The genomics of immortality in Hydra oligactis

The only animals repeatedly and extensively investigated and found to entirely lack an ageing phenotype, defined as a low, non-increasing mortality rate and a non-decreasing reproductive rate over time, are from the freshwater cnidarian genus, Hydra. This animal is composed of three multipotent stem cell populations, endodermal and ectodermal epithelial stem cells and interstitial stem cells which produce neurons, gland cells, and prey capture cells called nematocytes. Calculations from observed constant laboratory mortality rates indicate that 5% of an original Hydra population would still be alive after 3,500 years. Under benign conditions, laboratory Hydra species typically reproduce by asexual budding. However, in some strains of the species H. oligactis transferring animals from their typical laboratory rearing temperature of 18oC to 10oC causes budding to cease, gonads to form, containing gametes derived from interstitial stem cells, and ageing to begin. Signs of ageing, affecting all stem cell lineages, include general organ atrophy, reduced prey capture ability, slowed spontaneous contractions of the body wall, decreasing gamete production after the initial reproductive burst, disorganization of actin fibers in body wall cells, and an exponentially increasing mortality rate. To understand the nature of the switches responsible for this dramatic transformation from a non-ageing to an ageing phenotype, we sequenced the transcriptome of a male strain of H. oligactis during its non-ageing and ageing phases.