Abstract

Regardless of major differences in morphology and embryogenesis, modern Cnidaria and Bilateria share remarkable similarities in the molecular mechanisms controlling the development of their body axes. An essential factor implicated in setting-up the primary body axis in Metazoa is canonical Wnt signalling, which has been shown to be active both at the oral pole of Cnidaria and the posterior pole of Bilateria. Cnidarians, the likely sister group to Bilateria, hold an informative phylogenetic position for investigating metazoan evolution, and the sea anemone Nematostella vectensis has recently become an established model for developmental studies. Despite its simple external morphology, molecularly distinct territories have been identified along the oral-aboral axis, the primary body axis of the Nematostella larva. Investigating the establishment and the patterning of this axis during development, we could demonstrate the existence of a conserved module for aboral development involving orthologs of bilaterian anterior marker genes, including the transcription factor NvSix3/6 and the Wnt receptor NvFrizzled5/8. We showed also that the initial role of Wnt/β-catenin signalling in establishing the primary axis differs between Nematostella and other studied metazoan models, unveiling a previously uncharacterized mechanism for early patterning of the primary embryonic axis. Taken together, these data contribute to our understanding of the mechanisms responsible for the establishment and the progressive patterning of body axes in metazoans, shedding also new light on their evolution.