Ssdp influences Wg expression and embryonic somatic muscle identity in Drosophila melanogaster

The somatic muscles of the Drosophila embryo and larvae share structural and functional similarities with vertebrate skeletal muscles and serve as a powerful model for studying muscle development. Here we show that the evolutionarily conserved Ssdp protein is required for the correct patterning of somatic muscles. Ssdp is part of the conserved Chi/LDB-Ssdp (ChiLS) complex that is a core component of the conserved Wg/Wnt enhanceosome, which responds to Wg signals to regulate gene transcription. Ssdp shows isoform specific expression in developing somatic muscles and its loss of function leads to an aberrant somatic muscle pattern due to a deregulated muscle identity program. Ssdp mutant embryos fail to maintain adequate expression levels of muscle identity transcription factors and this results in aberrant muscle morphology, innervation, attachment and fusion. We also show that the epidermal expression of Wg is downregulated in Ssdp mutants and that Ssdp interacts with Wg to regulate the properties of a subset of ventral muscles. Thus, our data unveil the dual contribution of Ssdp contribution to muscle diversification by regulating the expression of muscle-intrinsic identity genes and by interacting with the extrinsic factor, Wg. The knowledge gained here about Ssdp and its interaction with Wg could be relevant to vertebrate muscle development.