— Some female birds, reptiles and other animals can make a baby on their own. But for mammals like us, eggs and sperm need each other.
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If Galentine’s Day had an animal mascot, it would have to be one of the species whose females can reproduce without a mate. Nearly all animals make more of themselves the traditional way, by combining eggs and sperm. But some have an alternative called parthenogenesis: no males needed.
No matter how many romantically frustrated mammals have wished they could truly go it alone, though, a genetic quirk means we still need sexual reproduction. For now, parthenogenesis is for the birds (and the bees), the fishes and the reptiles.
One of the most famous recent cases of parthenogenesis involved California condors, an endangered species. In 2013, Leona Chemnick, then a researcher at the San Diego Zoo Wildlife Alliance, discovered that two male chicks in the condor breeding program had DNA that didn’t match that of the fathers in their cages — or of any other male. The chicks’ DNA only matched their moms’.
Ms. Chemnick caught Oliver Ryder, the zoo’s director of conservation genetics, on the way to his car and asked him about the odd data she was seeing. He explained to Ms. Chemnick that any such condor chicks must have come from eggs that were not fertilized by sperm.
“We were literally walking out to the parking lot and had this eureka moment,” Dr. Ryder said. “We didn’t have time to dance or anything.”
By the time the two scientists and other colleagues published their parthenogenesis finding in 2021, the two unusual chicks, or parthenotes, were long gone. They’d both died young, at almost 2 years and almost 8. Their mothers both had many other offspring, though, conceived with their mates in the usual way (despite headlines declaring virgin births).
Every condor conception is a miracle of another kind. In 1982, when only 22 California condors remained on the planet, conservationists began trapping every bird and bringing them into captivity in a desperate bid to save the species. In 2022, the birds numbered 561, most of them free in the wild.
A crucial part of growing that healthy condor population has been tracking the birds’ genetics, which allowed the discovery of the parthenote chicks. Since finding the first two, Dr. Ryder said, his team has discovered two more, although they died before hatching.
How their moms made them is a bit murky.
Condors, like most animals, carry two copies of every gene — one copy from each parent. To make a sperm or egg cell, an animal must divide its genetic material in half. When egg and sperm meet during sexual reproduction, they combine their genes to create one complete new genome.
To make chicks without any sperm, the condor moms must have doubled the DNA from an egg. There are a few ways this could have happened, Dr. Ryder said, and his team is conducting a deeper analysis that should resolve the mystery.
Other birds, including chickens and turkeys, have also accomplished the feat. Then there are the reptiles, including Komodo dragons and other clever girls, that have been found to reproduce this way. Last year, scientists reported parthenogenesis in an American crocodile. There are even some snake and lizard species that reproduce only through parthenogenesis and have given up sex entirely.
Many insects and other invertebrates can reproduce without males. Certain sharks and other fishes can, too. One captive whitespotted bamboo shark bore several parthenotes, and one of those grew up to have her own fatherless offspring.
At Shedd Aquarium in Chicago, a female zebra shark named Bubbles had two parthenote pups in 2016, though both died shortly after hatching. Like the California condors, Bubbles surprised scientists with her quasi-virgin birth because she wasn’t alone at the time. She was living with two male sharks, which presumably wouldn’t have minded sharing their sperm.
No one knows whether a female can choose to reproduce on her own — say, if her current breeding options are unsatisfying — or whether parthenogenesis happens outside her control.
“It would be fascinating if they could willfully decide to do that,” Dr. Ryder said.
Humans have only noticed parthenogenesis when solo females had young or when researchers were monitoring a population’s genes. Given how many different branches of life have demonstrated the ability, though, many more kinds of female animals could be secretly reproducing on their own.
“It’s probably much more widespread than we think,” Dr. Ryder said.
Scientists are confident, though, that no mammal mother is having fatherless babies. We’re hampered by something called genomic imprinting.
To understand imprinting, recall that animals divide their paired genes in half to make a sperm or egg cell. Mammal parents add one more flourish to this process: They put chemical tags onto certain clusters of genes. The tags make those genes unreadable, as if the genetic instructions were struck through with a black marker.
After a mammal’s sperm and egg cells combine, those marked genes will stay silent. That means even though the offspring still has two copies of every gene, it may only use the copy from its mother or from its father, because the other copy is unreadable.
We can see imprinting in action when, for example, a lion and a tiger breed together in captivity. The resulting big cat looks different — a bulky liger or a petite tigon — depending on which species is the mom and which is the dad. At the imprinted sites, the hybrid is either all lion or all tiger.
“It’s really difficult to understand why this process evolved,” said Anne Ferguson-Smith, a developmental geneticist at the University of Cambridge.
Scientists have suggested that imprinting reflects a kind of evolutionary battle between the parents. That’s because many imprinted genes affect growth. The father’s modifications to the genome generally make his offspring grow bigger, while the mother’s changes keep the babies a more manageable size.
However, Dr. Ferguson-Smith suspects the true story is more complex. Some imprinted genes affect the offspring’s brain and behavior, or even how they’ll care for their own young in the future.
Regardless of why we imprint our genomes, the result is that mammals’ sperm and eggs need each other.
If a mammal mom tried to make a baby the way Bubbles the shark did, by doubling the genes from her own egg, her offspring wouldn’t develop. Genes that she silenced would be totally absent. Other genes would be present at twice the usual dose, because the offspring would be missing the usually silent copy from a father. This can also cause serious problems, Dr. Ferguson-Smith said.
Mammals, then, are stuck with sex. But some scientists are experimenting with ways to rescue endangered animals whose dating pools are small or nonexistent.
Dr. Ryder at the San Diego Zoo, for instance, is involved in efforts to create embryos using frozen cells and then put the cloned embryos into surrogates of closely related species. So far he has helped to create a black-footed ferret clone named Elizabeth Ann and two Przewalski’s horses. The younger cloned foal was born last year and named Ollie, in Dr. Ryder’s honor.
Dr. Ryder’s colleagues are also using genetic technology to try to save the northern white rhinoceros, a subspecies in dire trouble — only two are alive. A few years ago, he said, researchers at the San Diego Zoo Wildlife Alliance took a step in that direction.
They coaxed frozen cells from northern white rhinos to become stem cells. Ultimately, those stem cells could be turned into eggs and sperm. But first, as a test, the researchers told the cells to become heart muscle.
When Dr. Ryder saw northern white rhino heart cells beating in a dish, it was as good as a valentine.
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