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Scientifically Speaking | The secret to living forever, according to jellyfish

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Animals are born and then they grow up, get old, and die. This is a rule that is maintained across species. But there seems to be a diminutive and translucent kind of jellyfish found in parts of the Mediterranean that seems to defy this time-tested rule. When faced with difficult circumstances, mature jellyfish of a certain species can turn back the clock on their bodies and reverse metamorphose into younger, rejuvenated forms known as polyps.

Scientists have been studying these jellyfish (that are known by the scientific name, Turritopsis dohrnii) for decades now and what they have found is truly remarkable. There seems to be no end to the number of times this reversal of ageing can occur in these animals, and the process does not seem to cause any harm or outward change in appearance after multiple cycles. In essence, these jellyfish have cracked the problem of ageing and have found a way to become immortal.

Understandably, there is interest in figuring out just how these jellyfish defy normal biology. We shouldn’t harbour the illusion that figuring out how these animals defeat death will allow us to do the same in people — after all, we are hundreds of millions of years removed evolutionarily from jellyfish and we likely don’t have all the same biological parts to achieve this feat. But knowing their secrets could lead to advances in tissue regeneration and wound healing in humans.

Towards this goal, researchers led by Maria Pascual-Torner and Víctor Quesada at the University of Oviedo in Spain mapped the entire genome of the “immortal jellyfish” and compared it to another related jellyfish species incapable of reversing to a younger state after maturation. They published their findings last month in the scientific journal, The Proceedings of the National Academy of Sciences.

Since the genome is the entire genetic blueprint of an organism, the researchers reasoned that the ability to turn back to younger states found in some jellyfish, but not in others, could be characterised by differences in their genes. This is sound logic.

The researchers found that there probably isn’t a single switch that confers immortality, but a plethora of processes that contribute (many in unknown ways) to this biological feat. Some of these processes, however, can be triggered in other animals, too, to slow down ageing, though perhaps not to the same extent.

Among the genes that are likely to help in rejuvenation in the “immortal jellyfish” are multiple copies of genes that give rise to proteins that protect and repair DNA. There are also mechanisms that the jellyfish has acquired that help them to protect the ends of chromosomes (known as telomeres) which can fray like the ends of shoelaces. There are also extra genes for factors that allow certain kinds of cells to change back to other kinds of cells. From a biological perspective, all of these would seem to be required for an organism to transform from one form to another. So we know something about the parts involved, but not their relative effects and how they’re triggered in the right order.

This is a wonderful study that also leads to many more questions than it answers. How did this jellyfish acquire immortality and why is immortality so uncommon in biology? What are the minimal genes from the “immortal jellyfish” that a mortal kind of jellyfish would need to be able to reverse back to younger forms? And conversely, what would happen if these genes were gone from the “immortal jellyfish”? Would they lose the ability to regenerate over and over?

I think an interesting follow-experiment to the ones that the researchers have done will be to see how jellyfish that have gone through multiple rounds of regeneration from adult to juvenile forms vary over generations. They may appear to be identical, but on the genetic level, we would expect to see differences in their DNA sequences over time. After all, change is a constant in biology.

Anirban Mahapatra is a scientist by training and the author of a book on COVID-19. He’s writing a second popular-science book

The views expressed are personal

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