Toxin-like neuropeptides in the sea anemone Nematostella unravel recruitment from the nervous system to venom.

The sea anemone Nematostella vectensis (Anthozoa, Cnidaria) is a powerful model for characterizing the evolution of genes functioning in venom and nervous systems. Although venom has evolved independently numerous times in animals, the evolutionary origin of many toxins remains unknown. In this work, we pinpoint an ancestral gene giving rise to a new toxin and functionally characterize both genes in the same species. Thus, we report a case of protein recruitment from the cnidarian nervous to venom system. The ShK-like1 peptide has a ShKT cysteine motif, is lethal for fish larvae and packaged into nematocysts, the cnidarian venom-producing stinging capsules. Thus, ShK-like1 is a toxic venom component. Its paralog, ShK-like2, is a neuropeptide localized to neurons and is involved in development. Both peptides exhibit similarities in their functional activities: They provoke contraction in Nematostella polyps and are toxic to fish. Because ShK-like2 but not ShK-like1 is conserved throughout sea anemone phylogeny, we conclude that the two paralogs originated due to a Nematostella-specific duplication of a ShK-like2 ancestor, a neuropeptide-encoding gene, followed by diversification and partial functional specialization. ShK-like2 is represented by two gene isoforms controlled by alternative promoters conferring regulatory flexibility throughout development. Additionally, we characterized the expression patterns of four other peptides with structural similarities to studied venom components and revealed their unexpected neuronal localization. Thus, we employed genomics, transcriptomics, and functional approaches to reveal one venom component, five neuropeptides with two different cysteine motifs, and an evolutionary pathway from nervous to venom system in Cnidaria.

The Birth and Death of Toxins with Distinct Functions: A Case Study in the Sea Anemone Nematostella.

The cnidarian Nematostella vectensis has become an established lab model, providing unique opportunities for venom evolution research. The Nematostella venom system is multimodal: involving both nematocytes and ectodermal gland cells, which produce a toxin mixture whose composition changes throughout the life cycle. Additionally, their modes of interaction with predators and prey vary between eggs, larvae, and adults, which is likely shaped by the dynamics of the venom system. Nv1 is a major component of adult venom, with activity against arthropods (through specific inhibition of sodium channel inactivation) and fish. Nv1 is encoded by a cluster of at least 12 nearly identical genes that were proposed to be undergoing concerted evolution. Surprisingly, we found that Nematostella venom includes several Nv1 paralogs escaping a pattern of general concerted evolution, despite belonging to the Nv1-like family. Here, we show two of these new toxins, Nv4 and Nv5, are lethal for zebrafish larvae but harmless to arthropods, unlike Nv1. Furthermore, unlike Nv1, the newly identified toxins are expressed in early life stages. Using transgenesis and immunostaining, we demonstrate that Nv4 and Nv5 are localized to ectodermal gland cells in larvae. The evolution of Nv4 and Nv5 can be described either as neofunctionalization or as subfunctionalization. Additionally, the Nv1-like family includes several pseudogenes being an example of nonfunctionalization and venom evolution through birth-and-death mechanism. Our findings reveal the evolutionary history for a toxin radiation and point toward the ecological function of the novel toxins constituting a complex cnidarian venom.