Conserved and divergent elements in Torso RTK activation in Drosophila development

The repeated use of signalling pathways is a common phenomenon but little is known about how they become co-opted in different contexts. Here we examined this issue by analysing the activation of Drosophila Torso receptor in embryogenesis and in pupariation. While its putative ligand differs in each case, we show that Torso-like, but not other proteins required for Torso activation in embryogenesis, is also required for Torso activation in pupariation. In addition, we demonstrate that distinct enhancers control torso-like expression in both scenarios. We conclude that repeated Torso activation is linked to a duplication and differential expression of a ligand-encoding gene, the acquisition of distinct enhancers in the torso-like promoter and the recruitment of proteins independently required for embryogenesis. A combination of these mechanisms is likely to allow the repeated activation of a single receptor in different contexts.