Networks of epithelial and endothelial tubes are essential for the function of organs such as the lung, kidney, and vascular system. The sizes and shapes of these tubes are highly regulated to match their individual functions. Defects in tube size can cause debilitating diseases such as polycystic kidney disease (PKD) and ischemia1,2. It is therefore critical to understand how tube dimensions are regulated. Here we identify the tyrosine kinase Src as an instructive regulator of epithelial tube length in the Drosophila tracheal system. Loss-of-function Src42 mutations shorten tracheal tubes while Src42 over-expression elongates them. Surprisingly, Src42 acts distinctly from known tube size pathways and regulates both the amount of apical surface growth and, with the conserved formin dDaam, the direction of growth. Quantitative 3-D image analysis reveals that Src42 and dDaam mutant tracheal cells expand more in the circumferential than the axial dimension, resulting in tubes that are shorter in length – but larger in diameter – than WT tubes. Thus, Src42 and dDaam control tube dimensions by regulating the direction of anisotropic growth, a mechanism that has not previously been described.

During muscle development, myosin and actin containing filaments assemble into the highly organized sarcomeric structure critical for muscle function. Although sarcomerogenesis clearly involves the de novo formation of actin filaments, this process remained poorly understood. Here we show that mouse and Drosophila members of the DAAM formin family are sarcomere-associated actin assembly factors enriched at the Z-disc and M-band. Analysis of dDAAM mutants revealed a pivotal role in myofibrillogenesis of larval somatic muscles, indirect flight muscles and the heart. We found that loss of dDAAM function results in multiple defects in sarcomere development including thin and thick filament disorganization, Z-disc and M-band formation, and a near complete absence of the myofibrillar lattice. Collectively, our data suggest that dDAAM is required for the initial assembly of thin filaments, and subsequently it promotes filament elongation by assembling short actin polymers that anneal to the pointed end of the growing filaments, and by antagonizing the capping protein Tropomodulin.