Ever since the film Jurassic Park, the idea of bringing extinct species back to life has been fascinating. But where should scientists turn first?
Instead of focusing on iconic species like the woolly mammoth or the Tasmanian tiger, a team specializing in paleogenetics sought to determine the method that would bring the humble Rattus macleari back to life. This species of rat endemic to Christmas Island, in the Indian Ocean, disappeared regarding 120 years ago.
The researchers didn’t go so far as to actually recreate a living specimen, but say their study, published Wednesday in the journal Current Biology, shows how close scientists working on so-called “de-extinction” projects might come.
“I don’t do de-extinction, but I think it’s an interesting idea, and technically it’s very exciting,” the lead author of the work, Tom Gilbert, told AFP. University of Copenhagen.
Three techniques are explored to resurrect extinct animals: crossing close species in order to find certain lost traits; cloning; and finally genome editing. It is on the latter that Tom Gilbert and his colleagues looked.
The idea is to take DNA from the extinct species, and compare its genome to similar modern species. Once the nearest one has been selected, the CRISPR tool, dubbed molecular scissors, is used to edit the modern genome where it differs from the ancient one.
The cells thus modified might then be used to create an embryo, to be implanted in a surrogate mother.
According to Tom Gilbert, ancient DNA is like a shredded book. The genome of a modern species is the reference book, intact, which can be used to decipher the damaged fragments of its ancestor.
The researcher’s interest in these rats was sparked by a colleague who studied their skin, looking for traces of pathogens linked to their extinction around the year 1900.
The hypothesis is that black rats reported on board European ships decimated the local rat population. The latter were tall, with a long yellow tail, and small round ears, according to a description in a scientific journal dating from 1887.
– Key functions lost –
The researchers used Norway rats, commonly present in laboratories, as a modern reference species. They determined that they might reconstruct 95% of the genome of the Christmas Island rat.
Although this proportion may sound like a great success, the 5% lost are part of regions of the genome controlling the sense of smell and the immune system: the resuscitated rat would thus certainly resemble the old one, but it would lack certain key functions.
“Even though we have almost the perfect situation in terms of ancient DNA, with a really good sample, which we’ve sequenced as much as possible, we’re still missing that 5%,” he said.
The two species began to diverge 2.6 million years ago, evolutionarily not very long ago, but still too long ago to reconstruct the entire genome.
These results have important implications for ongoing “de-extinction” projects, such as that of an American company to revive a mammoth.
Mammoths, extinct around 4,000 years ago, are as distant from modern elephants as Rattus macleari is from brown rats.
In Australia, a team is trying to revive the Tasmanian tiger, the last specimen of which died in captivity in 1936.
But even if the gene editing technique were further improved, without being able to recompose an identical genome, the animals created would still present significant differences.
“If you’re bringing a mammoth back to life just to have a furry elephant in your zoo, to raise money or raise awareness for animal welfare, then that’s not really a problem,” says Tom Gilbert.
But if the goal is to bring it back to its exact form, for example to reintroduce it into the wild, “that will never happen,” he says.
Moreover, the researcher admits that even if this science is fascinating, these projects of “deextinction” cause in him mixed feelings.
“I’m not convinced it’s the best job you can do for your money,” he says. “If you had to choose between resurrecting something and protecting something that still exists, I would bet on protection.”
