FGF signalling controls formation of the apical sensory organ in the cnidarian Nematostella vectensis.

Fibroblast growth factor (FGF) signalling regulates essential developmental processes in vertebrates and invertebrates, but its role during early metazoan evolution remains obscure. Here, we analyse the function of FGF signalling in a non-bilaterian animal, the sea anemone Nematostella vectensis. We identified the complete set of FGF ligands and FGF receptors, of which two paralogous FGFs (NvFGFa1 and NvFGFa2) and one FGF receptor (NvFGFRa) are specifically coexpressed in the developing apical organ, a sensory structure located at the aboral pole of ciliated larvae from various phyla. Morpholino-mediated knockdown experiments reveal that NvFGFa1 and NvFGFRa are required for the formation of the apical organ, whereas NvFGFa2 counteracts NvFGFRa signalling to prevent precocious and ectopic apical organ development. Marker gene expression analysis shows that FGF signalling regulates local patterning in the aboral region. Furthermore, NvFGFa1 activates its own expression and that of the antagonistic NvFGFa2, thereby establishing positive- and negative-feedback loops. Finally, we show that loss of the apical organ upon NvFGFa1 knockdown blocks metamorphosis into polyps. We propose that the control of the development of sensory structures at the apical pole of ciliated larvae is an ancestral function of FGF signalling

The blastoporal organiser of a sea anemone

In 1924 Hilde Mangold and Hans Spemann transplanted the dorsal blastopore lip of an amphibian embryo to a host embryo's ventral side. This experiment revealed that the dorsal blastopore lip can act as an 'organiser' to induce a secondary body axis [1]. The organiser experiment has fueled research in vertebrate developmental biology until today [2,3]. While an organiser might have been present in the chordate ancestor [4], it is not clear how widespread the principle of the blastoporal organiser is and what its evolutionary roots are. Here, we examined the organising activity of different parts of embryos of the sea anemone Nematostella vectensis, a representative of the basal animal phylum Cnidaria, which has retained many ancestral traits. We show by transplantation of small parts of the gastrula embryo that the blastopore lip - but not tissue from other parts of the embryo - is able to act as an organiser and to induce the formation of a secondary body axis with high efficienc