Coordinating cell fate and morphogenesis in Drosophila renal tubules.

Using the renal tubules of Drosophila as an example, we explore how cell specification leads to the morphogenetic movements that underlie the generation of tissue architecture. Taking two stages of development, we show first that the tubule cells are allocated by signalling between the endodermal and ectodermal compartments of the posterior gut. Activation of the Wnt pathway patterns the ectodermal anlage, resulting in the expression of tubule genes in a subset of cells and their eversion from the hindgut to form the tubule primordia. We argue that early gene expression directs these morphogenetic movements but not the complete programme of tubule differentiation. In the second example we show that the allocation of the mitogenic tip cell lineage in each tubule is required not only for the normal pattern of cell division but also for the stereotyped three-dimensional arrangement of the mature tubules. Analysis of mutants in which the tip cell lineage is misspecified reveals that both daughters of the tip cell progenitor are required for the tubules to navigate through the body cavity, so that the distal tips locate in their characteristic positions. We show that the regulator of Rac, Myoblast city is essential for this second morphogenetic process.