Abstract
Collective migration of cohesive tissues is a fundamental process in morphogenesis, and is particularly well illustrated during gastrulation by the rapid and massive internalization of the mesoderm, which contrasts with the much more modest movements of the ectoderm. In the Xenopus embryo, the differences in morphogenetic capabilities of these two tissues can be traced down to the properties of individual cells, which, when studied in vitro, show opposite intrinsic organizations, cohesive for ectoderm, dispersive for mesoderm. Surprisingly, we find these seemingly deep differences can simply be accounted for by differences in Rock-dependent actomyosin contractility. We show that Rock inhibition is sufficient to rapidly unleash motility in the ectoderm and confer it with mesoderm-like properties. In the endogenous mesoderm, this motility is dependent on two RhoA negative regulators, the small GTPase Rnd1 and the RhoGAP Shirin/Dlc2/ArhGAP37. Both are absolutely essential for gastrulation. At the cellular and tissue level, the two regulators show overlapping yet distinct functions: They are both necessary and sufficient to confer motility, but Shirin tends to stimulate more dispersion, while Rnd1 tends to favour more compact collective migration, each able to contribute to a specific property of the migratory behaviour of the mesoderm.