Archimedes had his famous “Eureka!” moment about water displacement after stepping into a very full bath. Oliver Ryder and Leona Chemnick’s big “Aha!” arrived in a parking lot.
They were standing outside the San Diego Zoo Wildlife Alliance at the end of a work day in 2013, and Chemnick, a researcher in the alliance’s conservation genetics lab, was describing a puzzling situation. The zoo is home to dozens of California condors, part of a rehabilitation effort that began after the birds’ population plummeted during the 20th century. In 1987 conservationists captured the last 22 birds from the wild and slowly nursed the population back from the brink of extinction. There are now about 500 condors in California and Mexico, but the bird is still critically endangered, so scientists carefully track the parents and chicks in the zoo’s breeding program.
Chemnick had been double-checking the parentage of each chick that had hatched since the program began, using DNA obtained from blood samples. But she happened across a mystery. Actually, two of them. These two chicks, which had hatched about a decade apart in the early 2000s, had DNA that matched their mothers’ DNA, but they didn’t have a single gene from the male condors that were listed as their fathers. In fact, they didn’t have DNA from any registered male condors. They were also entirely homozygous: Instead of having a mix of dominant and recessive genes, all of their alleles were exactly the same. Chemnick wondered how this was possible.
“Are they both male?” asked Ryder, the director of the conservation genetics lab.
“Yes,” replied Chemnick.
It was then, standing next to his car, that Ryder’s realization hit with an intensity that still gives him goosebumps today: Chemnick had just described what scientists know as “facultative parthenogenesis”—in a sense, two virgin births. Instead of being conceived through sex and receiving a set of genes from a mother and father, these chicks were produced from cells that only came from a mother. “I went home and I went, ‘Well, son of a gun, this is an unusual day,’” recalls Ryder.
Today, the researchers’ paper documenting the discovery of the two chicks, coauthored with other scientists at the Zoo Alliance, appears in the Journal of Heredity. Facultative parthenogenesis has previously been observed in animals including Komodo dragons, zebra sharks, cobras, and other birds including chickens, but this is the first time the phenomenon has been recorded in condors. (In fact, there were so many firsts, it took a few years to write up the article.)
Specifically, it was also the first time scientists have observed avian parthenotes—the offspring—that reached maturity, though neither of these chicks lived long enough to reproduce. One chick that Chemnick discovered in the records, SB260, was born in 2001 and released into the wild about a year and half later. But it died in 2003, weak and probably undernourished, before it reached sexual maturity. The other, SB517, hatched in 2009 and remained in captivity. Caretakers at the zoo described him as small and submissive. He lived for nearly eight years before succumbing to complications from a foot infection.
Neither bird was particularly robust, but the fact that they survived beyond hatching is a big deal. “I think it's one of the most important studies in the field of parthenogenesis and birds in a long time,” says Warren Booth, an evolutionary biologist at the University of Tulsa who studies facultative parthenogenesis in snakes and was not involved in this paper. He says that although sharks and rays produced through asexual reproduction have survived and even thrived, the same hasn’t been seen in birds. Parthenotes born to domesticated turkeys, chickens, quails, zebra finches, and pigeons have almost all died before hatching.
Although these condors died young, Booth says, “This gives us some information that maybe within raptors, we might see the ability to produce healthy—or at least living and somewhat viable—parthenogens that could then potentially reproduce within that population.”
Most vertebrates reproduce sexually, mixing genetic information from male and female partners to create offspring with a new combination of genes. This arrangement has some utility: If an embryo inherits a faulty copy of a gene from one parent, the copy from the other parent can compensate.
But sometimes animals with particularly ancient genomes—including birds, lizards, sharks, and snakes—leave the male out of the equation and reproduce asexually. Like mammals, females of these species produce eggs through meiosis, the process in which chromosomes are pulled apart. The pieces are divvied up among four separate cells, only one of which is an egg. During sexual reproduction, an egg merges its genetic material with that of a sperm produced by a male. But during parthenogenesis, the egg instead merges back together with one of the other cells, creating a self-fertilized egg.
Parthenotes can only be one sex, although which sex depends on their species. For snakes like boas and pythons, parthenotes are all female: Their chromosomes are XX.
Unlike humans, for birds it’s the egg, not the sperm, that dictates the sex of the embryo. For that reason, scientists use a different naming system for their chromosomes. A female has ZW chromosomes, while a male has ZZ. If a female reproduces asexually, that means she can only create a WW or ZZ embryo. But a WW in birds wouldn’t create a viable embryo, so all avian parthenotes that survive to the egg phase and beyond have to be ZZ—male.
Usually, parthenogenesis happens among females when there’s no male mate available. In theory, this mechanism allows the female to keep the gene pool going until a suitable male comes along. But it’s not an ideal solution, says Booth. Because the egg is fusing with a cell that contains a nearly identical set of chromosomes, there’s almost no genetic diversity in the resulting offspring. “Across most of its genome, it lacks diversity, which is why we see in most cases of pathogenesis, the animals don't do well long term,” he says. “They're just the most inbred that you could be.”
But Demian Chapman, director of the Sharks and Rays Conservation Program at Mote Marine Laboratory in Florida who has identified several different parthenotes in sharks and rays, points out that while they are more likely to have genetic flaws, the ones that do survive may be free of some of the lethal gene variants common in a species. “They can’t possibly be carrying them, because if they were carrying them they would die because they don’t have the other one to compensate,” he says.
Booth and Chapman say that parthenogenesis is probably much more common than scientists realize, but it’s hard to study because it’s usually only identified by accident. It’s rare to have an extensive database of genetic information about all the members of any species living in the wild. Parthenogenesis is more likely to be spotted in animals in captivity, says Chapman, because they are more carefully monitored. “If you have a cage full of females and there’s a baby, that’s a big red flag that maybe you should do some genetic testing,” he says.
Plus, because parthenotes often look normal, there’s no easy way to identify them in the wild. Had the zoo’s chicks been born anywhere else, scientists would have never suspected the circumstances of their births.
Why their mothers reproduced asexually, even though they had access to male partners, isn’t clear. Animals that experience parthenogenesis often have reproduced sexually in the past and go on to do so again in the future. Chapman suspects it might just happen every now and then, producing a few chicks here and there in a population that’s also reproducing sexually. It’s just harder to notice it when male condors are present. “I don't want to call this an accident,” says Ryder. Because it happened in two different nests, in two different places, at two different times, he thinks this is a recurring phenomenon.
What this means for the California condor’s still-diminished population is hard to say. On the one hand, Ryder worries that the gene pool is losing its diversity. But with so few birds left, adding more chicks is critical. The fact that two parthenotes survived past hatching gives Ryder hope that others have survived in the past—unnoticed by scientists—and that condors could benefit from this phenomenon in the future. “It makes me feel more secure that even though their gene pool may be small, that it's sufficient,” he says.
Update 10-28-2021 5:27 PM: This story was updated to correct the year of Chemnick and Ryder's parking lot conversation.