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The world’s oldest known land animal – Jonathan the giant tortoise – has had his genome probed for the first time. This has revealed that parts of his genome to do with mitochondrial function are as efficient as those of a 5-year-old member of his species. Efficient mitochondria may give his DNA repair systems more energy to fight off the effects of ageing, enabling him to live to an estimated age of 194.
What’s more, some of his genetic changes seem similar to those of supercentenarians. “This study adds to the body of evidence that the mechanisms of ageing may be conserved across species,” says Stephen Clark at the Kallel Foundation, a non-profit organisation focused on interventions to extend human healthspan, in Nashville, Tennessee.
Jonathan, an Aldabra giant tortoise (Aldabrachelys gigantea), and Napoleon Bonaparte are the two most famous residents of St Helena island in the South Atlantic. Napoleon arrived at the remote British territory after he was exiled in 1815, while Jonathan is thought to have been shipped there, possibly from Aldabra Atoll, in 1882. Jonathan arrived as a fully grown adult that was thought to already have half a century under his belt, putting him at around 194 years old today.
The researchers calculated that Aldabra tortoises have an average lifespan of around 80 years, but they can live much longer. To understand the mechanisms behind Jonathan’s extraordinary longevity, Clark and his colleagues have studied his genome.
“I firmly believe we can learn about the global mechanisms of ageing by looking at the exceptions,” says Manel Esteller at the Josep Carreras Leukaemia Research Institute in Barcelona, Spain, who led a study into a 117-year-old woman, once officially the world’s oldest person.
Esteller and his team found that the woman had exceptionally efficient mitochondrial function, which led Clark and his colleagues to suspect Jonathan similarly had “very efficient and very young” mitochondria, says Clark.
They had to study his genome via saliva samples and cheek swabs. This was because St Helena authorities refused their requests to draw Jonathan’s blood, given his fame and the risk of infection.
This was a major limitation, because it meant the researchers were unable to isolate large pieces of DNA, which are more stable in blood. Instead, they had to assemble fragments from Jonathan’s saliva, where DNA is more prone to bacterial contamination, and fill in the gaps using DNA from another Aldabra tortoise.
Once they had made the best possible composite of his genome, they compared it to the DNA of four other Aldabra tortoises. “We found 287 genes that had changes in them that were unique to him,” says Clark.
Of these genes, 41 are though to be functionally significant, meaning they actively contribute to an organism’s biology, survival and regulation. A dozen of them were also found in ageing databases of the genes of other long-lived animal species.
Parts of Jonathan’s genome to do with mitochondrial function were as young as a 5-year-old Aldabra, says Clark. “This, in turn, has helped him manage the effects of ageing by giving the DNA repair systems more energy from the mitochondria to fight off the effects of ageing.”
Kousuke Hashimoto at the University of Osaka, Japan, says the work is “fascinating”, but notes that Jonathan’s genome was compared against that of only a few other tortoises, which may have unique genetic changes themselves.
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