In late March, the last male northern white rhinoceros – a gentle giant named Sudan – passed away at the Ol Pajeta Conservancy wildlife park in Kenya. Thanks to decades of poaching by horn traffickers, there are now only two members of this majestic species left on Earth – Sudan’s daughter Najin and granddaughter Fatu. They live under 24-7 armed guard.
Yet before his death, an international team of reproductive biologists and zoologists collected samples of Sudan’s DNA and sperm cells, adding them to the small repository of genetic material acquired from previous recently deceased northern white rhino (NWR) males in the hopes of breeding future generations through in vitro fertilization (IVF). The plan is to use the banked sperm – of which there is only a meager 300 ml – to fertilize eggs from the closely related southern white rhino (SWR) and implant them in a healthy SWR female who will carry the embryos to term.
On Monday, it was announced that the first phase of this undertaking has been achieved. As reported in an upcoming study in Nature Communications, Cesare Galli, Thomas Hildebrandt, and their collaborators injected NWR sperm into 13 eggs and painstakingly coaxed them to begin dividing. Four fertilized eggs successfully developed to the pre-implantation blastocyst stage and two of them now remain.
In a July 3 press conference, the team reported that they hope to have a northern white rhino hybrid calf born in three years.
“We came to the point, around 2008, where there was no chance to save the subspecies with the techniques we had at that time available,” Hildebrand said. He and his peers had been collecting genetic material to use in assisted reproductive technology (ART) since the early 2000s, but the sperm quality was too poor and the female reproductive tracts were too damaged for the current IVF methods.
Believing that there was nothing left to be done, the sole surviving trio of Sudan, Najin, and Fatu were sent to retire in Ol Pajeta. “[W]hen the rhinos went to Kenya, we thought the story was over,” explained Hildebrandt.
But then, in 2012, Japanese biologists were awarded the Nobel Prize for their breakthrough discovery of how to create induced pluripotent stem cells – a method that allows reproductive cells, or gametes, to be generated using normal body cells (somatic). Because Hildebrandt and his colleagues had amassed somatic cell samples from 12 unrelated northern white rhinos, and key advances in ART had been made in the intervening years, there was suddenly a potential avenue for creating NWR individuals with enough genetic diversity to someday form a stable population.
The researchers were thrilled when two viable stem cell lines from southern white rhino embryos were successfully established. They are currently working toward inducing them to differentiate into oocytes (egg cells). Sadly, the stem cell lines from the NWR x SWR embryos did not survive. Once they master the technique to produce gametes using embryonic stem cells, they hope to translate it to induced pluripotent stem cells (iPSCs).
Galli and Hildebrandt noted that this step could take upwards of a decade, but is essential to the long-term goal of moving past birthing hybrids to birthing pure NWR calves. As of now, this is impossible because there are no viable NWR oocytes to work with, though the team is waiting for permission to attempt retrieval from Najin and Fatu for the first time.
Hildebrandt explained that producing hybrids is a necessary step because these individuals will likely serve as much better surrogates for future generations than pure southern white rhinos. Moreover, as was done for the auroch, careful breeding of hybrids could in itself eventually lead to an animal that physically and genetically resembles the original species.
The team has already put in decades of effort and a daunting road remains ahead, but thanks to their tireless commitment, it appears that this beautiful animal may have been saved from the very brink of extinction.