EGF signalling regulates cell invagination as well as cell migration during formation of tracheal system in Drosophila

The Drosophila tracheal system is a network of epithelial tubes that arises from the tracheal placodes, lateral clusters of ectodermal cells in ten em bryonic segments. The cells of each cluster invaginate and subsequent forma tion of the tracheal tree occurs by cell migration and fusion of tracheal b ranches, without cell division. The combined action of the Decapentaplegic (Dpp), Epidermal growth factor (EGF) and breathless/branchless pathways are thought to be responsible for the pattern of tracheal branches. We ask how these transduction pathways regulate cell migration and we analyse the con sequences on cell behaviour of the Dpp and EGF pathways. We find that,rhomb oid (rho) mutant embryos display defects not only in tracheal cell migratio n but also in tracheal cell invagination unveiling a new role for EGF signa lling in the formation of the tracheal system. These results indicate that the transduction path ways that control tracheal cell migration are active in different steps of tracheal formation, beginning at invagination. We dis cuss how the consecutive steps of tracheal morphogenesis might affect the f inal branching pattern.