The elbow (elB) gene encodes a conserved nuclear protein with a single zinc finger. Expression of ElB

The Drosophila tracheal system is a stereotypical network of interconnected tubes that supplies air to all cells of the organism. Initially, ten tracheal placodes are defined on both sides of the embryo, each consisting of 20 cells. The placodes undergo two rounds of division, giving rise to the final number of tracheal cells. All subsequent events of tracheal morphogenesis and branch migration occur in the absence of any further cell division (reviewed by Manning and Krasnow, 1993). The final structure of the tracheal tree is elaborate. Each tracheal pit gives rise to five different branches: dorsal branch (DB), dorsal trunk (DT), visceral branch (VB), lateral trunk anterior (LTa) and lateral posterior/ganglionic branch (LTp/GB). The number of cells allocated to each branch is fixed and the final structure of each branch is stereotyped, reflecting established migration routes. Within each branch, different cell types are formed from an originally equipotent population of tracheal cells (Samakovlis et al., 1996a). The cells at the termini of the branches differentiate as terminal cells that send long hollow extensions to hypoxic tissues (Guillemin et al., 1996). Another group of specialized cells, termed fusion cells, establishes connections between branches from adjacent segments (Samakovlis et al., 1996b; TanakaMatakatsu et al., 1996). This elaborate tracheal structure is set up by the concerted activity of multiple signaling pathways, uncovered in the past decade (reviewed by Affolter and Shilo, 2000; Zelzer and Shilo, 2000b). The initial assignment of tracheal fates within the population of ectodermal cells is driven by the localized expression of the Trachealess and Drifter transcription factors (Anderson et al., 1995; Wilk et al., 1996; Llimargas and Casanova, 1997; Zelzer and Shilo, 2000a). Persistent expression of these genes in the trachea provides a ‘cell context’ for other signals that impinge on the trachea. Prior to the onset of tracheal migration, the precise number of cells must be allocated to each future branch. Several signaling pathways contribute to this decision, and in many cases parallel inputs from different pathways are responsible for the assignment of a particular branch fate (Wappner et al., 1997; Vincent et al., 1998; Llimargas, 2000; Llimargas and Lawrence, 2001; Chihara and Hayashi, 2000; Glazer and Shilo, 2001). The process of migration is guided by the FGF pathway. All tracheal cells express the FGF receptor, Breathless (Btl) 3585 Development 129, 3585-3596 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV7944