Representatives of a giant tortoise species that had apparently been driven to extinction by humans more than 150 years ago must be alive today, if in very small numbers. Researchers reporting in the January 10 issue of Current Biology, a Cell Press publication, have come to this conclusion based on the "genetic footprints" of the long-lost species Chelonoidis elephantopus in the DNA of their hybrid sons and daughters.
"To our knowledge, this is the first report of the rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring," said Ryan Garrick of Yale University. "These findings breathe new life into the conservation prospects for members of this flagship group."
The Galápagos tortoises are famous for their influence on Charles Darwin's ideas about evolution by natural selection. But they are also impressive in their own right: individuals can weigh nearly 900 pounds, reach almost six feet, and live for more than 100 years in the wild. Today, several of 13 remaining species are considered highly endangered.
C. elephantopus was originally found only on Floreana Island, where they were presumed extinct soon after Darwin's historic voyage to the Galápagos Islands in 1835. Still, genes from recently extinct species can live on in the genomes of individuals of mixed ancestry. The Yale group had earlier detected the first traces of the "extinct" C. elephantopus within eleven individuals that otherwise belonged to another species, C. becki, living on an active volcano on Isabela Island.
In fact, movement of tortoises from one island to another by pirate and whaling ships was not uncommon during the 1800s, Garrick says, and his team suspected that individuals from Floreana had been translocated to northern Isabela years before. Those eleven hybrids appeared to be the last genetic vestiges of a unique evolutionary lineage in the wild.
That earlier finding inspired Garrick and his colleagues to take a closer look at what was happening on Isabela Island's Wolf Volcano, home to a large population of perhaps 7,000 tortoises, mostly C. becki. They've now sampled about 2,000 of those tortoises to find evidence that purebred C. elephantopus must live.
Comparisons of living tortoises and museum specimens indicate that the genomes of 84 of the newly sampled individuals can only be explained if one of their two parents were C. elephantopus. Those purebreds apparently exist at numbers so low that researchers would have to be incredibly lucky to sample one of them, Garrick says.
Even if purebred individuals of C. elephantopus are never found, their direct descendants could prove to be key in the giant tortoises' conservation. "Hybridization is considered largely deleterious to biodiversity conservation," Garrick says. "But in this case, hybrids may provide opportunities to resuscitate an 'extinct' species through intensive targeted breeding efforts."