Flying Solo: Parthenogenesis Discovered in California Condors
Genetic analyses uncover asexual reproduction by two female California condors despite access to fertile mates.


Christie Wilcox
Oct 29, 2021
ABOVE: A California condor tending to a chick
JOSEPH BRANDT/USFWS

When the population of California condors (Gymnogyps californianus) dropped to fewer than two dozen birds in 1982, some conservationists thought the species was doomed. But the California Condor Recovery Program successfully bred the animals back from the brink. As a part of that program, researchers collected DNA samples from the birds to gain insights into the genetic diversity of the population and to reduce potential inbreeding. Those molecular samples have now revealed something completely unexpected: two of the California condor females produced young without a male partner, a phenomenon called parthenogenesis.

The asexually produced offspring were especially surprising to scientists because both female birds were housed with males that sired other offspring with them before and after the unfertilized yet viable eggs were produced (one in 2001 and one in 2009). “Why it happened? We just don’t know,” Oliver Ryder, study coauthor and director of conservation genetics for the San Diego Zoo Wildlife Alliance, tells National Geographic. “What we do know is that it happened more than once, and it happened to different females.”

“Will it happen again? I rather believe so,” he says.

According to the report of the discovery, published October 28 in the Journal of Heredity, both of the parthenogenically produced offspring were males. In condors, as with most birds, females possess two different chromosomes (ZW) and males two of the same (ZZ), so the fact that the offspring were males suggests they only received half of their dam’s genes. When researchers looked at 21 microsatellite markers—repetitive sequences with alleles of differing length in a population, which are often used in parentage analyses—each bird had two copies of its expected dam’s markers, and none of its presumed sire’s. Taken together, these data suggest that in both cases, the half of the female’s genome present in an unfertilized egg was duplicated to produce the chick’s genome. “The two chicks identified as parthenotes were the only individuals from among 911 condors in our genotype database that were homozygous at all examined loci,” the authors write.

Whether parthenogenesis contributes meaningfully to the condor population or can be exploited to aid in the animals’ conservation remains to be seen, experts say. Both of the parthenotes were relatively small and died before becoming sexually mature, at 1.9 and 7.9 years old. Condors can live into their 50s.

 
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Other known examples of parthenogenesis in birds have almost all died before hatching, University of Tulsa evolutionary biologist Warren Booth tells Wired. That these condors lived as long as they did might suggest viable parthenogenetic offspring are possible in the species or raptors more generally, he says, so he considers the paper “one of the most important studies in the field of parthenogenesis and birds in a long time.”

“These findings now raise questions about whether this might occur undetected in other species,” Ryder tells the Associated Press. “The only reason we were able to identify that this had happened [in the condors] is because of these detailed genetic studies,” he says to National Geographic. “So, the birds in your backyard, are they occasionally producing a parthenogenetic chick? Nobody’s looking in deep enough detail to answer that question.”