ABSTRACT
The two somite compartments, dorso-lateral dermomyotome and medio-ventral sclerotome are major vertebrate novelties, but little is known about their evolutionary origin. We determined that sclerotome cells in Xenopus come from lateral somitic frontier (LSF) by lineage tracing, ablation experiments and histological analysis. We identified Twist1 as marker of migrating sclerotome progenitors in two amphibians, Xenopus and axolotl. From these results, three conclusions can be drawn. First, LSF is made up of multipotent somitic cells (MSCs) since LSF gives rise to sclerotome but also to dermomytome as already shown in Xenopus. Second, the basic scheme of somite compartmentalization is conserved from cephalochordates to anamniotes since in both cases, lateral cells envelop dorsally and ventrally the ancestral myotome, suggesting that lateral MSCs should already exist in cephalochordates. Third, the transition from anamniote to amniote vertebrates is characterized by extension of the MSCs domain to the entire somite at the expense of ancestral myotome since amniote somite is a naive tissue that subdivides into sclerotome and dermomyotome. Like neural crest pluripotent cells, MSCs are at the origin of major vertebrate novelties, namely hypaxial region of the somite, dermomyotome and sclerotome compartments. Hence, change in MSCs properties and location is involved in somite evolution.
