It is considered the most expensive bird in the world: The southwestern US states have spent tens of millions of dollars since the 1980s to save the California condor. Hunting, lead poisoning, and habitat loss had brought North America’s largest land bird to the brink of extinction. The costly efforts were successful.
In the sky over California and some neighboring states, 30 years after they were almost extinct, around 500 condors are again circling from a breeding program. The expensive and sophisticated company has not only brought back to the skies over California a species of bird that has also been revered by indigenous peoples for millennia: it is currently causing a sensation among scientists. The genetic analysis of practically all condors living in this period as part of the breeding program, which has been running for almost 35 years, helped the vulture protectors of the San Diego Zoo Wildlife Alliance to make a discovery that not only gave them goose bumps, as Oliver said Ryder remembers. The head of nature conservation genetics at the San Diego Zoo, which is still quite young in the USA, and his colleagues discovered that two of the condor chicks from the offspring program were born biologically fatherless – by parthenogenesis or “maiden birth”.
“Neither chick showed any genetic contribution from a male bird.”
The researchers made the discovery while looking at the DNA profiles they created for each of the rare birds. This allows you to determine the sex of each bird, identify it individually and understand its ancestry. A routine job, actually. When analyzing the samples from two chicks, however, the results were very unusual, as Ryder reports. “Neither chick had any genetic contribution from a male bird, they were biologically fatherless,” he says. The genetic makeup of both birds, which had been conceived by different mothers in different years, was homozygous, i.e. genetically uniform, both had male sex chromosomes, but all markers were only inherited from their mothers. The goosebumps moment came when Ryder added up these details that only allowed one conclusion: “They were clearly parthenotes.”
Parthenotes are offspring that arise naturally from unisexual reproduction, in which an embryo that has not been fertilized by a sperm develops further and only contains the genetic material of the mother. The phenomenon is known from plants and from some animal groups, for example water fleas, some species of snails and lizards, and even species of sharks. So far, parthenogenesis has only been documented four times in birds, but always in domesticated animals such as turkeys, breeding pigeons or bred finches – but not in wild birds.
Over the years, the laboratories in San Diego have accumulated gene samples from over 900 different condors, which were obtained from the blood, eggshell membranes, tissue or feathers of the vultures. The genetic data serve to avoid inbreeding among the offspring and to determine how genetically diverse, and therefore viable, the slowly developing wild population is.
A look back to the 1980s helps to understand the immense effort involved. Back then, the feathered giant only began to be saved with the human-induced extinction of the species. When the wild population of the once widespread birds continued to shrink and in 1987 only a handful of birds were left in the wild, authorities and conservationists decided to capture all of the remaining condors. These birds formed the nucleus of the successful breeding program at the San Diego Zoo.
The case of sexually produced vultures is characterized by other peculiarities that make the professional world sit up and take notice. For one thing, the embryos in the eggs developed well and even hatched. Both birds were physically weak, but lived to be two and eight years old. So far, only chicken parthenotes have survived until hatching and later even reproduced, emphasize the researchers in their current genetics journal Journal of Heredity published first detailed analysis of the cases.
Sexless reproduction is probably more common in nature than previously thought
Even more exciting is the fact that the mothers of both birds had long-term partnerships with male condors in their breeding cages and in this way also produced numerous offspring in a sexual way. One gave birth to eleven chicks. The second female was mated to a male for more than 20 years and even hatched 23 chicks during this time. The discovery in San Diego thus overturns another tenet of biology: That unisex reproduction in birds only takes place when a female has no access to a male. According to this thesis, parthenogenesis could be a kind of emergency solution of nature to prevent the extinction of a species.
Even if the female condors in San Diego were not necessarily dependent on unisexual reproduction, they are among the rarest birds on earth. Is there an evolutionary survival strategy already at work here? “There is speculation in the literature that rarity could be a factor,” says Ryder. But this connection has not yet been really thoroughly investigated. However, the researchers are more certain that sexless reproduction occurs more frequently in nature than previously assumed. “I think that might be the case, and that is one of the other questions that emerged from our investigation,” says Ryder. He points out that in this case too, fatherless reproduction was only discovered because the entire population was genetically monitored without gaps – and not because vultures are genetically particularly capable of doing this.
Ryder is also certain that vulture Parthenotes, like chickens, are reproductive. This in turn opens up space for mental games to create very rare and difficult-to-breed bird species with the help of parthenogenesis in the laboratory. “This is not possible at the moment, but further studies could find the key to the phenomenon,” believes Ryder. “The best thing to do is to choose a common model species, not such an endangered species as the California condor.”
The decades-long struggle for the survival of the California condor attracted a lot of attention in the USA and has long since become a symbol of the determination not to simply accept the loss of an animal species. Now it could perhaps also become the starting point for completely new approaches in species protection. In any case, the researchers of the San Diego Zoo Wildlife Alliance have announced that they will continue the gene typing in order to discover further secrets of single sex reproduction.