A MODIFIER SCREEN IN THE EYE REVEALS CONTROL GENES FOR KRUPPEL ACTIVITY IN THE DROSOPHILA EMBRYO

Irregular facets (If) is a dominant mutation of Drosophila that result s in small eyes with fused ommatidia, Previous results showed that the gene Kruppel (Kr), which is best known for its early segmentation fun ction, is expressed ectopically in If mutant eye discs, However, it wa s not known whether ectopic Kr activity is either the cause or the res ult of the If mutation. Here, we show that If is a gain-of-function al lele of Kr, We then used the If mutation in a genetic screen to identi fy dominant enhancers and suppressors of Kr activity on the third chro mosome. Of 30 identified Kr-interacting loci, two were cloned, and we examined whether they also represent components of a natural Kr-depend ent developmental pathway of the embryo. We show that the two genes, e yelid (eld) and extramacrochaetae (emc), which encode a Bright family- type DNA binding protein and a helix-loop helix factor, respectively, are necessary to achieve the singling-out of a unique Kr-expressing ce ll during the development of the Malpighian tubules, the excretory org ans of the fly. The results indicate that the Kr gain-of-function muta tion If provides a tool to identify genes that are active during eye d evelopment and that a number of them function also in the control of K r-dependent developmental processes.