Sea anemone MACPF proteins demonstrate an evolutionary transitional state between venomous and developmental functions

Gene duplication is a major force driving evolutionary innovation. A classic example is generating new animal toxins via duplication of physiological protein-encoding genes and recruitment into venom. While this process drives the innovation of many animal venoms, reverse-recruitment of toxins into non-venomous cells remains unresolved. Using comparative genomics, we find members of the Membrane Attack Complex and Perforin Family (MACPF) have been recruited into venom-injecting cells (cnidocytes), in soft and stony corals and sea anemones, suggesting that the ancestral MACPF was a cnidocyte expressed toxin. Further investigation into the model sea anemone Nematostella vectensis, reveals that three members have undergone Nematostella -specific duplications leading to their reverse-recruitment into mesoendodermal cells. Furthermore, simultaneous knock-down of all three mesoendodermally-expressed MACPFs leads to mis-development, supporting that these paralogs have non-venomous function. By resolving the evolutionary history and function of MACPFs in Nematostella , we provide the first proof for reverse-recruitment from venom to organismal development. Significance statement In this study, we reveal how a gene can gain a new function, even from a most unexpected origin. Specifically, we report that in the last common ancestor of corals and sea anemones a member of the Membrane Attack Complex and Perforin Family (MACPF), which is commonly associated with the immune system, was recruited into venom-injecting cells called cnidocytes. Using the sea anemone Nematostella vectensis we find repeated gene duplication has occurred leading to the new copies adopting divergent functions including being retained in cnidocytes but also recruited into non-venomous mesoendodermal cells. Furthermore, when we deplete Nematostella of mesoendodermally-expressed MACPFs we disrupt normal embryonic development, supporting that these copies have indeed been recruited from venom into the developmental plan.