Synergistic signaling by two BMP ligands through the SAX and TKV receptorscontrols wing growth and patterning in Drosophila

In Drosophila wing discs, a morphogen gradient of DPP has been proposed to determine the transcriptional response thresholds of the downstream genes s al and or,lb, We present evidence that the concentration of the type I rece ptor TKV must be low to allow long-range DPP diffusion. Low TKV receptor co ncentrations result, hen-ever, in low signaling activity. To enhance signal ing at low DPP concentrations, we find that a second ligand, GBB, augments DFP/TKV activity. GBB signals primarily through the type I receptor SAX, wh ich synergistically enhances TKV signaling and is required for proper OMB e xpression. We show that OMB expression in wing discs requires synergistic s ignaling by multiple ligands and receptors to overcome the limitations impo sed on DPP morphogen function by receptor concentration levels.