Let’s Talk about Parthenogenesis
If a condor can give birth without a male — Can we?
This past week, a California condor surprised scientists by laying a fertilized egg and hatching a chick without any contribution from a Condor male. Or, as the National Geographic’s playful headline explains: “Endangered birds experience ‘virgin birth,’ a first for the species.” The phenomenon is called parthenogenesis, and it means the condor has managed, in a way, to ‘clone’ her genes. (Sort of. More on that later… And you thought the story couldn’t get weirder.)
It sounds a bit sci-fi, I know. How could something like this happen? There are plenty of species that clone themselves, actually, but they tend to escape our notice by being very small. Aphids, for instance, take advantage of the practice, giving birth to pregnant clones during high season. If you have every wondered how an infestation seemingly happens overnight, there you have it: if you needn’t waste time on courting or sex, and you have an astonishingly short gestation period, it’s fairly easy to multiply your numbers. There is, however, a problem. Perfect clones, by virtue of being the same, also have all the same weaknesses. That has been the whole point of evolutionary mutation and variation: in diversity we triumph. Aphids apparently know this, too, and sometimes they also reproduce sexually, just to spice things up (and give their genes a little kick).
In most cases, parthenogenesis isn’t the only or favored means of reproduction. Take the Komodo dragon, for instance. At London’s Chester Zoo, a large female named Flora hatched eight clones all by herself. In her case, it largely had to due with a lack of available males. Absent fathers (and sperm donors) mean trouble for any species, so it makes a certain sense if an animal can switch from sexual to asexual reproduction. We know of about 70 species of vertebrates who can pull off this trick — and now, the condor is numbered among them. It’s a good thing; their numbers were decimated by DDT until only 22 remained. Their comeback has been slow and full of obstacles, so some mother birds decided to do it for themselves (if you would like a comic summary, see Vinny Thomas doing a very good condor impression).
We do have an understanding of how this virgin-birth process comes about. It happens when a cell inside the female behaves like a sperm and fuses with an egg. “Both males and females carry out meiosis in which cells divide to form the respective sex cells, sperm or egg,” writes Jeanna Bryner at LiveSceince. In females, meiosis produces four “egg-progenitor cells,” basically egg-making material. One becomes the egg and the other three get reabsorbed by the female’s body. Except when they don’t. The result is a bit like the aphid clones, but not exactly. The condors and dragons will be related to their mothers, but not carbon copies. In fact, the condor hatched a male chick. Not all copies of genes are identical, and each gene has an alternate form (which is why we sometimes need two copies of a gene to ensure certain traits or characteristics). They are still less genetically diverse, since the shuffling of genes happened only from one parent, but the resulting offspring can grow up and breed sexually, too. It’s a species-preserving design.
So. What about humans? No mammals, alas, have reproduced through parthenogenesis. You might jump to the logical conclusion that this means we cannot. But it’s trickier than that. Mammals, like condors and lizards, also produce progenitor egg cells. And once in a while, one of those wily cells will fuse with the egg. In mammals, parthenogenesis can begin to occur— but this “parthenote” (cute name, isn’t it?) never lives longer that a few days. The reason is simple but a little hard to explain: “During sperm and egg formation in mammals, certain genes necessary for embryo development are shut down with a series of chemical marks, or imprints, some in the sperm, other in the egg.” That means normal growth and development requires both of these keys to be present, and that only happens when you have egg and sperm.
Except when it doesn’t. In 2004, the New Scientist published a study by Tokyo University of Agriculture in Tokyo, Japan. They had taken two female mice and, from them, created a third baby mouse: Kaguya. The scientists manipulated the nucleus of a female egg to make it more male-like, partly by genetically altering it to produce a protein called IGF-2.
There are, however, other means of reproducing without males — and it returns us to the idea of aphid clones. If you can manipulate genes, implanting a cloned nucleus into a hollowed out egg and then stimulating it to provoke growth, then you could implant that egg into a womb. In February of 2021, a team of researchers cloned the endangered black footed ferret, and before that, a Przewalski’s horse. San Diego Zoo scientists revive cells from a 30- and 40-year deep freeze respectively to achieve the seeming impossibility. In each case, all the genetic material necessary came from the original (deceased) animal, itself the result of sexual reproduction. No missing bits, no failed imprint. It’s not parthenogenesis, but it is another way to save a species.
Mammals like humans cannot take advantage of sperm-free reproduction as condors or dragons can. But humans could take advantage of cloning. Some already have; Panos Zavos, a maverick fertility doctor, claimed in 2009 to have cloned fourteen human embryos (though they were not brought to term). We don’t clone human beings, but not because we can’t. Unlike the natural processes developed by parthenogenic species, our attempts would require scientific experiment, and potential danger to mother, offspring, or both. Kaguya the mouse was one of only two successes among over 400 failures. Bioethics would never bet on such odds for human trials.
And yet, science does continue to make the process of cloning more efficient (Kurt the horse went from uncertain idea to playful colt in just over a year). If our environmental challenges, natural and man-made catastrophes, and shrinking birthrates continue, will we see increasing calls for human ‘virgin births’? It’s difficult to say. In the meanwhile, let’s hope the condors keep surprising us.