The concept of de-extinction, the process of generating an organism that is extinct, has so far largely focused on popular animals such as mammoths or even dinosaurs. But in a new study, a team of paleo-geneticists have turned their attention to Rattus macleari, the Christmas Island rat, which went extinct 199 years ago -- a case study that is much more realistic.
Tom Gilbert, an evolutionary geneticist at the University of Copenhagen, Denmark, and his team acknowledge their pick is not the most sensational, but they argue it’s the most feasible with today’s technology. They examined how the extinct rat could be revived as well as the limitations to de-extinction, addressing important questions about how something like this should be done.
When working with the genome of an extinct species, scientists face the challenge of working with degraded DNA, which doesn’t have all the genetic information needed to rebuild a full genome of the animal. But with the Christmas Island rat, believed to have gone extinct because of diseases brought on board European ships, the researchers were lucky.
They sequenced the genome of the rat and found they were able to recover almost all of the genetic information. As it turns out, the Christmas Island rat is closely related to a living rat species, the Norway brown rat, sharing around 95% of its genome. This became very handy, as when sequencing the genome, researchers have to compare it to a modern reference.
There also are ethical concerns with bringing back an extinct species, they argued, the main one being that the money could instead be invested in the conservation of the still-living animals. But whether something like this could be done is indeed interesting and could be important for particular key species.
The possibilities behind de-extinction
In their paper, the researchers described the three most touted methods for de-extinction: back breeding (breeding of ancestral traits in modern animals), cloning, and gene editing.
The researchers focused on gene editing. Basically, this means changing the genes of an existing animal to resemble those of the animal you're trying to bring back. They sequenced ancient DNA from two skin samples collected between 1900 and 1902. Comparting the genome to several modern rats, they could identify traits of the extinct rat they could replicate.
With this method, a resurrected animal wouldn’t be just like the extinct group, there would be some differences. But in this case, there are many similarities between the existing rat and the de-extinction rat, which would allow replicating several key aspects of the extinct species.
However, the genes involved in the Christmas Island rat’s sense of smell were so different that Norway’s rat olfactory genes didn’t make a good basis for reconstructing them. Genes linked to immune response also weren’t covered. The researchers argue that any resurrected species would benefit from the immune genes of the Norway rat.
The study was a proof of concept more than anything else. The researchers showed how de-extinction could be carried out using the gene-editing of an existing species to bring back a related one, but don't actually intend to bring back any rats yet.
While we won’t get to see the Christmas Island rat back anytime soon, the study opened the door to discussing de-extinction and its potential outcomes and benefits -- and maybe it's a conversation we should be having sooner rather than later.
“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 in a statement.
The study was published in the journal Cell.