Abstract
The vertebrate head and cardiac muscles share a common origin in the pharyngeal mesoderm, but the cellular and molecular details of the initial fate choices remain obscured by the complexity of vertebrate embryos. In ascidians, common progenitors of heart and atrial siphon muscle (ASM) undergo stereotyped asymmetric divisions to produce first heart precursors, second heart precursors and ASM precursors. The ASM‐specific transcription factor COE is necessary and sufficient to promote pharyngeal muscle specification and block the heart program. We found that a cross‐antagonism between Tbx1 and NK4/Nkx2‐5 activities acts upstream of COE and GATAa to direct pharyngeal muscle versus heart specification in separate ASM and second heart precursors, respectively.Using a genomics approach, we showed both the cardiac and pharyngeal muscle programs are primed in cardiopharyngeal progenitors. We show that, following fate restriction, ASM precursors produce the myofibers that differentiate, and Notch‐mediated lateral inhibition suppresses MyoD expression and promote “stemness” in stem‐like progenitors that produce most body wall muscles. Our analyses of the ascidian cardiopharyngeal mesoderm shed new light on the evolutionary and ontogenic origins of the second heart field and head muscle stem cells.
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