Scientists discover a virgin birth in a crocodile
In January 2018, a female crocodile laid a clutch of eggs at a Costa Rican zoo. That was strange: she had lived alone for 16 years.
Although crocodiles can lay sterile eggs that do not develop, part of this clutch looked quite normal. And one of them – in a plot twist familiar to anyone who has seen “Jurassic Park” – continued to grow up in an incubator. In this case, life found no waybecause the egg ended up yielding a perfectly formed but stillborn baby crocodile.
In a paper Wednesday in the journal Biology Letters, a team of researchers reports that the baby crocodile was a parthenogen – the product of a virgin birth, containing only genetic material from its mother. Although 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 may have been capable of such reproductive feats as well.
Here’s how virgin birth happens: As an egg matures in its mother’s body, it divides repeatedly to generate a final product with exactly half the genes an individual needs. Three smaller cellular sacs of chromosomes, known as polar bodies, are formed as byproducts. Polar bodies usually wither. But in vertebrates that can perform parthenogenesis, sometimes one polar body 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 who mainly focuses on bed bugs, but he does an extensive sideline in identifying parthenogenesis. Sequencing of the parthenogenetic crocodile’s genome suggests that its chromosomes differ from the mother’s at the ends, where her DNA has been rearranged a bit — a telltale sign of polar body fusion.
This is exactly what happens in parthenogenesis in birds, lizards and snakes, said Dr. Booth, suggesting that this group of animals inherited the ability from a common ancestor. But crocodiles evolved long before many other modern parthenogenetic animals, suggesting intriguing possibilities about the creatures that got in between.
“What this tells us is that it’s very likely that this also happened in pterosaurs and dinosaurs,” said Dr. booth.
Why do animals produce parthenogens? While some parthenogens can survive to adulthood and mate, they aren’t always the healthiest of creatures, said Dr. booth. But the increasing ease of DNA analysis, making animals born this way easier to identify, has shown that they are not so rare.
“It’s much more widespread than people think,” he said.
It is possible that parthenogenesis gives a species the ability to survive for extended periods when a mate is not available. A fresh individual, carrying largely the same genes as its parent, could live long enough for a mate to arrive, enabling sexual reproduction leading to stronger offspring.
But it’s also possible that parthenogenesis is just a trait that doesn’t have enough drawbacks for evolution to eradicate it, said Dr. booth. 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 some of which are normal offspring and some are parthenogens. This is the conjecture of Dr. Booth: It’s an ability that can be turned on or off and maybe 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 allows scientists to differentiate it from delayed conception, where a female stores sperm for up to six years before using it to fertilize eggs. Without the ability to recover the DNA of dinosaurs and pterosaurs, which is not present in fossils, there is no certainty.
“We’ll never be able to prove they can,” Dr. Booth said. “But it suggests they had the opportunity.”