The serine/threonine kinase Back seat driver prevents cell fusion to maintain cell identity

Cell fate specification is essential for every major event of embryogenesis, and subsequent cell maturation ensures individual cell types acquire specialized functions. The mechanisms that regulate cell fate specification have been studied exhaustively, and each technological advance in developmental biology ushers in a new era of studies aimed at uncovering the most fundamental processes by which cells acquire unique identities. What is less appreciated is that mechanisms are in place to ensure cell identity is maintained throughout the life of the organism. The body wall musculature in the Drosophila embryo is a well-established model to study cell fate specification, as each hemisegment in the embryo generates and maintains thirty muscles with distinct identities. Here we show the serine/threonine kinase Back seat driver (Bsd), which regulates muscle morphogenesis, also maintains cell identity. Once specified, the thirty body wall muscles fuse with mononucleate muscle precursors that lack a specific identity to form multinucleate striated muscles. Importantly, body wall muscles do not fuse with each other and thereby maintain distinct identities. We show that Bsd prevents inappropriate fusion among the thirty body wall muscles. Thus, the regulation of cell fusion is one mechanism that maintains cell identity.