Cardiac competence of the head mesoderm fades concomitant with a shift towards the head skeletal muscle programme

The vertebrate head mesoderm provides the heart, the great vessels, smooth and most head skeletal muscle, and parts of the skull base. The ability to generate cardiac and smooth muscle is thought to be the evolutionary ground-state of the tissue, and initially the head mesoderm has cardiac competence throughout, even in the paraxial region that normally does not engage in cardiogenesis. How long this competence lasts, and what happens as cardiac competence fades, is not clear. Using a wide palette of marker genes in the chicken embryo, we show that the paraxial head mesoderm has the ability to respond to Bmp, a known cardiac inducer, for a long time. However, Bmp signals are interpreted differently at different time points. Bmp triggers cardiogenesis up to early head fold stages; the ability to upregulate smooth muscle markers is retained slightly longer. Notably, as cardiac competence fades, Bmp activates the head skeletal muscle programme instead.