Miniature Jellyfish Reverses Its Life Cycle, and Scientists Are Mapping How Its Genes Make That Possible

In the Mediterranean, fleets of translucent umbrellas, each about the size of a lentil, drift through the water. These are Turritopsis dohrnii, miniature jellyfish that wave and grasp with pale tentacles and bring plankton to their mouths, much as many other jellyfish do. What sets them apart is what happens when an adult, called a medusa, is damaged.

Instead of dying, the injured medusa can turn back the clock. It sheds its limbs, becomes a drifting blob, and morphs into a polyp, a twiggy growth that attaches to rocks or plants. Eventually the polyp buds off a medusa again, rejuvenated. A predator or an injury can kill T. dohrnii, but old age does not, which makes the animal effectively immortal.

A paper published on a Monday in the Proceedings of the National Academy of Sciences now takes a detailed look at the jellyfish's genome, searching for the genes that run this process. By examining which genes were active at different phases of the life cycle, the researchers caught a glimpse of how the rejuvenation is orchestrated.

Getting enough animals to study is hard. Only one scientist, Shin Kubota at Kyoto University in Japan, has kept a colony alive in the lab over the long term; he has also written and performed songs inspired by the jellyfish. Maria Pascual-Torner, a scientist at Universidad de Oviedo in Spain, says they are very picky in an aquarium, and so small that they are hard to identify and sample in the field. To collect wild animals for this work, she and a colleague drove a specially equipped camper van to a coast in Italy, dove for jellyfish, and rushed them back to the lab.

When the team sequenced the genomes, they found extra copies of certain genes, a sign those genes may matter for survival. Among the duplicated genes were some that protect and repair DNA, which tends to erode with age in animals.

To trigger rejuvenation, the researchers stressed the jellyfish, among other methods by letting them go hungry. As medusas shrank into little balls, sprouted polyps and began rebuilding adult bodies, the team sampled animals at each phase, froze them and ground them to mush to extract mRNA, a record of which genes were actively making proteins.

The pattern was marked. Genes linked to DNA storage were highly active in adults, then grew quieter as the animals sank back toward the polyp stage, reaching their lowest protein levels in the floating ball form. Genes tied to pluripotency, a cell's ability to develop into many fully formed types, did the opposite: quiet in adults, surging as the body broke down and rebuilt, then dormant again once the process finished. Pascual-Torner said this suggests normally stored DNA is brought out during the transformation, while genes that coax cells to reset go into overdrive.

Maria Miglietta, a marine biology professor at Texas A&M University at Galveston who also studies the species, said the results corroborate her group's study from last year, which found genes for DNA repair and protection involved in rejuvenation. Together, the work suggests that when and how much genes are expressed matters as much as the genes themselves. There is no gene for immortality, but there is a procedure for it.

Researchers hope to learn more, including whether the jellyfish could still start over if the storage proteins were tweaked to stay active. Applying any of this to people is unlikely. Pascual-Torner said the goal is not a formula for human immortality, since jellyfish differ greatly from humans and the effect comes from a whole mechanism working together. Whether any of it has a human parallel remains an open question.