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.