The Drosophila dysfusion Basic Helix-Loop-Helix (bHLH)–PAS Gene Controls Tracheal Fusion and Levels of the Trachealess bHLH-PAS Protein

Abstract

The development of the mature insect trachea requires a complex series of cellular events, including tracheal cell specification, cell migration, tubule branching, and tubule fusion. Here we describe the identification of the Drosophila melanogaster dysfusion gene, which encodes a novel basic helix-loop-helix (bHLH)-PAS protein conserved between Caenorhabditis elegans, insects, and humans, and controls tracheal fusion events. The Dysfusion protein functions as a heterodimer with the Tango bHLH-PAS protein in vivo to form a putative DNA-binding complex. The dysfusion gene is expressed in a variety of embryonic cell types, including tracheal-fusion, leading-edge, foregut atrium cells, nervous system, hindgut, and anal pad cells. RNAi experiments indicate that dysfusion is required for dorsal branch, lateral trunk, and ganglionic branch fusion but not for fusion of the dorsal trunk. The escargot gene, which is also expressed in fusion cells and is required for tracheal fusion, precedes dysfusion expression. Analysis of escargot mutants indicates a complex pattern of dysfusion regulation, such that dysfusion expression is dependent on escargot in the dorsal and ganglionic branches but not the dorsal trunk. Early in tracheal development, the Trachealess bHLH-PAS protein is present at uniformly high levels in all tracheal cells, but since the levels of Dysfusion rise in wild-type fusion cells, the levels of Trachealess in fusion cells decline. The downregulation of Trachealess is dependent on dysfusion function. These results suggest the possibility that competitive interactions between basic helix-loop-helix-PAS proteins (Dysfusion, Trachealess, and possibly Similar) may be important for the proper development of the trachea.

ACKNOWLEDGMENTS

We thank Kim Kamdar for support of this project and JoAnne Powell-Coffman and Cam Patterson for useful discussions and generously providing the sequences of Dys subfamily proteins before publication. We also thank Dan Lau, Don McEwen, and Dan Kiehart for advice; Mark Krasnow and Robin Wharton for reagents; Volker Hartenstein with help in identifying Dys-positive cells; Bob Duronio for reading the manuscript; Brian Mitchell, who was instrumental in initially identifying dys; Scott Halbrook and Mukund Patel for advice on RNAi experiments; and Tony Perdue for help with microscopy. Additional Drosophila strains were obtained from the Bloomington Drosophila Stock Center.

Funding was provided by Syngenta Biotech, Inc., and a grant 5R37HD025251 from NICHD.