Parthenogenesis: the miracle of 'virgin births' in the animal kingdom

In Telford's Exotic Zoo, life found a way.

A casque-headed iguana has given birth to eight babies at the Shropshire wildlife park – without any contact with a male, via a phenomenon called parthenogenesis.

"When we confirmed the eggs were fertile without any contact with a male, our jaws hit the floor," zoo owner Scott Adams told the BBC. "For us, it's a powerful reminder that life finds a way in the most unexpected circumstances."

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What is parthenogenesis?

A type of asexual reproduction, in which females create offspring without fertilisation from male sex cells. The unfertilised eggs develop into embryos that are genetic clones of the mother. Basically, the female clones herself.

It actually predates sexual reproduction, which evolved to introduce more genetic variation. It's more common in plants or invertebrates than vertebrates – but it has been observed among fish, snakes, sharks, lizards, and even birds. What is "mind-boggling is that parthenogenesis isn't even that rare", said the Smithsonian Institution's National Museum of Natural History. It was first documented in Komodo dragons in 2006, but has since been seen in all "vertebrate lineages" except mammals.

"Komodos are famous for it," Colin Stevenson, head of education at Crocodiles of the World, told The Guardian. "They can reproduce normally, but every now and again, they pop out a parthenogenetic one. The trick is to work out what kicks it off."

Are there other cases?

In January, a baby swell shark, Yoko, was born in a Louisiana aquarium – although the two females in the tank "had not been in contact with a male in over three years", said a Shreveport Aquarium spokesperson. Yoko's birth could be the result of either parthenogenesis or delayed fertilisation – females of many shark species can store sperm inside themselves for months or even years.

Kevin Feldheim of Chicago's Field Museum told NPR that both explanations were possible, and that genetic testing on Yoko would be necessary to compare her to the sharks in the tank. But there have been documented cases of parthenogenesis involving more than half a dozen shark species, he said.

Recently, a new parthenogenetic species of spider was discovered in the Czech Republic. Dysdera parthenogenetica uses "thelytoky" parthenogenetic reproduction, in which "generations are formed by females arising from unfertilised eggs", said the study published in the Journal of Zoological Systematics and Evolutionary Research.

This species is similar to another, Dysdera hungarica, but when researchers put the two together, the parthenogenetic females refused to mate with males. They "did not respond to their courtship behaviour", said the study, and "their copulatory organs were reduced".

So what kicks it off?

"In general, we think parthenogenesis is a last-ditch effort for a female to pass on its genes, so when a female is isolated from conspecific males, she is able to undergo parthenogenesis," Feldheim told NPR.

"How parthenogenesis kicks in or what cues the females use to begin the process remains to be discovered."

What are the downsides?

Organisms born via parthenogenesis, known as parthenotes, "don't have the best track record when it comes to survivorship or fitness", said the Smithsonian. Every zebra shark parthenote has died before reaching sexual maturity. However, one female white-spotted bamboo shark not only survived to sexual maturity, but also gave birth through parthenogenesis.

Yoko is "thriving", said the aquarium, but shark pups born from rare reproductive events usually suffer health issues. "Should Yoko's time with us be brief, it will still leave an unforgettable legacy, contributing invaluable insights to the study of shark reproduction and conservation efforts."