DEVELOPMENT OF THE DROSOPHILA TRACHEAL SYSTEM OCCURS BY A SERIES OF MORPHOLOGICALLY DISTINCT BUT GENETICALLY COUPLED BRANCHING EVENTS

The tracheal (respiratory) system of Drosophila melanogaster is a bran ched network of epithelial tubes that ramifies throughout the body and transports oxygen to the tissues, It forms by a series of sequential branching events in each hemisegment from T2 to A8. Here we present a cellular and initial genetic analysis of the branching process, We sho w that although branching is sequential it is not iterative, The three levels of branching that we distinguish involve different cellular me chanisms of tube formation. Primary branches are multicellular tubes t hat arise by cell migration and intercalation; secondary branches are unicellular tubes formed by individual tracheal cells; terminal branch es are subcellular tubes formed within long cytoplasmic extensions, Ea ch level of branching is accompanied by expression of a different set of enhancer trap markers. These sets of markers are sequentially activ ated in progressively restricted domains and ultimately individual tra cheal cells that are actively forming new branches. A clonal analysis demonstrates that branching fates are not assigned to tracheal cells u ntil after cell division ceases and branching begins, We further show that the breathless FGF receptor, a tracheal gene required for primary branching, is also required to activate expression of markers involve d in secondary branching and that the pointed ETS-domain transcription factor is required for secondary branching and also to activate expre ssion of terminal branch markers, The combined morphological, marker e xpression and genetic data support a model in which successive branchi ng events are mechanistically and genetically distinct but coupled thr ough the action of a tracheal gene regulatory hierarchy.