Identification of cis-regulatory modules responding to positional cues regulating the patterned expression of the Drosophila follicle cell fate determinants mid and mirr.

Abstract

In the Drosophila ovary, the developing oocyte secretes a localized inductive signal, Gurken (Grk), that activates the Drosophila epidermal growth factor receptor (EGFR) within the overlying follicular epithelium. Grk localization is dynamic, and Grk/EGFR signaling leads to different outcomes depending on the timing and location of signaling. In early stages, Grk is localized at the posterior and induces a posterior fate characterized by expression of the T-box transcription factors midline (mid) and H15. Later, Grk adopts a dorsal anterior localization and induces the dorsal anterior fate determinant mirror (mirr), a homeodomain transcription factor. These context-dependent outcomes of Grk/EGFR signaling are mutually exclusive, raising the question of how the choice of target gene expression is regulated. We showed previously that these alternative signalling outcomes are determined by localized antiparallel gradients of posterior JAK/STAT and anterior BMP pathway activity, and mutual repression between Mid/H15 and Mirr. These regulatory interactions define a genetic network that acts as a bistable switch that toggles between alternative EGFR signaling outcomes. We hypothesize that this spatial information is ultimately interpreted by regulatory elements associated with the mid, H15 and mirr loci, but at what level these multiple cues are integrated to generate these outcomes is unclear. To approach this question, here we screen two genome-wide collections of transcriptional reporters for those bearing genomic regions from the H15, mid and mirr loci and expressed in a pattern resembling that of the endogenous gene. Here we characterize the expression of four such putative cis-regulatory regions, two from the mid locus and two from the mirr locus and test their response to the known upstream regulators in this patterning network. We find that each reproduces distinct aspects of the expression and response of the endogenous gene, with some emerging features that suggest that these regions do not function strictly modularly. These data identify genomic sequences that drive patterned expression in this tissue and provide a simplified system for asking whether these sequences are directly regulated by the upstream spatial cues in this patterning network.