Leg development in flies versus grasshoppers: differences in dpp expression do not lead to differences in the expression of downstream components of the leg patterning pathway

All insect legs are structurally similar, characterized by five primary seg ments. However, this final form is achieved in different ways, Primitively, the legs developed as direct outgrowths of the body wall, a condition reta ined in most insect species, In some groups, including the lineage containi ng the genus Drosophila, legs develop indirectly from imaginal discs, Our u nderstanding of the molecular mechanisms regulating leg development is base d largely on analysis of this derived mode of leg development in the specie s D. melanogaster, The current model for Drosophila leg development is divi ded into two phases, embryonic allocation and imaginal disc patterning, whi ch are distinguished by interactions among the genes wingless (wg), decapen taplegic (dpp) and distalless (dll). In the allocation phase, dll is activa ted by wg but repressed by dpp, During imaginal disc patterning, dpp and wg cooperatively activate dll and also indirectly inhibit the nuclear localiz ation of Extradenticle (Exd), which divide the leg into distal and proximal domains, In the grasshopper Schistocerca americana, the early expression p attern of dpp differs radically from the Drosophila pattern, suggesting tha t the genetic interactions that allocate the leg differ between the two spe cies. Despite early differences in dpp expression, wg, Dll and Exd are expr essed in similar patterns throughout the development of grasshopper and fly legs, suggesting that some aspects of proximodistal (P/D) patterning are e volutionarily conserved. We also detect differences in later dpp expression , which suggests that dpp likely plays a role in limb segmentation in Schis tocerca, but not in Drosophila, The divergence in dpp expression is surpris ing given that all other comparative data on gene expression during insect leg development indicate that the molecular pathways regulating this proces s are conserved. However, it is consistent with the early divergence in dev elopmental mode between fly and grasshopper limbs.