Reproductive biologists are developing an unusual way to produce farm animals with desirable traits: injecting surrogate fathers — whose own sperm production has been crippled by gene editing — with sperm-producing stem cells from another male that pass along ‘elite’ genes to offspring. From then on, the surrogate sire’s offspring will not be his own, but the donor’s.
The goal is to spread genes for desirable traits, such as disease resistance or heat tolerance, through a population of animals in fewer generations than is possible with conventional breeding. If scientists can surmount lingering technical hurdles, the technique could prove invaluable for pigs, chickens and other livestock that are tricky to breed using artificial insemination. “There’s a lost opportunity to improve genetics,” says Jon Oatley, a reproductive biologist at Washington State University in Pullman.
The technique could also aid efforts to conserve species for which semen storage is difficult, including many birds.
In the US dairy industry, the practice of artificially inseminating cows with sperm collected from elite bulls, as well as careful genetic selection, has yielded cows that produce four times more milk than animals did in the 1940s, before the practice was introduced. But artificial insemination is not often used in beef cattle, because the animals are allowed to roam freely over pasture, making it hard to track down cows at the right stage of their reproductive cycle. And the technique doesn’t work well for pigs because their sperm often die in storage.
Who's your daddy?
Oatley and his colleagues are now working to develop surrogate pig sires. In 2017, the researchers reported that they had used the gene-editing tool CRISPR–Cas9 to disable a gene called NANOS2 in pigs. Pigs that carry two copies of the disabled gene can’t produce sperm, but are otherwise normal — ideal surrogate sires1.
That same year, another team led by Michael McGrew of the University of Edinburgh’s Roslin Institute said it had created sterile female chickens using a different gene-editing system to disable a gene called DDX42. Females that inherit the disabled gene from their fathers would be sterile, and could be transformed into surrogate mothers.
McGrew and his team have gone on to transplant stem cells into developing female embryos that carry the disabled DDX4 gene. The once-sterile recipients went on to lay eggs, McGrew says, and his team is now verifying that the offspring from those eggs came from the transplanted cells.
McGrew hopes to apply the technique in the next year to chicken species whose small populations are highly adapted to local conditions in African countries such as Ethiopia and Ghana. He also hopes to use it to conserve rare breeds of chicken in India and the United Kingdom.
Chicken eggs are an easier target than mammals for transplanting cells, McGrew notes, because it is comparatively easy for researchers to access chicken embryos. All a geneticist needs to do is make a small hole in an egg’s shell, he says, and inject the cells into the vasculature of the developing embryo. From there, the cells migrate to the right location and multiply.
Pigs pose a greater technical challenge. At the Plant and Animal Genome meeting in San Diego, California, in January, Oatley presented the results of his efforts to transplant sperm-producing stem cells into his surrogate pig sires. The cells survived and generated sperm that seemed normal — but there were far fewer than would be expected from a typical sire.
“It’s obviously not enough sperm to do the job,” says Alison van Eenennaam, an animal geneticist at the University of California, Davis, who attended the talk. “But it showed that you could generate sperm and that really is the proof of concept.”
Next month, at the Transgenic Technology Meeting in Kobe, Japan, Oatley plans to present additional data showing that he can achieve normal fertility in surrogate mouse sires, even when he transplants the sperm-producing stem cells from a genetically dissimilar strain of mice. The trick now, he says, will be to make the system work in livestock.
That could still present a formidable challenge, says Ina Dobrinski, a reproductive biologist at the University of Calgary in Canada. Researchers have ways to expand the number of mouse and rat stem cells that give rise to sperm when grown in cultures. But the techniques have not worked well for larger animals, including people, Dobrinski says — despite fervent research aimed at finding ways to restore fertility for boys who have been treated for cancer.
Oatley acknowledges these challenges, but says that a small number of stem cells might be sufficient if they multiply enough after the transplantation. And Bhanu Telugu, a reproductive biologist at the University of Maryland in College Park, says that tweaking the procedure to create a surrogate pig sire — such as transplanting the cells when the surrogate is younger — could boost the number of sperm produced.
Oatley estimates that his technique is about five years from the farmyard. But it is unclear whether the approach will be embraced by the public and regulators. Oatley has twice travelled to present his work to the US Food and Drug Administration, and McGrew’s team has discussed the matter with regulators in India. The offspring of a surrogate sire would not be gene edited, but some governments might still regulate them as though they were, McGrew cautions. In some countries, that could mean a lengthy and expensive approval process.
“You and I know the recipient is genetically edited and the sperm are not, but explain that to the regulatory agencies or the consumers,” says Dobrinski. “I’m not sure how a knockout animal would fly.”
Nature 567, 292-293 (2019)