Charlotte the stingray was the subject of the most famous fake pregnancy. Some claimed she was pregnant all by her lonesome—which is called parthenogenesis. Her caretaker thought she’d been impregnated by a shark. Last week, poor Charlotte was diagnosed as just plain sick. But her case ignited a great debate over why animals have ‘virgin births’.
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How does that even happen—the virgin birth?
First, a definition: In normal circumstances, there is sperm and an egg. And when the twain meet, there is often a baby—with half the genetic information of each parent. The so-called virgin birth is called parthenogenesis: “Stemming from the Greek, parthénos, meaning ‘virgin’, and genesis, meaning ‘creation’, it is where an egg develops into an embryo without being fertilised by sperm.” In parthenogenesis, one of two things happen (brace yourself for Bio 101 deja vu):
One: The ovaries produce eggs by the process of meiosis—so each cell divides, but has half the mum’s genetic info. The byproduct of that process are smaller cells called polar bodies—that ‘mate’ with the eggs:
In one version of parthenogenesis called automixis, an animal can merge a polar body with an egg to produce offspring. This process, which has been documented in sharks, slightly shuffles the mother’s genes to create offspring that are similar to the mother but not exact clones.
Two: In the other version, the cells divide and create exact copies of themselves—in a process called mitosis:
Because these cells never undergo the gene-jumbling process of meiosis, offspring produced this way are clones of their parent, genetically identical. This form of parthenogenesis is more common in plants.
FYI: Parthenogenesis is not rare at all among insects. And it has been observed in 80 vertebrate species—about half of which are fish or lizards. Scientists also discovered two cases of California condor chicks hatching from unfertilized eggs—which is remarkable among birds. They think there may be a whole lot more virgin births happening in the wild than we suspect.
The downer: Because both the offspring contain the genetic information of a single parent they are a "highly inbred individual.” While pathogenesis sounds cool, it isn’t ideal from an evolutionary point of view. If so, why do we have virgin births at all? There are two schools of thought on that question.
Theory #1: A response to scarcity
The theory of evolution relies on the core premise that all living things want to pass on their genes—and will do all sorts of things just to achieve that goal. Females resort to parthenogenesis as a “last ditch” attempt to pass on their genes—when there’s no male around. That was one hypothesis about Charlotte—who had been swimming in a tank with no male stingray in the vicinity. Another example, a zebra shark named Leonie who had two rounds of parthenogenetic eggs.
The interesting bit: Leonie had babies the old-fashioned way before she ended up in a male-less desert. So researchers hypothesise that parthenogenesis is not an accident—but an evolutionary strategy:
For most animals, sexual reproduction is a better option than parthenogenesis. It gives their babies more diverse genes, and that makes them stronger. But if there are no males around and sexual reproduction is off the table, then maybe something can be triggered in some females’ bodies, letting them pursue this alternative. So a shark like Leonie, removed from males for a long time, could start taking new measures.
That’s why it may often be a response to the threat of extinction—as with female smalltooth sawfish in Florida.
But, but, but: Because Mother Nature just loves spitting up exceptions to the rule—scarcity isn’t always a factor:
In certain insects, salamanders, and flatworms, the presence of sperm serves to trigger parthenogenesis. Sperm cells launch the process by penetrating the egg, but the sperm later degenerates, leaving only the maternal chromosomes. In this case, sperm only sparks an egg’s development––it makes no genetic contribution.
In this case, the role of the sperm is to play catalyst for a parthenogenetic birth.
Separate point to note: In a recent study, scientists genetically engineered a sexually reproducing fruit-fly species to deliver virgin larvae. This has huge implications for recent efforts to eliminate pests like mosquitoes. In Florida and other places, researchers are tampering with the genes of male mosquitoes to kill future generations (explained here). But this tactic may not work in the long run—if the females (who spread the disease) begin birthing offspring by themselves.
Theory #2: It’s meaningless
Another set of scientists point out that babies produced by ‘virgin births’ are usually duds. That was the conclusion of an artificial insemination trial in zebra sharks, where scientists studied the difference between normal and parthenogenetic pups:
The study found parthenotes (individuals generated by parthenogenesis) lived on average for a year less, with many displaying behaviour that weakened their chances of survival, such as lopsided swimming, spinning, headstands, and difficulty feeding.
That doesn’t sound like an ideal evolutionary outcome for any species.
Kinda like the tailbone: According to some scientists like Warren Booth, parthenogenesis is a feature that has stuck around long past its ‘use by’ date. From an evolutionary perspective: “It makes [parthenogenetic snakes] the most inbred thing that you can think of in a vertebrate system… So they’re ... they’re not that great.” That’s why they are not any kind of ‘Hail Mary’—but just a quirk of evolution:
At least in the snakes he’s looked at, he thinks these offspring are just too inbred to meaningfully carry along the torch to another generation. Instead, he thinks that this ability to sort of randomly, occasionally reproduce parthenogenetically is genetic… If that’s the case, he says, then this is potentially just a vestigial thing that popped out in some ancient vertebrate ancestor and that it’s being passed along from generation to generation. But the species would be fine if it eventually faded out.
But, but, but: Because nature is maddening, the parthenogenetic offspring of those endangered sawfish seem to be doing just fine:
There was a general feeling that vertebrate parthenogenesis was a curiosity that didn’t usually lead to viable offspring… The seven parthenogens we found looked to be in perfect health and were normal size for their age. This suggests parthenogenesis is not a reproductive dead end, assuming they grow to maturity and reproduce.
Hmm, maybe not exactly like a tailbone.
Umm, what about humans?
Nope, it can’t happen among mammals—with apologies to the Virgin Mary. And here’s why:
No mammals are known to reproduce this way because unlike simpler organisms, mammals rely on a process called genomic imprinting. Like a molecular stamp, imprinting labels which genes are from mom and which are from dad. For mammals such as humans, this means that certain genes are switched on or off depending on the contributing parent. If there were only a single parent, some genes would fail to activate altogether, making viable offspring impossible.
Point to note: There are more than 30 imprinted genes that become active only if they come from sperm. And there are another 30-plus that only work if they come from the mother. So any sci-fi scenario of women spontaneously reproducing by themselves will remain the stuff of fantasy—at least without the intervention of technology.
The bottomline: Conservationists are increasingly considering it as a solution to save animals from extinction—especially in extreme conditions produced by climate change. After all, virgin births offer all sorts of benefits:
Parthenogenesis enables a population to expand twice as rapidly as a sexually reproducing population. It allows for fast reproduction and population growth without the requirement for fertilisation. In fact, females who reproduce via parthenogenesis can generate the same number of children as sexually reproduced females while using half the resources.
Good thing it isn’t possible for humans lol!
Reading list
Vox has the best overview of parthenogenesis. Washington Post has the sawfish study—while the New York Times has more on those condors. BBC News and The Conversation look at why this happens. Earth.org looks at whether parthenogenesis can protect endangered species. LiveScience has a list of all the species that can have virgin births.
