At 194, Jonathan the Giant Tortoise Shows Youthful Gene Regulation, Science Advances Study Finds
The world's oldest known land animal carries 287 unique gene variants and an epigenome that looks younger than his years, offering clues to aging, though researchers stress the study covers a single animal.
Jonathan, an Aldabra giant tortoise estimated to be about 194 years old, is the oldest known living land animal in the world. A study published Wednesday in the journal Science Advances suggests his longevity comes from a combination of genetic traits and an unusual ability to keep key cellular systems working in a youthful way.
Jonathan was brought to the remote South Atlantic island of St Helena in 1882 as a fully grown animal, likely from Aldabra Atoll or the Seychelles. He lives in a paddock at Plantation House, the governor's residence. He stands about 4 feet high and is 5 to 6 feet long. "Other Aldabra tortoises have been claimed to have lived as long as Jonathan, but he has the most documentary evidence supporting his age," said Benjamin Vaisvil, a researcher and co-lead author of the study.
The team sequenced Jonathan's genome and analyzed his epigenome, the chemical markers that control which genes switch on and off. "We found changes in Jonathan's genome in pathways associated with longevity, like DNA repair, mitochondrial function, insulin regulation and telomere function," Vaisvil said. The researchers identified 287 genes with variants unique to Jonathan, including some linked to tumor suppression and DNA repair. He also carries extra copies of a gene that encodes a protein that hunts down aging and cancerous cells.
The epigenome result was the bigger surprise. Vaisvil compared the genome to a cookbook and the epigenome to handwritten notes in the margins. As the book ages, the notes get harder to read, a disorder the team measures as methylation entropy. In stretches of DNA that act as on-off switches for longevity-related genes, Jonathan's methylation entropy was lower than in every tortoise studied except a five-year-old juvenile. Co-author Justin Gerlach, of Peterhouse at the University of Cambridge, told NPR that Jonathan's gene regulation resembles that of a very young animal. "He's actually genuinely aging more slowly than we would expect," Gerlach said. As regulation breaks down with age, genes get expressed incorrectly, and cancer is an obvious consequence.
Collecting the data took some ingenuity. Blood draws are restricted on St Helena to prevent infection risks, so the researchers pretended to feed Jonathan, waited until he began biting the air, wedged his mouth open with a gloved hand and scraped under his tongue with a specialized kit. They compared his epigenome with four other Aldabra tortoises, ranging from juveniles to older adults.
The authors are careful about what the work proves. It involved a single exceptionally old animal, so it is hard to tell whether the traits are unique to Jonathan or common among long-lived tortoises. "We don't know at the moment whether that's because he is a really exceptional individual or whether actually quite a lot of giant tortoises have this," Gerlach told NPR. Age estimates for old tortoises are also hard to confirm because records before the 20th century are scarce.
Jonathan, now mostly blind from cataracts, is described by longtime veterinarian and co-author Joe Hollins as "a gentle giant," though in his youth he was known for overturning chairs and lying on croquet balls. Gerlach noted that Jonathan has never fathered offspring in his 144 years on the island, so there is no way to test whether the traits are inherited.
