March 1, 2024

Self-love is important, but we mammals are stuck on sex

If Galentine’s Day had an animal mascot, it would have to be one of the species whose females can reproduce without a mate. Almost all animals make the most of it the traditional way, combining eggs and sperm. But some have an alternative called parthenogenesis: no males needed.

No matter how many romantically frustrated mammals wish they could actually go it alone, a genetic quirk means we still need sexual reproduction. For now, parthenogenesis is for birds (and bees), fish and 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 parents in their cages — or any other males. The puppies’ DNA only matched that of their mothers.

Chemnick ran into Oliver Ryder, the zoo’s director of conservation genetics, on the way to her car and asked him about the strange data she was seeing. He explained to Mrs. Chemnick that such condor chicks must have come from eggs that were not fertilized by sperm.

“We were literally walking to the parking lot and had a eureka moment,” Dr. Ryder said. “We didn’t have time to dance or anything.”

When the two scientists and other colleagues published their discovery about parthenogenesis in 2021, the two unusual chicks, or parthenotes, were long gone. Both died young, aged almost 2 and almost 8. Their mothers had many other children, however, conceived with their partners in the usual way (despite the headlines declaring virgin births).

Each condor conception is a miracle of another kind. In 1982, when there were only 22 California condors left on the planet, conservationists began capturing all the birds and taking them into captivity in a desperate attempt to save the species. In 2022, there were 561 birds, most of them free in the wild.

A crucial part of growing this healthy condor population has been tracking the birds’ genetics, which allowed the discovery of the parthenote chicks. Since finding the first two, Ryder said, his team has discovered two more, although they died before they hatched.

How their mothers raised them is a bit unclear.

Condors, like most animals, carry two copies of each gene – one copy from each parent. To produce a sperm or egg, the animal must divide its genetic material in half. When the egg and sperm meet during sexual reproduction, they combine their genes to create a complete new genome.

To produce offspring without sperm, condor mothers must have duplicated the DNA of an egg. There are a few ways this could have happened, Dr. Ryder said, and his team is conducting a deeper analysis that should solve the mystery.

Other birds, including chickens and turkeys, have also achieved the feat. Then there are the reptiles, including Komodo dragons and other smart girls, who reproduce this way. Last year, scientists reported parthenogenesis in an American crocodile. There are even some species of snakes and lizards that reproduce only through parthenogenesis and have abandoned sex altogether.

Many insects and other invertebrates can reproduce without males. Certain sharks and other fish can, too. A captive white-spotted bamboo shark gave birth to several parthenotes, and one of them grew up to have fatherless children of his own.

At the Shedd Aquarium in Chicago, a female zebra shark named Bubbles had two parthenote pups in 2016, though both died shortly after hatching. Like California condors, Bubbles surprised scientists with her near-virgin birth because she was not alone at the time. She was living with two male sharks, who probably wouldn’t mind sharing their sperm.

No one knows whether a female can choose to reproduce on her own – say, if her current reproductive options are unsatisfactory – or whether parthenogenesis happens outside of her control.

“It would be fascinating if they could decide to do this voluntarily,” Ryder said.

Humans only noticed parthenogenesis when females alone had offspring or when researchers were monitoring the genes of a population. Given that many different branches of life have demonstrated this ability, many other types of females could be secretly reproducing on their own.

“It’s probably a lot more widespread than we think,” Ryder said.

Scientists are confident, however, that no mammalian mother will have fatherless children. We are harmed by something called genomic imprinting.

To understand imprinting, remember that animals split their paired genes in half to form a sperm or egg cell. Mammalian parents add one more detail to this process: they place chemical tags on certain groups of genes. The tags make these genes unreadable, as if the genetic instructions had been marked with a black marker.

Once a mammal’s sperm and eggs combine, these marked genes will remain silent. This means that even if the offspring still have two copies of each gene, they can only use their mother’s or father’s copy because the other copy is unreadable.

We can see printing in action when, for example, a lion and a tiger breed in captivity. The resulting big cat looks different – ​​a bulky liger or a small tigon – depending on which species is the mother and which is the father. In printed places, the hybrid is 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 parents. This is because many imprinted genes affect growth. The father’s genome modifications generally make his offspring grow larger, while the mother’s changes keep the babies at 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 will care for their offspring in the future.

Regardless of why we printed our genomes, the bottom line is that mammalian sperm and eggs need each other.

If a mother mammal tried to make a baby like Bubbles the shark did, by duplicating the genes in her own egg, her offspring would not develop. The genes she silenced would be completely absent. Other genes would be present at twice the usual dose, because the offspring would not have the father’s usually silent copy. This can also cause serious problems, Dr. Ferguson-Smith said.

Mammals, then, are trapped by sex. But some scientists are experimenting with ways to rescue endangered animals whose encounters are few or non-existent.

Dr. Ryder of the San Diego Zoo, for example, is involved in efforts to create embryos using frozen cells and then place the cloned embryos in surrogates from closely related species. So far, he has helped create a black-footed ferret clone named Elizabeth Ann and two Przewalski’s horses. The youngest cloned foal was born last year and was named Ollie, after Dr. Ryder.

Dr. Ryder’s colleagues are also using genetic technology to try to save the northern white rhino, a subspecies in serious 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 persuaded frozen cells from northern white rhinos to become stem cells. Ultimately, these stem cells could be transformed into eggs and sperm. But first, as a test, the researchers told the cells to turn into heart muscle.

When Dr. Ryder saw northern white rhino heart cells beating in a dish, it was as good as a Valentine’s Day.

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