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

Skeletal muscle formation involves the fusion of myocytes into precisely aligned, multinucleated myofibres. These fibres continue to grow through reiterative rounds of myocyte fusion, incorporating new myonuclei and supporting muscle growth, repair and regeneration over organismal life span. The vertebrate-specific myocyte fusogens, Myomaker (Mymk) and Myomixer (Mymx), are crucial for generating multinucleated skeletal muscles. Here, using quantitative imaging and a mymx knockout strain, we explored the impact on myogenesis at different life stages of the zebrafish. We demonstrate that during the initial phase of muscle formation, mymx has a spatiotemporally varied expression across all axes of the developing myotome, not just along the anterior-posterior axis. On Mymx loss, myotome morphogenesis is disrupted, with both cell and tissue structure impacted. In particular, the shape of the resulting myotome segments is altered. Moreover, we show differential effects of Mymk versus Mymx loss on myocyte fusion and muscle growth. Finally, we report that perturbation to adult muscle multinucleation and size impacted bone development, again with different phenotypic severities among the two fusogen mutants. Together, our work provides insights into the interplay between myocyte fusion, myotome morphogenesis and acquisition of final adult form.