Hydra, a fruitful model system for 270 years.

The discovery of Hydra regeneration by Abraham Trembley in 1744 promoted much scientific curiosity thanks to his clever design of experimental strategies away from the natural environment. Since then, this little freshwater cnidarian polyp flourished as a potent and fruitful model system. Here, we review some general biological questions that benefitted from Hydra research, such as the nature of embryogenesis, neurogenesis, induction by organizers, sex reversal, symbiosis, aging, feeding behavior, light regulation, multipotency of somatic stem cells, temperature-induced cell death, neuronal transdifferentiation, to cite only a few. To understand how phenotypes arise, theoricists also chose Hydra to model patterning and morphogenetic events, providing helpful concepts such as reaction-diffusion, positional information, and autocatalysis combined with lateral inhibition. Indeed, throughout these past 270 years, scientists used transplantation and grafting experiments, together with tissue, cell and molecular labelings, as well as biochemical procedures, in order to establish the solid foundations of cell and developmental biology. Nowadays, thanks to transgenic, genomic and proteomic tools, Hydra remains a promising model for these fields, but also for addressing novel questions such as evolutionary mechanisms, maintenance of dynamic homeostasis, regulation of stemness, functions of autophagy, cell death, stress response, innate immunity, bioactive compounds in ecosystems, ecotoxicant sensing and science communication.