California Condors Are Capable of Asexual Reproduction
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.