Could California Condor ‘Virgin Birth’ save the species?
Virgin birth—which involves the development of an unfertilized egg—has preoccupied humans for eons. And even if it can’t occur in mammalsthis appears to be possible in other backbone (vertebrate) animals, such as birds and lizards.
A recent article led by researchers at the San Diego Zoo reports that two fatherless male chicks were raised as part of a program to save the California condor from extinction. Could the species be restored by a single surviving female?
Sexual reproduction is fundamental in all vertebrates. Normally, an egg from a female needs to be fertilized by a sperm from a male, so each parent contributes a copy of the genome.
Violation of this rule, as with fatherless condor chicks, tells us a lot about why sexual reproduction is such a good biological strategy – as well as how sex works in all animals, including animals. humans.
How Fatherless Chicks Were Identified
The magnificent California condor, a type of vulture, is the largest flying bird in North America. In 1982, the species fell to a population of barely 22 peoplesparking an ambitious captive breeding program run by the San Diego Zoo, which has seen its numbers begin to increase.
With so few birds, the team had to be careful not to choose closely related parents, because a lack of genetic variation would produce less vigorous offspring and worsen the trend towards extinction.
The researchers conducted a detailed genetic study of the birds to avoid this, using condor-specific DNA markers that varied from bird to bird. They collected feathers, blood and eggshells from nearly 1,000 birds over 30 years.
By analyzing this data, they established parentage, confirming that half of each chick’s DNA markers came from a female and the other half from a male, as one would expect. They continued to follow the fate of hundreds of captive-bred chicks in the colony and after releasing them into the wild.
But there was something unusual about two male chicks, as detailed in the recent article. These chicks, which hatched several years apart from eggs laid by different females, had DNA markers that all came from the female parent. There was no trace of markers from the male she had been paired with.
The development of unfertilized eggs is called “parthenogenesis” (Greek words that literally mean “virgin creation”). It is quite common in insects and other invertebrates such as aphids and starfish, and can be accomplished by several different mechanisms. But it’s very rare in vertebrates.
Cases of parthenogenesis have been reported in housed fish and reptiles without males. In Tennessee, a lone female Komodo dragon held in captivity for many years has given up on finding a mate and produces three viable offspring alone. So did a female python and a boaalthough these parthenogenic offspring all died prematurely.
Some lizards, however, have adopted parthenogenesis as a way of life. There are exclusively female species in Australia and the United States where females lay eggs carrying only combinations of their own genes.
Parthenogenesis also occurs in domestic chickens and turkeys reared without a male, but the embryo usually dies. There are only a few reports of fatherless male turkeys reaching adulthood, and only one or two producing sperm.
How’s it going ?
In birds, parthenogenesis always results from an egg carrying a single copy of the genome (haploid). Eggs are made in a female’s ovary by a special kind of cell division called meiosis, which reshuffles the genome and also halves the number of chromosomes. Sperm cells are made by the same process in a man’s testicle.
Normally, an egg and a sperm fuse together (fertilization), incorporating the genomes of both parents and restoring the usual (diploid) number of chromosomes.
But in parthenogenesis, the egg is not fertilized. Instead, it achieves a diploid state either by fusing with another cell of the same division – which is normally abandoned – or by replicating its genome without the cell being divided.
So, rather than getting one genome from the mother and another from the father, the resulting egg only contains a subset of the mother’s genes in double dose.
Fatherless birds will always be males
Condors, like other birds, determine sex by the Z and W sex chromosomes. These work the opposite of the human XX (female) and XY (male) system, in which the SRY gene on the Y chromosome determines masculinity.
However, in birds, males are ZZ and females are ZW. Sex is determined by the assay of a gene (DMRT1) on the Z chromosome. The ZZ combination has two copies of the DMRT1 gene and results in a male, while the ZW combination has only one copy and results in a female .
Haploid eggs receive either a Z or a W from the ZW mother. Their diploid derivatives will therefore be ZZ (normal male) or WW (dead). The reason WW embryos cannot develop is that the W chromosome contains almost no genes, while the Z chromosome contains 900 genes that are vital for development.
Fatherless chicks must therefore be ZZ males, as has been observed.
Why Virgin Birth Fails
Is it possible that an endangered species of bird such as the condor could be resurrected from a single female survivor, by hatching a fatherless male chick and breeding with it?
Well not quite. Turns out parthenogens (fatherless animals) aren’t doing so well. Neither of the two fatherless condors produced offspring of their own. One died before reaching sexual maturity and the other was weak and submissive, making him a poor prospect for fatherhood.
In chickens and turkeys, parthenogenesis produces either dead embryos or weak hatchlings. Even female-only lizard species, although they appear hardy, are usually the product of a recent mixing of two species that messed up meiosis and left them with no other options. These species don’t seem to last long.
Why do parthenogens fare so badly? The answer goes to the heart of a fundamental biological question. That is: why do we have sex? One would think it would be more efficient if the mother’s genome was simply passed on to her clonal offspring without worrying about meiosis.
Variation is key
But the evidence indicates that it is not healthy to have a genome made up entirely of the mother’s genes. Genetic variation is critical to the health of an individual and their species. Mixing the genetic variants of the male and female parents is vital.
In diploid offspring with two parental genomes, good variants can span mutants. Individuals who inherit genes only from the mother may have two copies of a maternal mutant gene that weakens them – without a healthy version from a male parent to compensate.
The variation also helps protect populations from deadly viruses, bacteria and parasites. Meiosis and fertilization provide many rearrangements of different gene variants, which can confuse pathogens. Without this additional protection, pathogens could run amok in a population of clones, and a genetically similar population would contain no resistant animals.
So the ability of female condors to hatch fatherless chicks is unlikely to save the species. On the bright side, human efforts have now led to hundreds of women – and men – flying the skies of California.
Jenny Graves is the Distinguished Professor of Genetics and Vice-Chancellor of La Trobe University in Melbourne, Australia.