SAN DIEGO – He has his mother’s eyes, and his father’s… nothing.
A science team led by the San Diego Zoo Wildlife Alliance announced Thursday that California condors can mate without having sex. The researchers made the discovery after genetic tests showed that two condors born in captivity did not have a father.
Yes, the fathers. And this is not a misspelling of “feathers”.
This phenomenon, called parthenogenesis, has been observed in some insects, fish and reptiles. There have been a few cases in birds as well. But no one until now knew that female California condors could have offspring without males, a finding that raises questions about how often this happens and if it matters in the wild.
Another unanswered question: why did this happen? The mothers of the two condors had mated successfully before and were housed with a male at the time of their asexual reproduction.
“I thought it was quite remarkable,” said Kevin Burns, an ornithologist at San Diego State University, who was not involved in the study. “It raises the issue that we should be researching this more, I think.”
It is no coincidence that the discovery was made by the San Diego Zoo Wildlife Alliance, which operates the zoo and Safari Park. The nonprofit has played a key role in helping California condors navigate their way to the brink of extinction.
In 1982, only 22 of the iconic birds remained. By the end of 2019, that number had risen to 525, with 306 condors flying freely across California, Arizona, Utah, and Baja California. A captive breeding program run from Safari Park has been a big part of that rebound, and the Zoo and Safari Park have hatched more than 160 condors over the years.
Throughout this ongoing effort, researchers collected a large repository of condor blood, feathers, and tissue. DNA from these samples can reveal how closely related two condors are and how traits are passed down from generation to generation.
The researchers weren’t expecting any major surprises when they began testing samples from over 900 condors around 2013. That all changed when Leona Chemnick, then a researcher at the zoo’s Conservation Research Institute , told the director of genetics, Dr Oliver Ryder, that she needed to tell him about some puzzling findings.
Ryder, who was leaving the office, asked Chemnick to walk and talk as he made his way to his car. As they did, she explained that two captive-born condors didn’t appear to have genetic material from their fathers in any of the 21 different DNA regions that Chemnick checked (and rechecked).
What was also strange, she added, was that although each condor had two copies of DNA at each site, those copies were all identical.
This stopped Ryder dead in his tracks.
Did birds inherit only maternal copies, he asked?
Were they males?
“I said, ‘You just showed that there is parthenogenetic development in California condors,’ Ryder recalls. “I always get cold and goosebumps when I tell this story.”
Ryder and his colleagues reported their findings in the Journal of Heredity, the official publication of the American Genetic Association, a century old.
Most animals (including humans) inherit two sets of genes, one from mom and one from dad. That the researchers couldn’t find evidence that the birds had DNA from a father was a telltale sign that they didn’t.
The sex of hatched condors is also suitable. This is because while it may seem odd for a female to have male chicks on her own, the sex in birds and humans is not determined in the same way.
In men, women have two X chromosomes, while men have an X and a Y. There are exceptions, such as people with Klinefelter syndrome, who are XXY men, but this is the general rule. . In birds, it is the opposite. Males have two Z chromosomes while females have a Z and a W, which means that an unfertilized egg already has the genetic material to form a male chick.
It’s happened before, says Reshma Ramachandran, a researcher in the Department of Poultry Science at Mississippi State University, noting that reports of parthenogenesis in pigeons, quails, chickens and turkeys date back to the 1960s. these cases, the researchers believe there is a problem in the normal stages of cell division and development that causes eggs to contain only half of the DNA they need to form a living creature, which yields an egg with a full set of DNA from just one parent, although scientists haven’t fully worked out all the details.
“It was very interesting. I wasn’t totally surprised,” she said of the recent study. “Based on my research experience, I expect more incidents like the latter in nature and in domestic herds. “
But such cases are likely to be rare, adds Ramachandran, who has been part of teams that have researched parthenogenesis in turkeys and quails.
Despite 15 years of testing, scientists have never hatched a single quail through parthenogenesis, although they have seen clear signs of early embryo development in unfertilized eggs. However, they did grow a male turkey into adulthood, which other researchers have also observed in turkeys and chickens. But the bird was small and failed to fertilize the hens even after researchers tried artificial insemination.
These problems echo the fate of the two condors. One of the birds, hatched at Safari Park in 2001, was released back into the wild but died at Big Sur’s Ventana Condor Sanctuary in 2003. The other, hatched at the Los Angeles Zoo in 2009, died at the Big Sur Zoo. Oregon in 2017. The latter bird was underweight and suffered from a curved spine, and no condors reproduced before they died.
This raises the question of whether parthenogenesis in birds is important in nature. At present, scientists are not sure. Some speculate that rare and scattered species such as condors could benefit from asexual reproduction if they cannot find a mate. But this is only true if the offspring survive and pass their genes on to the next generation.
In other species, however, it is clear that parthenogenesis is an effective strategy. Some exclusively female lizard species only breed this way. In other cases, sharks, snakes and scorpions can alternate between sexual and asexual reproduction, using parthenogenesis when males are hard to find.
There have been a number of reports of zoos where Komodo dragons, pythons and other creatures have had offspring without ever seeing a male. But scientists would not have spotted the condor cases without their detailed genetic analysis. After all, says Ryder, if you see two birds tending to a brood of chicks, it’s natural to assume that the male is the father. This new discovery calls this hypothesis into question.
“One of the meanings of this is that we might think we understand life, but we shouldn’t take it for granted,” he said. “Maybe there is more going on than we know. “
– By Jonathan Wosen, The San Diego Union-Tribune