Molecular evolution of the ligands of the insulin-signaling pathway: dilp genes in the genus Drosophila

Drosophila melanogaster, unlike mammals, has seven insulin-like peptides (DILPS). In Drosophila, all seven genes (dilp1-7) are single copy in the 12 species studied, except for D. grimshawi with two tandem copies of dilp2. Our comparative analysis revealed that genes dilp1-dilp7 exhibit differential functional constraint, which is indicative of some functional divergence. Species of the subgenera Sophophora and Drosophila differ in some traits likely affected by the insulin-signaling pathway, such as adult body size. It is in the branch connecting the two subgenera that we found the footprint left by positive selection driving nonsynonymous changes at some dilp1 codons to fixation. Finally, the similar rate at which the two dilp2 copies of D. grimshawi have evolved since their duplication and the presence of a putative regulatory region highly conserved between the two paralogs would suggest that both copies were preserved either because of subfunctionalization or dose dependency rather than by the neofunctionalization of one of the two copies.