With only two living females, the Northern white rhino is functionally extinct. Now, scientists are attempting to use assisted reproductive technology to save itby Philip Ball / July 5, 2018 / Leave a comment
Photo: Jan Husar/Zuma Press/PA Images The Northern white rhinoceros of the grasslands and savannah of southern Africa has been hunted to extinction. But modern assisted reproduction technology might yet rescue it. That’s according to a new study which demonstrates a dramatic possibility for developments in embryology to transform nature conservation. The population of Northern white rhinos has been tiny since the 1980s, rarely exceeding 30 or so. Currently, there are just two alive. Both were born in captivity and belong to the Dvůr Králové Zoo in the Czech Republic but are kept under armed guard in the Ol Pejata Conservancy of Kenya. Both, however, are females—a mother and daughter The last surviving male died of natural causes in March. So there is no way, now, for the species to persist by natural means; it is “functionally” extinct. The reason for that is poaching. Rhino horn fetches huge prices on the black market, and conservators have been taking to sawing off the horns of rhinos under sedation to eliminate poachers’ key motivation. To Jan Stejskal of the Dvůr Králové Zoo, the plight of the Northern white rhino is “a symbol of the human disregard for nature.” To make matters worse, it’s rare for the rhinos to reproduce in captivity—which is why the Czech animals were originally returned to the wild. Faced with this dire situation, in 2015 scientists at Berlin’s Leibniz Institute for Zoo and Wildlife Research, in collaboration with others, hatched a plan to maintain the endangered rhinos using reproductive technology. The rhinos’ DNA was sampled and stored in gene banks, and the researchers decided to try to create rhino embryos using egg cells (oocytes) from the surviving females. Sperm had been taken, and frozen, from males of the species when they still survived. This is, basically, emergency IVF—with scant and imperfect materials. With just two remaining females, the options both for collecting oocytes and gestating any embryos are severely limited. But there are alternatives. One is cloning. This involves transferring the genetic material from an animal’s fully developed tissue cells into an oocyte that has been emptied of its own chromosomes, and then stimulating the egg to grow into an embryo by artificial means. That’s how Dolly the Sheep was created in 1997, using chromosomes taken from the mammary cells of an adult ewe. In this case, oocytes could be taken from Southern white rhinos, a closely-related subspecies not currently at threat of extinction. By replacing the genes with those of a Northern white rhino, a cloned embryo gestated in a female Southern white rhino would still be genetically a Northern white rhino. It would be a bold strategy—sounding almost akin to something from Jurassic Park—but it could be feasible. A team led by one of the researchers in the current study, Cesare Galli of the Avantea Laboratory of Reproductive Studies in Cremona, Italy, has already demonstrated the cloning technique for horses. But cloning would be an extreme option—and, Galli says, one that many conservationists would regard with horror. For now, variants of standard IVF look more promising. Galli and his colleagues, in conjunction with Thomas Hildebrandt of the Leibniz Institute, Stejskal of the Czech zoo, and an international team of other collaborators, have now reported the creation of Northern and Southern white rhino “hybrid” embryos this way. (Such hybrids can arise naturally by crossbreeding in the wild.) The researchers collected oocytes from Southern white rhinos, allowing the egg cells to “mature” in vitro to prepare them for fertilisation. They then set about fertilising the oocytes with cryo-preserved Northern white rhino sperm. “The astonishing possibility exists to take a random bit of tissue—even from a long-dead rhino—and from it grow egg cells and sperm” In this way, the researchers were able to produce six embryos, four of which developed to the “blastocyst” stage where they are ready to implant into the uterus wall. At this stage, embryos have a 50 per cent chance of becoming a baby when implanted into the uterus, says Galli. The next step? “We need now to show that we can deliver a baby.” The problem is there isn’t yet a reliable embryo transfer process for rhinoceroses. For now, the hybrid blastocyst embryos have been frozen for later use. Even if a reliable method was developed, the transfer would ultimately have to be made into Southern white rhino surrogate mothers, as the two remaining female Northern white rhinos are no longer reproductively competent. Yet if the hybrid embryos are carried through to live birth and grown to adulthood, they might make better surrogate mothers for gestating pure Northern white rhinos embryos further down the line, Hildebrandt says. There are other reasons to hope that Northern white rhinos might yet survive. The team also extracted some cells from the hybrid embryos to develop into embryonic stem cell lines, which can give rise to all tissue types in the body. If the same IVF procedures could be used to make just a few pure Northern white rhino embryos, their stem cells can thus potentially be used to grow “artificial” sperm and eggs to compensate for the paucity of these cells available from adult animals. In fact, you may not even need embryonic stem cells to do that. Healthy mice pups have, for example, been born from embryos created by using cell-transformation techniques to turn adult mouse skin cells into egg cells that were fertilised and implanted in female mice for gestation. That process hasn’t been demonstrated for larger mammals, but there seems no reason in principle why it couldn’t eventually be made to work. In other words, the astonishing possibility exists to take a random bit of tissue—even from a long-dead rhino, if well preserved—and from it grow egg cells and (pending a few technical problems still to be cracked) sperm that could be combined to make viable embryos for surrogacy. With this in mind, the researchers are hoping to make induced stem cells from preserved Northern white rhino tissue samples. Using such specimens to make gametes would broaden the genetic diversity of rhino calves produced that way, so as to avoid problems of inbreeding. Making oocytes from stem cells is a long-term and uncertain strategy though, and it would be quicker and easier to use ones collected from the surviving Northern white rhinos, for which the researchers are seeking permission. Hildebrandt says they hope to have the first pure Northern white rhino calf born within three years. While all this shows what astonishing feats of cell manipulation and embryo production are now possible, it is also evidently a high-risk strategy—a last-ditch effort to save the Northernwhite rhino from vanishing off the face of the earth. Some might see it as a technofix that glosses over the deeper problem: the human malpractice that has endangered the subspecies in the first place. That’s one reason why some conservationists vehemently oppose the use of biotechnological approaches. What, after all, is the point of preserving the rhinos if it remains too hazardous to release them into the wild? But no one is pretending that the science can somehow fix that problem. It’s a hugely complex one: a social malaise born of poverty, avarice, corruption and limited conservation resources. Even if those challenges could be effectively tackled—and they are of course immense—it would be too late to save the Northern white rhino by such means alone. “We are not claiming to have solved all the problems,” says Galli, “but to have shown that assisted reproduction technology can be an effective option.” To regard this as a zero-sum game with exclusive outcomes—to seek just scientific or field conservation or sociopolitical solutions—is mistaken. We need them all.