Scientists discover virgin birth in crocodile – 06/08/2023 – Science

Scientists discover virgin birth in crocodile – 06/08/2023 – Science


In January 2018, a female crocodile at a Costa Rican zoo laid a clutch of eggs. This was peculiar, because she had been living in isolation for 16 years.

Although crocodiles can lay sterile eggs that fail to develop, some of this clutch looked pretty normal. And one of them — in a twist familiar to anyone who watched “Jurassic Park” — continued to mature in an incubator. In this case, the egg ended up producing a perfectly formed but stillborn baby crocodile.

In an article published on Wednesday (7) in the journal Biology Letters, a team of researchers reports that the baby crocodile was parthenogenetic – the product of a virgin birth, containing only genetic material from the mother. While parthenogenesis has been identified in creatures as diverse as king cobras, sawfish and California condors, this is the first time it has been found in crocodiles.

And given the position of crocodiles on the tree of life it implies that pterosaurs and dinosaurs may also have been capable of such reproductive feats.

How does a virgin birth happen? As an egg matures in the female’s body, it repeatedly divides to generate a final product with exactly half the genes needed to create an individual. Three smaller cell sacs containing chromosomes, known as polar bodies, are formed as a by-product. Polar bodies usually disappear.

But in vertebrates that can perform parthenogenesis, a polar body sometimes fuses with the egg, creating a cell with the necessary complement of chromosomes to form an individual.

That’s what appears to have happened in the case of the crocodile, according to Warren Booth, an associate professor at Virginia Tech who has studied the eggs. Booth is an entomologist whose primary focus is bed bugs, but he has extensive parallel work in identifying parthenogenesis. Sequencing of the crocodile’s parthenogenetic genome suggests that its chromosomes differ from those of the mother at their ends, where there has been a small reorganization of the DNA – a telltale sign of polar body fusion.

That’s exactly what happens in parthenogenesis in birds, lizards and snakes, Booth said, suggesting that this group of animals inherited the ability from a common ancestor. But crocodiles evolved long before many other modern parthenogenetic animals, which suggests intriguing possibilities about midway creatures.

“What this tells us is that it most likely happened to pterosaurs and dinosaurs as well,” Booth said.

Why do animals produce parthenogens? While some parthenogenes can survive into adulthood and mate, they are not always healthy creatures, Booth said. But the increasing ease of DNA analysis, which makes it possible to better identify animals born this way, has shown that they are not so rare.

“It’s much more widespread than you think,” he said.

It is possible that parthenogenesis gives a species the ability to survive for long periods when no mate is available. A new individual, carrying basically the same genes as its parent, may live long enough for a mate to appear, thus allowing sexual reproduction, which tends to generate more resistant offspring.

But it’s also possible that parthenogenesis is simply a trait that doesn’t have enough downsides for evolution to eliminate it, Booth said. It’s not necessarily a response to stress or even a lack of partners.

In 2020, scientists discovered that lizards can mate and then lay eggs where some are normal offspring and others are parthenogenetic. This is Booth’s hunch: it’s an ability that can be turned on or off, and perhaps it’s controlled by a single gene.

So did dinosaurs do this, as the discovery of parthenogenesis in crocodiles suggests? Parthenogenesis is best confirmed with DNA analysis, a process that has allowed scientists to differentiate it from delayed conception, in which a female stores sperm for up to six years before using it to fertilize eggs. Without the ability to recover dinosaur and pterosaur DNA, which does not persist in fossils, there is no certainty.

“We’ll never be able to prove that they had that ability,” Booth said. “But this suggests so.”

Translated by Luiz Roberto M. Gonçalves.



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