Rescue of a decapentaplegic hypomorphic wing growth phenotype by expression of the signalling ligands Hedgehog, Semphorin a1 and Wnt homolog 2 in <i>Drosophila melanogaster</i>

An expectation of a hypothesis that proposes cell-to-cell signalling pathways are redundant due to the redundancy of pathway terminal transcription factors (TFs) was tested by screening 35 signalling ligands for rescue of a decapentaplegic (dpp) hypomorphic wing growth phenotype. Three examples of partial rescue were identified: Hedgehog (HH), Semphorin 1a (SEMA1A) and Wnt ortholog 2 (WNT2). HH overexpression with dppGAL4 may increase the expression of DPP from the hypomorphic dpp alleles. However, SEMA1A and WNT2 did not phenocopy ectopic expression of HH or DPP and neither SEMA1A nor WNT2 were required for wing growth suggesting substitution of DPP for restoration of wing growth. The WNT2 rescue was dependent on the Frizzled 4 (FZ4) WNT receptor excluding the possibility that WNT2 weakly binds the DPP receptor. Although examples of phenotypic nonspecificity of signalling ligand function were identified, this is an expectation, and not direct proof, of the hypothesis of TF redundancy.