Cardiac neural crest is dispensable for outflow tract septation in Xenopus

In vertebrate embryos, cardiac precursor cells of the primary heart field give rise to the atria and the left ventricle. The right ventricle and the outflow tract are later evolutionary innovations derived from an adjacent population of precursors known as the second heart field. In addition, the cardiac neural crest contributes cells to the septum of the outflow tract to separate the systemic and the pulmonary circulations. The amphibian heart lacks a right ventricle but has a rudimentary outflow tract and a spiral septum, however it is unknown whether the cardiac neural crest is also involved in outflow tract septation as seen in amniotes. Using a combination of tissue transplantations and molecular analyses in Xenopus we show that the amphibian outflow tract is derived from a second heart field equivalent to the one described in birds and mammals. However, in contrast to what we see in amniotes it is the second heart field and not the cardiac neural crest that forms the septum of the amphibian outflow tract. In Xenopus cardiac neural crest cells remain confined to the aortic sac and arch arteries and never populate the outflow tract cushions. This significant difference suggests that cardiac neural crest cell migration into the cardiac cushions is an amniote‐specific characteristic, presumably acquired to increase the mass of the outflow tract septum with the evolutionary need for a fully divided circulation.