Genetic regulation of amphioxus somitogenesis informs the evolution of the vertebrate head mesoderm

The evolution of vertebrates from an ancestral chordate was accompanied by the acquisition of a predatory lifestyle closely associated to the origin of a novel anterior structure, the highly specialized head. While the vertebrate head mesoderm is unsegmented, the paraxial mesoderm of the earliest divergent chordate clade, the cephalochordates (amphioxus), is fully segmented in somites. We have previously shown that fibroblast growth factor signalling controls the formation of the most anterior somites in amphioxus; therefore, unravelling the fibroblast growth factor signalling downstream effectors is of crucial importance to shed light on the evolutionary origin of vertebrate head muscles. By using a comparative RNA sequencing approach and genetic functional analyses, we show that several transcription factors, such as Six1/2, Pax3/7 and Zic, act in combination to ensure the formation of three different somite populations. Interestingly, these proteins are orthologous to key regulators of trunk, and not head, muscle formation in vertebrates. Contrary to prevailing thinking, our results suggest that the vertebrate head mesoderm is of visceral and not paraxial origin and support a multistep evolutionary scenario for the appearance of the unsegmented mesoderm of the vertebrates new ‘head’.The laboratory of H.E. was supported by the Centre national de la recherche scientifique and Agence nationale de la recherche (ANR) grant no. ANR-16-CE12-0008-01; S.B. was supported by the Institut Universitaire de France. D.A. holds a fellowship from Conicyt Becas Chile. J-L.G-S. was supported by the Spanish government (grant no. BFU2016-74961-P), the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant no. 740041) and the institutional grant Unidad de Excelencia María de Maeztu (no. MDM-2016-0687). I.M. was funded by an EMBO short-term fellowship (ASTF 377-2014) and by the Spanish Government with a Juan de la Cierva postdoctoral contract (JCI-2012-13495). H.E. and S.M. were supported by the ECOS-CONICYT C15S02 grant.Peer reviewe