DOWNSTREAM OF SEVEN-UP FOR THE DEVELOPMENT OF PHOTORECEPTORS R3 AND R4

Photoreceptor induction in the Drosophila eye is mediated by activatio n of the Ras signal transduction cascade, Although this process is wel l understood, little is known about how the diversity of photoreceptor subtypes is generated, The pipsqueak (psq) gene is expressed at high levels in the R3/R4 precursors during eye development and this express ion depends on seven-up (svp) gene function. Moreover, strong psq alle les are dominant suppressors of a svp-induced cone cell transformation phenotype, Although the gene was previously identified and described as a member of the maternal posterior group of genes, the strong semil ethal alleles isolated here demonstrate a specific requirement for psq function downstream of svp for the development of photoreceptors R3/R 4. The gene has three independent 5' ends and codes for several nuclea r protein isoforms, some containing the POZ domain which has been impl icated in protein-protein interactions. Interestingly, all viable alle les with a maternal posterior group phenotype cluster around one speci fic 5' exon, while all semilethal alleles have lesions which map to a different alternative 5' exon.