INTRODUCTION 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; Tanaka- Matakatsu 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 The elbow (elB) gene encodes a conserved nuclear protein with a single zinc finger. Expression of ElB is restricted to a specific subset of tracheal cells, namely the dorsal branch and the lateral trunks. Stalled or aberrant migration of these branches is observed in elB mutant embryos. Conversely, ElB misexpression in the trachea gave rise to absence of the visceral branch and an increase in the number of cells forming the dorsal branch. These results imply that the restricted expression of ElB contributes to the specification of distinct branch fates, as reflected in their stereotypic pattern of migration. As elB loss-of- function tracheal phenotypes are reminiscent of defects in Dpp signaling, the relationship between ElB and the Dpp pathway was examined. By using pMad antibodies that detect the activation pattern of the Dpp pathway, we show that Dpp signaling in the trachea is not impaired in elB mutants. In addition, expression of the Dpp target gene kni was unaltered. The opposite is true as well, because expression of elB is independent of Dpp signaling. ElB thus defines a parallel input, which determines the identity of the lateral trunk and dorsal branch cells. No ocelli (Noc) is the Drosophila protein most similar to ElB. Mutations in noc give rise to a similar tracheal phenotype. Noc is capable of associating with ElB, suggesting that they can function as a heterodimer. ElB also associates with the Groucho protein, indicating that the complex has the capacity to repress transcription of target genes. Indeed, in elB or noc mutants, expanded expression of tracheal branch-specific genes was observed. Key words: Elbow, Noc, Zinc finger, Trachea, Branch migration, Groucho, Drosophila SUMMARY Elbow and Noc define a family of zinc finger proteins controlling morphogenesis of specific tracheal branches Ruslan Dorfman 1 , Lillian Glazer 1 , Ulrich Weihe 2 , Mathias F. Wernet 3 and Ben-Zion Shilo 1, * 1 Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel 2 EMBL, Heidelberg, Germany 3 Department of Biology, New York University, New York, USA *Author for correspondence (e-mail: benny.shilo@weizmann.ac.il) Accepted 13 May 2002