Functional definition of the Drosophila airway progenitor field through overlapping compensatory regulators

Tubular organs present a common solution to fluid transport in multicellular organisms. They often arise by an initial bulging of flat epithelial progenitor cells, which then undergo branching morphogenesis. Here, we present 3 cooperative programs fully defining the Drosophila airway progenitor field and their roles in early morphogenesis linking the radial pattern of the 2-dimensional (2D) field to the proximo-distally patterning of the 3D tubes. We previously showed that extrinsic Hedgehog (Hh) and intrinsic POU-Homeobox TF Ventral-veinless (Vvl)/Drifter/U-turn dominantly drive the transcriptional program toward the distal airway cell identity at the expense of a proximal program specified by the GATA TF grain (grn). Both programs require the basic-HLH-POU TF trachealess (trh) (Matsuda et. al, 2015). Whereas trh is not essential for primordia invagination, we show that in hh vvl double mutants, the oval-shaped primordia frequently remain at the 2D plane, retaining trh expression in a grn dependent manner. Therefore, hh and vvl are the principal regulators of progenitor invagination independent of trh. Each of the 3 regulators, Trh, Vvl and Grn fulfills only complementary or compensatory functions in transcription and morphogenesis but their combinations functionally define the airway progenitor field. We further provide a comprehensive description for allocating the airway progenitors on the body coordinates, involving dorsal Decapentaplegic/BMP signaling along the dorso-ventral axis and subsequent radial EGFR signaling along the proximo-distal axis. The presence of 3 complementary, regulatory programs in early gene expression and morphogenesis of the simple Drosophila airways may reflect the vital needs for respiration, and their influence on the evolution of various strategies in tubular organ development.