Downstream of identity genes: dCryAB and Gelsolin control discrete growth-related properties of muscle subsets in Drosophila

A combinatorial code of identity transcription factors (iTFs) specifies the diversity of muscle types in Drosophila. However, how individual muscle properties are acquired downstream of the iTF code remains elusive. Here, we applied a cell type specific translational profiling (TRAP) to identify gene expression signatures underlying muscle identity in the Drosophila embryo. We found that dCryAB and Gelsolin (Gel), both encoding actin-interacting proteins, displayed lateral transverse (LT) muscle prevailing expression positively regulated by, the LT iTFs. Loss of dCryAB function resulted in LTs with irregular shape and occasional forked/branched ends associated with affected LTs innervation. In contrast, enlarged and then split LTs with a greater number of myonuclei formed in Gel mutants while Gel gain of function resulted in unfused myoblasts, collectively indicating that Gel regulates LTs size by limiting myoblast fusion. Thus, dCryAB and Gel act as muscle identity realisator genes setting discrete growth-related properties of LT muscles and respectively contribute to preventing their branching and splitting. Our findings offer clues to still unknown mechanisms of pathological muscle fiber branching and splitting in vertebrates.