Fig. 5

Vertebrate head mesoderm evolved through polarization of ancestral mesodermal patterning. a In amphioxus and vertebrates, the dorsal mesoderm has two groups of genes that encode positional value (Group 1, orange; Group 2, light blue). At stage I (early-to mid-gastrula stages), group 1 and group 2 are co-expressed both in amphioxus and vertebrate dorsal mesoderm. At stage II (late-gastrula stage), group 1 genes shift anteriorly and group 2 genes shift posteriorly in vertebrates, whereas the amphioxus mesoderm retains the overlapped condition. Numbers indicate positional value of the dorsal mesoderm along the A/P axis. c chordate, a amphioxus, v vertebrates. b The dorsal mesoderm of vertebrates evolved by the reorganization of mesodermal genes under the control of Wnt/β-catenin-signalling input. Notch/Delta signalling regulates somitogenesis both in amphioxus and vertebrates. s somite, nt notochord, tb tail bud, psm presomitic mesoderm, cnh chordoneural hinge, m mesenchyme, hm head mesoderm. hairy genes are key components that regulate somite formation during development [16]. pitx2 and en2 are expressed in head mesoderm of vertebrate embryos [10]