Buffered EGFR signaling regulated by spitz to argos expression ratio is critical for patterning the Drosophila eye

The Epidermal Growth Factor Receptor (EGFR) signaling pathway plays a critical role in regulating tissue patterning. Drosophila EGFR (DER) signaling achieves specificity through multiple ligands and feedback loops to finetune signaling spatiotemporally. The principal Drosophila EGF, cleaved Spitz, and the negative feedback molecule, Argos are diffusible and can act both in a cell autonomous and non-autonomous manner. The relative expression dose of Spitz and Argos early in development has been shown to be critical in patterning the Drosophila eye, but the exact identity of the cells expressing these genes in the larval eyedisc has been elusive. Using single molecule RNA Fluorescence in situ Hybridization (smFISH), we reveal an intriguing differential expression of spitz and argos in the Drosophila third instar eye imaginal disc indicative of directional DER signaling. By genetically tuning DER signaling, we show that rather than absolute levels of expression, the ratio of expression to be critical for determining the adult eye phenotype. Proper ommatidial patterning is robust to thresholds around a tightly maintained wildtype ratio, and breaks down beyond. This provides a powerful instance of developmental buffering.