Scientists sequence genome of Jonathan, the tortoise thought to be 194
Gene switches controlling DNA repair and metabolism resemble those of younger tortoises of the same species, the University of Cambridge says.
Scientists have sequenced the genome of Jonathan, a giant tortoise on St Helena estimated to be 194 years old, and report that, compared with other giant tortoises, Jonathan has unique gene variants in most ageing pathways, according to a paper in Science Advances.
The University of Cambridge, whose researcher Justin Gerlach took part, said in a 7 October press release that the paper was published that day; the journal’s record carries the issue date 9 October. The work was led by Benjamin Vaisvil of the Kallel Foundation in Nashville and the company Igenbio in Chicago, with Stephen Clark, Kallel’s founder, as senior author.
Jonathan’s age is an estimate. Guinness World Records, which lists Jonathan as the oldest-known living terrestrial animal, says the tortoise was born around 1832. The figure comes from the fact that Jonathan was said to be “fully mature” when brought to St Helena in 1882, and so was at least 50 even then. It also notes that some experts think the shell points to a separate species or subspecies of Seychelles tortoise, a debate it says is unsettled.
The researchers sequenced both Jonathan’s genome and methylome. The Guardian, which reported the work on 7 October, described methylation as chemical tags that attach to DNA and typically silence the genes they latch on to. It said it is unclear whether, or how much, the chemical patterning contributes to Jonathan’s lifespan.
Cambridge said the team identified 287 unique gene variants that reduce the usual effects of ageing, and that the study is the first to examine a giant tortoise’s epigenome.
The team compared Jonathan’s DNA methylation with that of four other Aldabra tortoises, ranging from a 5-year-old juvenile to older adults. They found substantial differences in both methylation and in a measure the authors call methylation entropy. Cambridge said, however, that the switches controlling DNA repair and metabolism genes were very similar to those in younger giant tortoises of the same species.
Regions where Jonathan’s methylation entropy was lower were enriched for the promoters of genes involved in the mitochondrial electron transport chain and in RNA metabolism, the authors write. They say this “suggests that high-fidelity transcription of the genes in these pathways may be crucial for long-lived species”.
“We found that the gene regulators involved in energy production and DNA repair have remained incredibly stable in Jonathan over almost two centuries,” Gerlach said. The abstract’s closing sentence says the findings support a model for ageing in which low methylation entropy in gene promoters is coupled to efficient mitochondrial energy production, efficient RNA processing and efficient genomic repair.
