The Hydra Holobiont: A Tale of Several Symbiotic Lineages

Abstract

The personal journey of one of us (TB) toward the realization that beneficial microbes are important began in summer 2000 when Jens Schröder, a dermatologist at Kiel University, on the occasion of the inauguration of the new chair of Zoology asked if Hydra were not a good model system to investigate the biochemistry of epithelial defenses. Nobody at that time would have anticipated that this triggered the development of a novel model system in comparative and evolutionary immunology. Up to that moment, Hydra was all for examining developmental mechanisms in an evolutionary context and to uncover basic principles of pattern formation and stem cell regulation. To think of immune reactions as equally important features of an animal did not come to our mind. That evolution of a simple multicellular animal such as Hydra means both invention of developmental pathways to shape and maintain a given body plan and also to protect this body all life long, and understanding that part of that context requires understanding the biotic and abiotic environment in which Hydra evolved turned out to be enlightening and exciting. This chapter will show just how much we know about host–microbe interactions in Hydra and what these findings mean in a more general context of holobiont research.