Probing for gene specificity in epithelial development

We surveyed a total of 228 random insertions of a P[GawB] element to determine the fraction of regulatory regions in the Drosophila genome that activate gene expression specifically in follicle cells versus producing more complex patterns of expression. We monitored the GAL4 expression encoded by this construct in the ovarian follicle cells by crossing the lines to a strain containing a lacZ reporter construct. Sixty four per cent of the insertions showed ovarian expression. To assess the specificity of this expression, 124 of the 228 lines were crossed to strains containing either an activated form of Armadillo, the Drosophila homolog of beta-catenin, or an activated form of Torpedo/Egfr, the Drosophila homolog of the Epidermal Growth Factor receptor, under the control of GAL4 target sites. The lethality and imaginal disc phenotypes observed in these crosses suggest that most random insertions cause GAL4 expression in a variety of tissues. Very few insertions appear to drive expression only in follicle cells. Although the activated form of Armadillo produced higher frequencies of lethality and disk phenotypes, expression in the follicle cell epithelium at later stages of oogenesis did not lead to a visible phenotype. This contrasts with the dorsalized phenotypes observed in the combination of the same GAL4 lines with the activated Torpedo construct.