Evolution of Somite Compartmentalization: A View From Xenopus

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Chordate Embryogenesis

Background:

• Somites are transient, segmented structures crucial for vertebrate axial musculoskeletal system development.
• Somite compartmentalization into dermomyotome, myotome, and sclerotome is a defining vertebrate feature.
• Understanding somite evolution requires comparative analysis across vertebrate and chordate lineages.

Purpose of the Study:

• To review and synthesize literature on somite compartmentalization, focusing on *Xenopus*.
• To compare somite development in anamniote and amniote vertebrates.
• To propose a model for the evolutionary history of somite compartmentalization from ancestral chordates to amniotes.

Main Methods:

• Literature review and synthesis of existing research on somite development and evolution.
• Comparative analysis of somite compartmentalization across diverse vertebrate groups.
• Development of a theoretical model for evolutionary transitions in somite organization.

Main Results:

• Ancestral chordate somites likely comprised a lateral compartment of multipotent somitic cells (MSCs) and a medial primitive myotome.
• The transition from ancestral chordates to vertebrates involved MSCs evolving potency, giving rise to dermomyome, hypaxial region, and sclerotome.
• The transition from anamniotes to amniotes may involve lateral MSC territory expansion, potentially aiding sclerotome formation.

Conclusions:

• Somite compartmentalization evolved through significant modifications of cell potency in embryonic progenitors.
• Two major evolutionary transitions shaped vertebrate somite organization: the origin of vertebrates and the divergence of anamniotes/amniotes.
• Understanding the plasticity of multipotent somitic cells (MSCs) is key to deciphering vertebrate evolutionary history.