A change incis-regulatory logic underlying obligate versus facultative muscle multinucleation in chordates

Abstract

AbstractVertebrates and tunicates are sister groups that share a common fusogenic factor, Myomaker (Mymk), that drives myoblast fusion and muscle multinucleation. Yet they are divergent in when and where they express Mymk. In vertebrates, all developing skeletal muscles express Mymk and are obligately multinucleated. In tunicates, Mymk is only expressed in post-metamorphic multinucleated muscles, but is absent from mononucleated larval muscles. In this study, we demonstrate thatcis-regulatory sequence differences in the promoter region ofMymkunderlie the different spatiotemporal patterns of its transcriptional activation in tunicates and vertebrates. While in vertebrates Myogenic Regulatory Factors (MRFs) like MyoD1 alone are required and sufficient forMymktranscription in all skeletal muscles, we show that transcription ofMymkin post-metamorphic muscles of the tunicateCionarequires the combinatorial activity of MRF/MyoD and Early B-Cell Factor (Ebf). This macroevolutionary difference appears to be encoded incis,likely due to the presence of a putative Ebf binding site adjacent to predicted MRF binding sites in theCiona Mymkpromoter. We further discuss howMymkand myoblast fusion might have been regulated in the last common ancestor of tunicates and vertebrates, for which we propose two models.