Bristles induce bracts via the EGFR pathway on Drosophila legs

A long-standing mystery in Drosophila has been: how do certain bristles induce adjacent cells to make bracts (a type of thick hair) on their proximal side? The apparent answer, based on loss- and gain-of-function studies, is that they emit a signal that neighbors then transduce via the epidermal growth factor receptor pathway. Suppressing this pathway removes bracts, while hyperactivating it evokes bracts indiscriminately on distal leg segments. Misexpression of the diffusible ligand Spitz (but not its membrane-bound precursor) elicits extra bracts at normal sites. What remains unclear is how a secreted signal can have effects in one specific direction.