Extrinsic branching factors promote the elongation and migration of tubular organs. In the Drosophila tracheal system, Branchless/Fibroblast Growth Factor (FGF) stimulates the branching program by specifying tip cells that acquire motility and lead branch migration to a specific destination. Tip cells have two alternative cell fates: the terminal cell (TC), which elongates the cytoplasmic extension with intracellular lumen, and the fusion cell (FC), which mediates branch connections to form tubular networks. How Branchless/FGF controls this specification of cells with distinct shapes and behaviors is unknown. Here we report that this cell-type diversification involves the modulation of FGF signaling by the zinc-finger protein Escargot (Esg), which is expressed in the FC and is essential for its specification. The dorsal branch begins elongation with a pair of tip cells with high FGF signaling. When the branch tip reaches its final destination, one of the tip cells become an FC and expresses Esg. FCs and TCs differ in their response to FGF: TCs are attracted by FGF, while FCs are repelled. Esg suppresses ERK signaling in FCs to control this differential migratory behavior.
- Received December 18, 2015.
- Accepted October 4, 2016.