Dynamic expression and differential requirement of the myocyte fusogen Myomixer during distinct myogenic episodes in the zebrafish

Skeletal muscle formation involves elongation and fusion of myocytes into aligned, multinucleated myofibres. These fibres continue to grow by addition of new myonuclei through reiterative rounds of myocyte fusion to support muscle growth, repair and regeneration over the organism's life span. The vertebrate-specific myocyte fusogens, Myomaker (Mymk) and Myomixer (Mymx), are crucial for generating multinucleated skeletal muscles. While the role of the transmembrane protein Mymk in myocyte fusion is well established, expression dynamics of mymx and role of the Mymx micropeptide, is less understood. Here, using quantitative imaging and a mymx knockout strain, we have explored the impact of on skeletal muscle formation at different life stages of the zebrafish. We demonstrate that during the initial phase of myogenesis, mymx has a spatiotemporally varying expression across all axes of the developing myotome. On Mymx loss, myotome morphogenesis is disrupted, with both cell and tissue structure impacted. Moreover, we could show differential effects of Mymk versus Mymx loss on myocyte fusion and muscle growth. Finally, we report that perturbation to adult muscle size and strength impacts bone development, again with different phenotypic severities among the two fusogen mutants. Together, our work provides key insights into the interplay between myocyte fusion, myotome morphogenesis and acquisition of the final adult form.