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 embryonic segments. The cells of each cluster invaginate and subsequent formation of the tracheal tree occurs by cell migration and fusion of tracheal branches, 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 consequences on cell behaviour of the Dpp and EGF pathways. We find that rhomboid (rho) mutant embryos display defects not only in tracheal cell migration but also in tracheal cell invagination unveiling a new role for EGF signalling in the formation of the tracheal system. These results indicate that the transduction pathways that control tracheal cell migration are active in different steps of tracheal formation, beginning at invagination. We discuss how the consecutive steps of tracheal morphogenesis might affect the final branching pattern.