(MENAFN – The Conversation) Virgin birth – which involves the development of an unfertilized egg – has preoccupied humans for eons. And while this can’t happen in mammals, it seems to be possible in other backbone (vertebrate) animals, such as birds and lizards.
A recent article edited by researchers at the San Diego Zoo in the United States reports two orphaned male chicks being 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 must be fertilized by a sperm from a male, so each parent contributes a copy of the genome.
Violating this rule, as with orphaned 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 humans.
How orphaned chicks were identified
The magnificent California condor, a kind of vulture, is the largest flying bird in North America. In 1982, the species fell to a population of just 22 individuals, triggering an ambitious captive breeding program led by the San Diego Zoo, which began to increase in numbers.
With so few birds, the team had to be careful not to choose closely related parents, as a lack of genetic variation would produce less vigorous offspring and accentuate the slide to extinction.
The researchers conducted a detailed genetic study of birds to avoid this, using DNA markers specific to condors and which 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 were from a female and the other half from a male, as you 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.
There are currently around 500 living condors in the world. Shutterstock Virgin birth
The development of unfertilized eggs is called “parthenogenesis” (Greek words which literally mean “virgin creation”). This is quite common in insects and other invertebrates like aphids and starfish, and can be accomplished by several different mechanisms. But it is very rare in vertebrates.
Cases of parthenogenesis have been reported in fish and reptiles housed without males. In Tennessee, a solitary female Komodo dragon held in captivity for many years gave up on finding a mate and produced three viable offspring on her own. The same goes for a female python and a boa, although these parthenogenic descendants 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 in which females lay eggs carrying only combinations of their own genes.
Parthenogenesis also occurs in domesticated chickens and turkeys raised in the absence of a male, but the embryo usually dies. There are only a few reports of fatherless male turkeys that have reached adulthood, and only one or two that have produced 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 shuffles the genome and also halves the number of chromosomes. Sperm are made by the same process in a man’s testicles.
Normally, an egg and a sperm merge (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 dumped – 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 through the Z and W sex chromosomes. These function in opposition to 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. Gender is determined by the assay of a gene (DMRT1) on the Z chromosome. The ZZ combination has two copies of the DMRT1 gene and makes a male, while the ZW combination has only one copy and makes a female.
Read more: How Birds Become Male or Female, and Sometimes Both
Haploid eggs are given either a Z or a W from mother ZW. Their diploid derivatives will therefore be ZZ (normal male) or WW (dead). The reason WW embryos cannot develop is that the W chromosome contains virtually no genes, while the Z chromosome has 900 genes which are vital for development.
Orphan chicks must therefore be ZZ males, as has been observed.
Why the virgin birth fails
Is it possible that an endangered bird species such as the condor could be resuscitated from a single survivor, hatching an orphaned male chick and breeding with it?
Well, not exactly. It turns out that parthenogens (fatherless animals) don’t do so well. Neither of the two orphaned condors produced their own offspring. One died before reaching sexual maturity and the other was weak and submissive, making it a bad prospect of fatherhood.
In chickens and turkeys, parthenogenesis produces either dead embryos or weak newborns. Even the only female lizard species, although they seem hardy, are usually the product of a recent mix of two species that spoiled meiosis and left them with no other options. These species do not seem to last long.
Why do parthenogens hurt so much? The answer goes to the heart of a fundamental biological question. That is to say: why do we have sex? One would think that it would be more efficient if the mother’s genome was simply passed on to her clonal offspring without worrying about meiosis.
Variation is the 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 essential for the health of an individual and their species. The mixing of genetic variants of male and female parents is vital.
Read more: These sex-hungry creatures pick up new genes from other ponds
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 mutant maternal 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 genetic variants, which can outwit pathogens. Without this extra protection, pathogens could unleash themselves in a population of clones, and a genetically similar population would not contain resistant animals.
It is therefore unlikely that the ability of female condors to hatch fatherless chicks will save the species. On the bright side, human efforts have now led hundreds of women – and men – to fly through the Californian skies.
Legal warning: MENAFN provides the information “as is” without warranty of any kind. We do not accept any responsibility for the accuracy, content, images, videos, licenses, completeness, legality or reliability of the information contained in this article. If you have any complaints or copyright issues related to this item, please contact the supplier above.