Epidermal growth factor receptor (EGFR) signalling regulates numerous processes throughout Drosophila development. For example, during oogenesis, an EGFR activation gradient induced by Gurken (a TGFα-like ligand secreted from the oocyte) patterns the follicular epithelium. On p. , Stanislav Shvartsman and colleagues present a revised mathematical model for this important process, which initiates the formation of two dorsal eggshell appendages. Each of these appendages is derived from a primordium that comprises a patch of cells expressing the transcription factor gene broad (br) and an adjacent strip of cells expressing rhomboid (rho), which encodes a protease in the EGFR pathway. Previous models of eggshell patterning have not fully accounted for the coordinated expression of br and rho. The new model, however, proposes that the sequential action of feed-forward loops and Notch-mediated juxtacrine signals activated by the EGFR signalling gradient establishes rho expression, successfully describes the wild-type br and rho expression patterns, and accounts for changes in these patterns in response to genetic perturbations.
