Hydra, the everlasting embryo, confronts aging.

Existing data imply that the cnidarian Hydra vulgaris does not undergo senescence. In contrast, the related species Hydra oligactis shows increased mortality and physiological deterioration following sexual reproduction. Hydra thus offers the chance to study a striking difference in lifespan in members of the same genus. Adult Hydra possess three well-characterized stem cell populations, one of which gives rise to both somatic cells and gametes. The lack of senescence in Hydra vulgaris raises the question of how these stem cell populations are maintained over long periods of time. Investigation of the roles in Hydra of proteins involved in cellular stress responses in other organisms should provide insight into this issue. Proteins of particular interest include the Hsp70 family proteins and the transcription factor FoxO.