Who Wants To Live Forever?
by Kiko Matsing
While colonial animals can have their immortality, solitary individuals are doomed to die. Hydrozoan cnidarians usually have a complex life cycle, wherein a colonial stage leads to the sexually mature, solitary, adult stage. Eggs and sperm from solitary, sexual, adult medusa (jellyfish) develop into an embryo and planula larva, and they then form the colonial polyp stage. Medusae are formed asexually from polyps. These medusae have a limited lifespan and die shortly after releasing their gametes.
The hydrozoan Turritopsis nutricula has evolved a remarkable variation on this theme, and in so doing appears to have achieved immortality. The solitary medusa of this species can revert to its polyp stage after becoming sexually mature (Bavestrello et al., 1992; Piraino et al., 1996). In the laboratory, 100% of these medusae regularly undergo this change. Thus, it is possible that organismic death does not occur in this species!
How does Turritopsis accomplish this feat? It can do this because it can alter the differentiated state of a cell, transforming it into another cell type. Such a phenomenon is called transdifferentiation, and it is usually seen only when parts of an organ regenerate. However, it appears to occur normally in the Turritopsis life cycle. In this transdifferentiation process, the medusa is transformed into the stolons and polyps of a hydroid colony…
Turritopsis nutricula is the first case in which a metazoan is capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. Thus, it appears that it has cheated death and is a potentially immortal, solitary metazoan.
(from the online companion to Developmental Biology, by Scott F. Gilbert)