A modifier screen of ectopic Krüppel activity identifies autosomal Drosophila chromosomal sites and genes required for normal eye development.

Irregular facets (If) is a dominant gain-of-function allele of the Drosophila segmentation gene Krüppel (Kr) that interferes with eye development. In a search for genes that interact with Kr activity, we recently performed a systematic genetic screen to identify dominant enhancers and suppressors of the If eye phenotype that are located on the third chromosome. Here we describe locations and candidate genes of the second chromosome that act as dominant modifiers of ectopic Kr activity during eye development. The collection of more than 40 modifiers of Kr activity located on the second and third chromosomes, from which a total of 16 genes were identified, includes genes encoding transcription factors and components of signal transduction pathways that may regulate or be regulated by Kr activity. We also identified genes coding for more general cellular factors that could interfere with the intracellular transport or the half-life of the Kr protein. The data demonstrate that the If mutation provides a means to screen the Drosophila genome for functional components of developmental pathways that depend on or can be modified by Kr activity. Owing to the bias of the screening system applied, these modifier genes will be expressed and are likely to be required during Drosophila wild-type eye development.