Reproduction without sex is more common than scientists thought

Sex is a tricky business, evolutionarily speaking. One problem is that sexually reproducing organisms must suffer the considerable faff of securing a mate (for the males of some species, the struggle to do so can be fatal). Another is that the mixing of two genomes into one offspring means that, per child, each parent gets only half its genes into the next generation rather than the full complement.

The fact that it is nonetheless widespread suggests that sex must have big advantages, too. One concerns genetic variety. In asexually reproducing species, the only source of variation is mutation. Sex, by contrast, produces genetically unique individuals every time. That may increase the chance that at least some survive a disease, or a change in environmental conditions, that prematurely kills the others.

Some animals, though, like to have things both ways. American crocodiles, for example, usually reproduce sexually. But in a paper published in Biology Letters on June 7th, a team led by Warren Booth, an entomologist at Virginia Polytechnic Institute and State University, show that this is not always true. It seems that female crocodiles can, under some conditions, reproduce via “parthenogenesis”—the production of fertile eggs without the involvement of a male.

The female crocodile in Dr Booth’s paper lives in a Costa Rican reptile park. Now 18 years old, she was obtained by the park at the age of two, and has been kept by herself ever since. The park staff were therefore surprised when, in 2018, she laid a clutch of 14 eggs. When workers examined the eggs—by holding them in front of a bright light, giving a murky view of the contents—seven appeared to be fertile.

Intrigued, the park incubated the fertile eggs. None hatched. Six contained embryos that had died early in their development. But one contained a fully developed crocodile fetus that was almost ready to hatch. The mother, it seemed, had given parthenogenetic reproduction a go, and very nearly succeeded.

Parthenogenesis is fairly common. Some insects, scorpions and worms, among others, are known to do it. But it was thought to be rare among vertebrates. That assumption, though, is changing. In the 1950s turkey farmers discovered that some of their hens had laid viable eggs despite never having had access to males. Lizards and snakes were added to the list in the 1960s and 1990s respectively. In 2021 researchers monitoring the critically endangered California condor noticed that some of the birds lacked genes from the males that were supposed to be their fathers.

Crocodiles are the newest members of the vertebrate-parthenogenesis club. Its growing membership raises questions about just how widespread the ability might turn out to be. Despite their differences crocodiles, lizards, snakes and birds (which are descended from dinosaurs) are all members of the clade Reptilia. The evolutionary distance between those species suggests the ability is an ancient one. Might that mean that other members of Reptilia—turtles, for example, or chameleons—could have it too? ■

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