Gene regulatory network that shaped the evolution of larval sensory organ in Cnidaria

Among non-bilaterian animals, a larval apical sensory organ with integrated neurons is only found in cnidarians. Within cnidarians, an apical organ with a ciliary tuft is mainly found in Actiniaria. Whether this apical tuft has evolved independently in Actiniaria or alternatively originated in the common ancestor of Cnidaria and Bilateria and was lost in specific groups is uncertain. We generated transcriptomes of the apical domain during the planula stage of four species representing three key groups of cnidarians: Aurelia aurita (Scyphozoa), Nematostella vectensis (Actiniaria), and Acropora millepora & Acropora tenuis (Scleractinia). We showed that the canonical genes implicated in patterning the apical domain of Nematostella are largely absent in Aurelia, indicating that scyphozoans and anthozoans do not share apical organ homology. In contrast, the apical domain of the scleractinian planula shares gene expression pattern with Nematostella. By comparing the larval single-cell transcriptomes, we revealed the apical organ cell type of Scleractinia and confirmed its homology to Actiniaria. However, Fgfa2, a vital regulator of the regionalisation of the Nematostella apical organ, is absent in the scleractinian genome. Likewise, we found that FoxJ1 and 245 genes associated with cilia are exclusively expressed in the Nematostella apical domain, which is in line with the presence of ciliary apical tuft in Actiniaria and its absence in Scleractinia and Scyphozoa. Our findings suggest that the common ancestor of cnidarians and bilaterians lacked an apical organ with a ciliary tuft, and it could have evolved independently in several clades of cnidarians and bilaterians.