Many of the signaling pathways shared between vertebrates and invertebrates were present in the eumetazoan, the ancestor of all multicellular animals, as evidenced by their presence in the sea anemone Nematostella vectensis. Fibroblast growth factor (FGF) signaling performs varied functions throughout an animal’s lifetime and is required during early embryogenesis for gastrulation and neural cell fate specification, including the specification of sensory structures. Rentzsch et al. suggest that sensory organ specification was one of its ancestral roles. Nematostella has only one larval sensory organ, the apical organ, which forms at the aboral pole of the embryo (opposite the mouth) and allows the animal to find a suitable place for settlement and metamorphosis. Expression analysis revealed that genes encoding two FGF ligands, NvFGFa1 and NvFGFa2, and one encoding a FGF receptor, NvFGFRa, were expressed at the embryonic apical pole. Knockdown with morpholino oligonucleotides of either NvFGFa1 or NvFGFRa produced larvae that lacked an apical organ and that were able to swim but failed to settle and metamorphose. Conversely, knockdown of NvFGFa2 generated larvae with expanded apical organs. Signaling by NvFGFa1 was required to maintain expression of both NvFGFa1 and NvFGFa2, thereby establishing both positive and negative feedback mechanisms. Pharmacological experiments suggested that NvFGFa1 signaled through a mitogen-activated protein kinase (MAPK) cascade. The region of the embryo in which NvFGFa1 and NvFGFRa were expressed expanded in the absence of NvFGFa2. Thus, two FGFs antagonize one another to induce the apical organ and at least NvFGFa1 signals through a pathway that is very much like the canonical FGF pathway of more complex animals. Although sea anemones do not have a centralized nervous system, the larval sensory organ is induced by the same pathway that contributes to neural cell fating and sensory organ induction in more complex nervous systems.
F. Rentzsch, J. H. Fritzenwanker, C. B. Scholz, U. Technau, FGF signalling controls formation of the apical sensory organ in the cnidarian Nematostella vectensis. Development 135, 1761-1769 (2008). [Abstract] [Full Text]