Caitlin Looby, Mongabay | January 5, 2022
For many people, extinction is a term that refers to the past. It’s meant for dinosaurs, woolly mammoths and so many other species only found in textbooks or on display at museums.
“Extinction is an abstract concept to many people. It was to me as a kid,” says Oliver Ryder, director of conservation genetics at the San Diego Zoo Wildlife Alliance.
But the planet us growing less biodiverse each year. And extinction is a very real and present crisis. In September 2021, the U.S. Fish and Wildlife Service declared 23 species to be newly extinct. And many species aren’t technically considered extinct because they live in captivity, even though they no longer exist in the wild. Where conventional conservation methods have stalled, there may be another option, one that’s highly contentious, to pull a species back from the brink: cloning.
“Right now, conventional conservation measures are struggling to keep up with the pace of the threats that face the world. We’ve crippled populations to such fragmented and small sizes,” says Ben Novak, lead scientist at Revive and Restore, a nonprofit organization that works to boost biodiversity through what’s known as genetic rescue of endangered and extinct animals.
“Biotechnologies, like cloning and gene editing, now give us a chance to accelerate the evolution of species so they can actually cope with change and survive it.”
‘Providing genetic fodder’
Cloning is one type of genetic rescue, an approach used to restore genetic diversity in a population and lower the risk of extinction. Today, we can clone some mammals, fish and amphibians, and potentially even marsupials, Novak says.
Often, when a species’ population crashes to very low levels, it undergoes a genetic bottleneck, where only a few individuals remain to pass on genes to future generations. That reduces the overall genetic diversity of the population, increasing the likelihood of inbreeding, which in turn raises the chances of offspring carrying recessive traits, further undermining the species’ survivability.
Where the conventional conservation methods of captive and assisted breeding are limited to working with the genetic material currently circulating within a species, cloning can reinject lost genetic diversity into the population, says Beth Shapiro, an evolutionary molecular biologist at the University of California, Santa Cruz, who also serves on the board of directors at Revive and Restore.
Increasing genetic diversity “is likely to benefit by providing genetic fodder for natural selection,” she says.
Right now, cloning is the only way to produce an actual copy of an animal from a non-reproductive cell, Novak says.
In December 2020, a team of researchers led by Revive and Restore cloned the first black-footed ferret (Mustela nigripes), which they named Elizabeth Ann. Today, all black-footed ferrets descend from only seven individuals, so every ferret is related in some way. Elizabeth Ann is a clone from a different population and has three times more genetic diversity than any other ferret alive. So if she breeds successfully, she will contribute new genetic diversity to the population.
Earlier in 2020, scientists from Revive and Restore also successfully cloned a Przewalski’s horse (Equus ferus przewalskii), which they named Kurt.
Cloning can’t happen unless tissues samples, like skin, are biobanked, which means they’re frozen for preservation and stored in a biorepository. As a species declines, it loses genetic diversity within the populations. That’s why it’s important for researchers to collect tissue samples whenever possible to leave options open for the future, Novak says.
“For any species on the brink of extinction, what we can be doing right now is saving as many tissue samples as possible so that in the future if they go extinct or they get really rare we can … reach into the past and get something back that we’ve lost,” he says.
However, an animal can only be cloned if the technology already exists to breed it in captivity, because researchers need to know how to handle it and how it reproduces, Novak says. So cloning should never be aimed at replacing captive-breeding programs or nudging out other reproductive technologies, he says. But there will be different scenarios that favor different approaches.
Cloning and stem cell technology
With cloning, scientists take cells from tissues, like the skin, and a reproductive egg from a common ancestor. They remove the nucleus, the part that holds the genetic material, and then fuse the ancestor cell with the skin cell, so that the fused cell then only contains the genes from the animal they’re cloning. The fused cell is now an embryo that a surrogate will carry.
Proponents of cloning say such a method could work to revive the northern white rhino (Ceratotherium simum cottoni), a subspecies down to its last two individuals. There’s currently another project underway to save the northern white rhino, not with cloning but with stem cell technology. In this case, scientists revert cells from tissues samples to stem cells. Once they are stem cells, they have the potential to become any kind of cell, like reproductive egg and sperm cells.
The difference between cloning and stem cell technologies is that, with the latter, scientists aren’t making an identical copy of the donor. Instead, they are creating the circumstances so that cells can produce offspring through sexual reproduction, says the Wildlife Alliance’s Ryder.
There are only two northern white rhinos left in the world: Najin and her daughter, Fatu. In October 2021, Najin was retired as a potential egg donor due to age and illness, so now Fatu is the only donor left.
Experts like Ryder and Shapiro say they’re confident that one day they will successfully create a northern white rhino, but it will take a long time and there are a lot of challenges and unknowns along the way.
In mid-2021, the German government-funded initiative BioRescue announced that it had successfully created 12 northern white rhino embryos.
But still, the embryos need to be successfully transferred to a surrogate that is a common ancestor with the same gestational period. And in this case, that would be the southern white rhino (Ceratotherium simum simum).
However, there has never been a successful embryo transfer in a rhino to date, Ryder says.
Southern white rhinos also faced a population decline and subsequent genetic bottleneck due to poaching. The population crashed to as low as 30 individuals, limiting genetic diversity, before bouncing back to a population today of more than 10,000 individuals, but with much less diversity than before.
In fact, researchers at the Wildlife Alliance sequenced the 12 cell cultures that they have in their Frozen Zoo and found more genetic diversity in those 12 cultures than in the entire population of southern white rhinos, Ryder says.
So genetically, they have the potential to recover, he says. The technology just isn’t there yet to make the jump from frozen embryo to live rhino calf.
To clone or not to clone?
Although some may argue that intervening in the evolution of plants and animals is unnatural, Shapiro points out that humans have been intervening for tens of thousands of years already. She says we shouldn’t reject new technologies until the risks and potential are fully evaluated.
“I think in biotechnology, sometimes we get lost in the ‘can we do this?’ And the goal of an ethics analysis is to answer the question, should we do this biotechnology?” says Samantha Wisely, a conservation geneticist at the University of Florida.
Wisely was a part of a team that recently published an ethics analysis of cloning as a form of genetic rescue and looked at black-footed ferrets as a case study.
The ethical analysis took into consideration whether the goals of the program are justified, whether cloning can be done responsibly, and whether there’s support from the public and within the conservation community.
For the ferrets, cloning was really the only way, Wisely says. The remaining individuals were highly inbred and there were no new individuals to bring in and add to that genetic diversity.
Revive and Restore’s Novak points out that captive-breeding programs are already heavily managed. Animals don’t get to choose who they mate with; they’re paired together based on who’s related to who. With cloning, scientists are just expanding their options, he says.
The use of somatic cells, like skin cells, is also much less invasive than collecting reproductive cells, the analysis points out. Using somatic cells to create genetically identical animals has been used in the livestock industry in the U.S. for more than a decade.
And ultimately, whether or not cloning should be done as a means of genetic rescue should be considered on a case-by-case basis, Wisely says.
“The way to save species is to save them in their habitats,” Ryder says. “But the fact that we have species going extinct means that … it’s not being realized to the fullest extent.
“An additional set of options offers the future the possibility that wouldn’t otherwise have existed,” he adds.
‘Too fast for natural selection’
Over the next few years, Revive and Restore hopes to make more clones of Elizabeth Ann and Kurt as well as some new, unique Przewalski’s horses, Novak says.
He says this also gives them an opening to use gene editing or genetic engineering. For instance, they can sequence the genomes of ferrets that died in the early 1900s and find genes that are potentially beneficial, like ones for disease resistance. Scientists can “write” those genes back into living ferrets, providing a genetic boost to the current population.
The technology is unfolding in real time. And it’s important to engage global stakeholders who will be affected by the adoption of these approaches, while the technology is still developing, Shapiro says.
“The pace of change is too fast today for evolution by natural selection to keep up,” she says.
“If we want to live in a future that is both biodiverse and filled with people, then we have no choice but to continue to intervene as we always have … in more deliberate, thoughtful, and careful ways.”