Hydra Module

Hydra is a freshwater cnidarian polyp, easily reared in the lab, which was identified by Abraham Trembley (1744) as the first animal able to fully regenerate any missing part of its body. Trembley also found that Hydra is a carnivorous animal that actively catches its food, walks and reacts to physical stimulation by contracting its body. Since then Hydra proved to be a powerful model system to investigate the molecular and cellular basis of stem cell biology, asexual reproduction (budding), body regeneration and low senescence in a non-bilaterian context. One major discovery was made by Ethel Browne (1909) who used pigmented and unpigmented animals to show that restricted regions in intact or budding or regenerating animals, nowadays named organizers, induce ectopic axis formation upon transplantation by recruiting cells from the host. Organizers were later evidenced in developing chordates such as the gastrula organizer (Spemann and Mangold, 1924 – Nobel Prize 1935). The cell biology is equally fascinating in these animals as three continuously cycling stem cell populations exhibit an unusual cell cycling behavior and a tight spatial regulation. Epithelial stem cells provide a high resistance to starvation and stress, while interstitial stem cells allow de novo neurogenesis and gametogenesis whatever the age of the animal. Modellers of developmental processes were inspired by Hydra to propose as general model of patterning the reaction-diffusion model (Turing, 1952, Meinhardt and Gierer, 1972) or the French flag model (Wolpert, 1969). Over the past 15 years, beside omics data, functional genomics were made possible with stable transgenic lines, gene silencing through RNAi and Crispr-Cas9 strategies.

In this module, students will observe the natural behavior and developmental processes in intact and bisected Hydra, identify the aging phenotype developed by some strains, follow the autophagy flux with the help of an autophagy sensor. They will also use transgenic lines to perform transplantation experiments and evidence the activity of organizers, to produce reaggregates– natural organoids – that undergo regeneration, to monitor through live imaging the wound healing process when ROS production is altered. Although the constraints of a short module will not allow sufficient time to perform gene silencing experiments, the students will have the opportunity to be creative in their experimental design to investigate the amazing developmental plasticity of this animal.