Ancestral neuronal receptors are bacterial accessory toxins

Symbiosis and horizontal gene transfer events were crucial in the emergence of eukaryotic and multicellular life. A striking example is the acquisition of Teneurins, putative surface-exposed toxins in bacteria that function as cell adhesion receptors in metazoan neuronal development. Here, we demonstrate the evolutionary relationships between metazoan and bacterial Teneurins. We show that bacterial Teneurins are accessory proteins, that harbour diverse encapsulated toxins. They predominate in species with complex social behaviours, suggesting roles in both cell-to-cell competition and cooperation. Metazoan Teneurins are therefore repurposed bacterial toxins central to cell-to-cell communication in nervous systems. Their acquisition was a foundational event in the evolution of metazoans.