Scientists Discover a Virgin Birth in a Crocodile

In January 2018, a female alligator at a Costa Rica zoo laid her eggs. It was strange: He had been living alone for 16 years.

While crocodiles may lay undeveloped sterile eggs, some of these nests look quite normal. And one of them – in a storyline familiar to anyone who has watched “Jurassic Park” – continues to mature in an incubator. In this case, life doesn’t, uh, find a waybecause the egg eventually produced a perfect baby crocodile but was stillborn.

In the papers out Wednesday in the journal Biology Letters, the research team reports that baby crocodiles are parthenogens – the product of virgin births, containing only genetic material from their parents. While parthenogenesis has been identified in creatures as diverse as king cobra, sawfish And California condors, it was first discovered in crocodiles. And because of the crocodile’s place on the tree of life, it implies that pterosaurs and dinosaurs may have been capable of such reproduction as well.

This is how a virgin birth occurs: As the egg matures in its mother’s body, it divides repeatedly to produce the final product with exactly half the genes needed for an individual. Three small cellular sacs containing chromosomes, known as polar bodies, form as a by-product. Polar bodies usually wither. But in vertebrates capable of parthenogenesis, a polar body sometimes fuses with the egg, creating a cell with the chromosomal complement necessary to form an individual.

That’s what appears to be happening in the alligator case, said Warren Booth, a professor at Virginia Tech who has studied the eggs. Booth is an entomologist whose main focus is bedbugs, but he does sideways in identifying parthenogenesis. Sequencing of the parthenogenetic crocodile genome showed that its chromosomes differ from those of the mother at the ends, where there is a slight rearrangement of its DNA – a sign of polar body fusion.

This is exactly what happens in parthenogenesis in birds, lizards and snakes, says Dr. Booth, showed that this group of animals inherited abilities from a common ancestor. But crocodiles evolved long before many other modern parthenogenetic animals, which suggests interesting possibilities about creatures that arose in between.

“This tells us that it was most likely the same for pterosaurs and dinosaurs,” said Dr. Booth.

Why do animals produce parthenogens? Although some parthenogens survive to maturity and mate, they are not always the healthiest creatures, says Dr. Booth. But the increasing ease of DNA analysis, which makes animals born this way easier to identify, has shown they are not all that rare.

“It’s much broader than people think,” he said.

It is possible that parthenogenesis gave a species the ability to survive long periods when no mates were available. A fresh individual, carrying most of the same genes as its parents, may live long enough to mate, allowing sexual reproduction, which tends to produce more violent offspring.

But it’s also possible that parthenogenesis is simply a trait that doesn’t have enough drawbacks to be ruled out by evolution, says Dr. Booth. It’s not necessarily a response to stress or even a lack of a partner. In 2020, scientists discovered that lizards can mate and then lay eggs in which some offspring are normal and some are parthenogeneous. This is Dr. hunch. Booth: This is an ability that can be turned on or off, and may be controlled by a single gene.

So did dinosaurs do it, as the discovery of parthenogenesis in crocodiles shows? Parthenogenesis is best confirmed by DNA analysis, a process that allows scientists to distinguish it from delayed conception. where the female stores sperm for six years before using it to fertilize eggs. Without the ability to retrieve dinosaur and pterosaur DNA, which do not survive in fossils, certainty is not available.

“We will never be able to prove that they can do it,” said Dr. Booth. “But it shows they have the ability.”

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