In his lab at the University of Edinburgh, Mike McGrew is turning commercial egg-laying hens into surrogates for rare chicken breeds revived from frozen cells—a technique that he hopes one day might also be used to revive endangered bird species.
The problem with pulling this off has always been the egg. Chicken eggs are huge, relatively speaking. They are full of water and encased in a hard, brittle shell. All of which means they are impossible to preserve long-term. “We just can’t freeze down bird’s eggs,” says McGrew. In “frozen zoos,” scientists have started to save eggs, sperm, and embryos to safeguard species against extinction, but there is just no way to save bird eggs for future revival.
The solution, it turns out, is to dispense with freezing eggs altogether, by constructing an elaborate work-around on the limits of chicken biology.
First, McGrew’s team carefully extracted a microliter of blood from the early embryo of a rare chicken breed. This blood contains “primordial germ cells,” which are chicken cells that will eventually develop into eggs or sperm. Scientists have found that primordial germ cells can be made to multiply in a lab and then easily frozen for long-term preservation. But McGrew’s team still had to figure out how to turn these frozen primordial germ cells into a live chick. In other words, they had to create a surrogate chicken.
To do this, they took a commercial egg-laying chicken known as Hy-Line Brown, which is optimized to lay eggs. (These chickens can lay more than 200 eggs a year.) But the team also genetically modified these egg-laying chickens to disable a gene called DDX4. Without this gene, the hens do not develop primordial germ cells.
The next step is to put the two pieces together, by replacing the missing cells with those from a rare breed. McGrew’s team took the primordial germ cells extracted from the rare breeds and injected them into an egg containing a modified Hy-Line Brown embryo, which lacked primordial germ cells of its own. The chick hatched from this egg can become a surrogate mother. While it grows into a Hy-Line Brown layer chicken, its eggs in turn contain genetic material from the rare breed. When the eggs are fertilized with sperm—frozen or fresh—from a male of the same breed, the result is, finally, a new chick. (Semen from birds doesn’t freeze as well as that from mammals either, but it can be done.)
The team ultimately got chicks this way from four rare British breeds: Cream Legbar, Marsh Daisy, Scots Dumpy, and Scots Grey. All were hatched from eggs laid by genetically modified Hy-Line Brown layer hens.
“That’s been a goal for a long time. I’m glad to see it done,” says Jim Petitte, a professor of poultry science at North Carolina State University. The poultry industry has long sought a way to maintain and revive breeds from frozen cells, as a means of genetic backup. If a new illness strikes or farmers want to breed a new trait, they dip into the genetic catalog of heritage breeds.
Currently, however, the only way to maintain a genetic stock of birds is to continuously keep a flock of birds alive. This is expensive, which means it is precarious. Drew Benson, a professor of poultry science at the University of Georgia, says a turkey line he studied in his dissertation is no longer being maintained. It’s essentially gone from the world. A frozen aviary would be much cheaper to maintain. “All you need essentially is a nitrogen storage tank, and you don’t have the recurring expense of feedstock,” he says.
McGrew’s work is partially funded by agriculture grants in the U.K., but he’s also interested in how this technique could be applied to endangered birds. In 2013, he gave a TEDx talk about the possibility of reviving extinct passenger pigeons. Elsewhere in the world, the San Diego Zoo, which famously maintains a “frozen zoo,” has also investigated the use of germ cells for preserving exotic birds. The big, unanswered question, McGrew says, is whether female birds of not just different breeds but different species can act as surrogates.