TY - JOUR T1 - Cell shape changes during larval body plan development in <em>Clytia hemisphaerica</em> JF - bioRxiv DO - 10.1101/864223 SP - 864223 AU - Yulia Kraus AU - Sandra Chevalier AU - Evelyn Houliston Y1 - 2019/01/01 UR - http://biorxiv.org/content/early/2019/12/03/864223.abstract N2 - The cnidarian “planula” larva shows radial symmetry around a polarized, oral-aboral, body axis and comprises two epithelia cell layers, ectodermal and endodermal. This simple body plan is set up during gastrulation, a process which proceeds by a variety of modes amongst the diverse cnidarian species. In the hydrozoan laboratory model Clytia hemisphaerica, gastrulation involves a process termed unipolar cell ingression, in which the endoderm derives from mass epithelial-mesenchymal transition (EMT) at the future oral pole of a monoepithelial blastula. This contrasts markedly from the gastrulation mode in the anthozoan cnidarian Nematostella vectensis, in which endoderm formation primarily relies on cell sheet invagination. To understand the cellular basis of gastrulation in Clytia we have characterized in detail successive cell morphology changes during planula formation by Scanning and Transmission Electron Microscopy combined with confocal imaging. These changes accompany epithialization of the blastoderm, bottle cell formation followed by EMT in the oral domain, cohesive migration and intercalation of ingressed mesenchymal cells, and re-epithelialization of the endoderm. From our data we have reconstructed the cascade of morphogenetic events leading to the formation of planula larva. We also matched the domains of cell morphology changes to the expression of selected regulatory and marker genes expressed during gastrulation. We propose that cell ingression in Clytia not only provides the endoderm, but generates internal forces that shape the embryo in the course of gastrulation. These observations help build a more complete understanding of the cellular basis of morphogenesis and of the evolutionary plasticity of cnidarian gastrulation modes. ER -