The evolutionary origin of the central nervous system (CNS) is one of the major unresolved questions in evolutionary biology. It is assumed that the nervous system began to function as a net-like structure similar to that seen in modern cnidarians, and that subsequent neural centralization led to the development of an information processing system that connects various input and output systems. However, the early evolutionary process of how and why integrated systems developed in simple neural networks still largely remains to be explored.
In recent years, comparisons of the expression patterns of genes involved in bilaterian CNS development during embryonic and larval stages have revealed widely conserved genetic features. To understand the role of these genes in the early neural centralization, it is necessary to clarify their functions in the neural development of the lineage that diverged before the emergence of Bilateria. Using Cnidaria, the closest sister to all Bilateria, and we have been analyzing the developmental and cell physiological characteristics of their nervous systems. In this symposium, I present our latest findings on the developmental control mechanisms and functions of the Pharyngeal Nervous System (PhNS), which constitutes a subgroup of the neural network of the cnidarian Nematostella vectensis.
By using RNASeq data during regeneration of the oral region, and single-cell RNASeq data, we identified 7 types of PhNS neuronal clusters and 356 PhNS genes including the orthopedia homeobox (Otp) and neuropeptide RFamide. Inhibition of Otp function in Nematostella developing embryos resulted in reduced expression of 193 PhNS genes, including RFamide. Furthermore, we found that the uptake of food (rotifers) was reduced in Otp-deficient Nematostella polyps, which was recovered by the addition of synthetic RFamide peptide, indicating that PhNS is necessary for controlling feeding behavior. Otp is required for the development of hypothalamus in the vertebrate CNS. Furthermore, hypothalamic neurons expressing RFamide-type neuropeptides are known to play a role in controlling feeding behavior. The above findings obtained in the Nematostella indicate that the cnidarian PhNS may have evolutionarily conserved characteristics at the molecular level as a feeding control center.