FIGURE 8.

(A) Summary of the main changes during the evolutionary history of somites compartmentalization. The last common ancestor of bilaterians, Urbilateria, possesses neither somites nor notochord, but probably transverse muscles and a medial mesodermal tissue according to the axochord hypothesis (Brunnet al, 2015; Yasuoka, 2020). Satellite-like cells and cartilage-like cells are also probably already present in Urbilateria. Regarding the transcription factors expressed in vertebrate somites, results from Drosophila and Xenopus suggest that Mef2 and Twist could act upstream of muscle identity genes in Urbilateria. The notochord and the somites appear in chordates. The somite is made up of the primitive myotome and probably multipotent progenitors which give rise to satellite cells and muscle-associated tissues ventrally and dorsally. The existence of sclerotome-like cells in cephalochordates suggests that the somitic progenitors can already give rise to specialized connective tissue cells. In vertebrate, the somites compartmentalize mainly into the myotome, the dermomyotome, and the sclerotome. In gnathostome vertebrates, the three populations of slow, lateral fast, and medial fast muscle fibers has been characterized. The genome possesses both Scleraxis and Tcf15 genes, but also four MRFs and four Mef2 genes. The non-conservation of Mef2 function in the paraxial mesoderm and the changes in compartmentalization mode between zebrafish and Xenopus raise the question of the origin of these variations. (B) Evolution of somite compartmentalization based on axochord hypothesis (Brunet et al., 2015; Yasuoka, 2020). The axochord hypothesis (the axochord in annelids and the notochord in chordate are homologs) proposes that the notochord evolves from a medial mesodermal tissue present in Urbilateria, the last common ancestor of all bilaterians, and suggests that transverse muscles attached to it, could give rise to the primitive myotome in ancestral chordates. The origin of MSCs in Urbilateria is unknown. Proto-MSCs probably already exist in last chordate ancestor. The transition from ancestral chordates to vertebrates allowed MSCs to give rise to all new somite structures, i.e., the dermomyotome, its hypaxial region, and the sclerotome. The transition from anamniote to amniote vertebrates is characterized by expansion of the MSCs domain at the expense of the primitive myotome. The chordate dorso-ventral axis is inverted compared with Urbilateria. Anamniote vertebrate is used in Figure 8B as the somite organization of the extant anamniote vertebrates are considered to be closed to the primitive one. VM, ventro-medial mesodermal tissue; TM, transverse muscle; M, medial somite region; L, lateral somite region.