Scientists discover a virgin birth in a crocodile

COSTA RICA – In January 2018, a female crocodile in a Costa Rican zoo laid a clutch of eggs. The peculiar thing: She had been living alone for 16 years.

While crocodiles can lay sterile eggs that do not develop, some of this clutch looked quite normal.

And one of them – in a development reminiscent of the movie Jurassic Park – continued to mature in an incubator. This egg eventually yielded a perfectly formed but stillborn baby crocodile.

In a paper out on Wednesday in the journal Biology Letters, a team of researchers reported 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 is 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 by-products. 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 is what appears to have happened in the case of the crocodile, said Associate Professor Warren Booth of Virginia Tech who has studied the eggs.

Prof 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 has 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, Prof 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,” Prof Booth said.

But why do animals produce parthenogens? Though some parthenogens can survive to adulthood and mate, they are not always the healthiest creatures, Prof Booth added.