In vivo replication of a fully functional nerve net in a clonally reproducing animal

Nerve nets are considered the evolutionarily oldest form of nervous system. How such networks develop and how complex nervous systems become fully functional is largely unknown. In a phylogenetically ancient animal, the freshwater polyp Hydra, we demonstrate how a nerve net consisting of distinct neuronal populations is precisely replicated over the course of asexual proliferation by budding. During this process, precursor cells first accumulate at the site of the emerging neural network, followed by a stepwise formation of functional neural circuits composed of distinct neuronal populations. These circuits become arranged according to an accurately controlled spatiotemporal pattern and take control of behaviors independently of the existing parental network. Since similar processes also govern the formation of more complex nervous systems in other invertebrates and vertebrates, we suggest that the basic requirements for a neuronal network to replicate were already in place at the beginning of the evolution of the nervous system.