A team of scientists recently modeled the likelihood of hauling a species back from dead, using the Christmas Island rat, a large rodent that went extinct between 1902 and 1908, as their hypothetical subject.
The researchers sequenced the long-gone rat’s DNA and mapped it to some of the animal’s closest living relatives. They managed to recover 95% of the rat’s genome, though they believe the missing 5% significantly complicates any future attempts to resurrect the species in a process called de-extinction. The research was published today in Current Biology.
“I’m interested in how easy or hard will de-extinction by editing be. And in our article, we computationally come up with an idea of what we think might go wrong,” said Tom Gilbert, an evolutionary geneticist at the University of Copenhagen and a co-author of the paper, in an email to Gizmodo.
Christmas Island rats (Rattus macleari, also known as Maclear’s rat) were one of two rat species endemic to Christmas Island, a 52-square-mile landmass about 200 miles southwest of Indonesia. (The other species, the bulldog rat or Rattus nativitatis, went extinct around the same time.)
Theories abound as to what drove the rats to extinction; one of the more recent is that black rats (Rattus rattus) introduced by European settlers had fleas that themselves carried a pathogen, Trypanosoma lewisi, which caused mass die-offs of the native rodents in the early 20th century.
In the paper, the researchers note the three most touted methods for de-extinction: back-breeding (the selective breeding of ancestral traits in modern animals), cloning, and gene editing. There are many ethical concerns with bringing back extinct species, chief among them that the money could be spent on conservation of the creatures still with us.
The team was focused on showing how de-extinction via gene editing would work—but they don’t actually intend to bring back any rats. (“I certainly have no goal of reintroducing a Xmas rat,” Gilbert said. “Seems like not the best use of money.”)
The researchers sequenced ancient DNA from two skin samples of the Christmas Island rat collected between 1900 and 1902 and currently held in the Oxford University Museum of Natural History. Comparing the genome of the extinct rat to several modern counterparts—most importantly the Norway brown rat (Rattus norvegicus), its closest living relative—the researchers were able to identify traits of the extinct rat they were confident they could replicate.
A resurrected animal wouldn’t be genetically identical to the extinct group it represents. But in this case study, genes involving keratin and proteins associated with keratin, vital ingredients in hair and whiskers, were well covered across the two rat genomes. So two were genes involved in the shape of the Christmas Island rat’s ears. Combined, the researchers posited that if they edited the Norway rat’s genome, they could replicate the fur color and ear shape of the Christmas Island rat.
The genes involved in the Christmas Island rat’s sense of smell, however, were different enough that the Norway rat’s olfactory genes don’t make a good basis for reconstructing them, according to the study. Genes related to immune response were also not covered by the Norway rat. But as the researchers point out in their paper, given the running theory about the Christmas Island rat’s extinction, any resurrected version of the species could theoretically benefit from using the immune genes of the Norway rat.
This was a proof-of-principle model, meaning the researchers were showing how one could go about de-extinction (if they intended to) using the gene editing of an extant species to bring back a related one, like the woolly mammoth or the thylacine.
“We aren’t actually planning to do it, as probably the world doesn’t need any more rats, and probably the money it would take to do the best job possible could be spent on better things, e.g., conserving living things,” Gilbert said.