In January 2018, a female crocodile in a Costa Rican zoo laid a clutch of eggs. That was peculiar: She’d been living alone for 16 years.
While crocodiles can lay sterile eggs that don’t develop, some of this clutch looked quite normal. And one of them — in a plot twist familiar to anyone who has watched “Jurassic Park” — continued to mature in an incubator. In this case, life did not, uh, find a way, as the egg eventually yielded a perfectly formed but stillborn baby crocodile.
In a paper out Wednesday in the journal Biology Letters, a team of researchers report that the baby crocodile was a parthenogen — the product of a virgin birth, containing only genetic material from its 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 because of where crocodiles fall on the tree of life, it implies that pterosaurs and dinosaurs might also have been capable of such reproductive feats.
Here’s how a virgin birth happens: As an egg cell matures in its mother’s body, it divides repeatedly to generate a final product with exactly half the genes needed for an individual. Three smaller cellular sacs containing chromosomes, known as polar bodies, are formed as byproducts. Polar bodies usually wither away. But in vertebrates that can perform parthenogenesis, one 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, said Warren Booth, an associate professor at Virginia Tech who has studied the eggs. Dr. Booth is an entomologist whose main focus is bedbugs, but he has an extensive sideline in identifying parthenogenesis. Sequencing of the parthenogenetic crocodile’s genome suggests that its chromosomes differ from the mother’s at their tips, where there’s been a little reshuffling of her DNA — a telltale sign of polar body fusion.
This is precisely what happens in parthenogenesis in birds, lizards and snakes, Dr. 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 the creatures that came in between.
“What this tells us is it’s very likely that this also happened in pterosaurs and dinosaurs,” Dr. Booth said.
Why do animals produce parthenogens? Though some parthenogens can survive to adulthood and mate, they are not always the healthiest creatures, Dr. Booth said. But the increasing ease of DNA analysis, which makes animals born this way easier to identify, has shown that they are not all that rare.
“It’s much more widespread than people think,” he said.
It’s possible that parthenogenesis gives a species the ability to survive extended periods when no mate is available. A fresh individual, bearing largely the same genes as its parent, might live long enough for a mate to arrive, thus allowing sexual reproduction, which tends to produce hardier offspring.
But it is also possible that parthenogenesis is simply a trait that doesn’t have enough downsides for evolution to weed it out, Dr. Booth said. It is not necessarily a response to stress or even a lack of mates. In 2020, scientists discovered that lizards can mate and then lay clutches of eggs where some are normal offspring and some are parthenogens. This is Dr. Booth’s hunch: It’s an ability that can be switched on or off, and it is perhaps controlled by a single gene.
So, did dinosaurs do it, as the discovery of parthenogenesis in crocodiles suggests? Parthenogenesis is best confirmed with DNA analysis, a process that has allowed scientists to tell it apart from delayed conception, where a female stores sperm for as long as six years before using it to fertilize eggs. Without the ability to retrieve dinosaurs’ and pterosaurs’ DNA, which does not persist in fossils, certainty is not available.
“We’ll never be able to prove they could do it,” Dr. Booth said. “But it suggests they had the ability.”
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