Leonie the zebra shark (Stegostoma fasciatum) met her male partner at an aquarium in Townsville, Australia, in 1999. They had more than two dozen offspring together before he was moved to another tank in 2012.

From then on, Leonie did not have any male contact. But in early 2016, she had three baby sharks.

Intrigued, Christine Dudgeon at the University of Queensland in Brisbane, Australia, and her colleagues began fishing for answers.

One possibility was that Leonie had been storing sperm from her ex and using it to fertilise her eggs. But genetic testing showed that the babies only carried DNA from their mum, indicating they had been conceived via asexual reproduction.

Some vertebrate species have the ability to reproduce asexually even though they normally reproduce sexually. These include certain sharks, turkeys, Komodo dragons, snakes and rays.

However, most reports have been in females who have never had male partners.

There are very few reports of asexual reproduction occurring in females with previous sexual histories, says Dudgeon. An eagle ray and a boa constrictor, both in captivity, are the only other female animals that have been documented switching from sexual to asexual reproduction.

“In species that are capable of both reproductive modes, there are quite a few observations of switches from asexual to sexual reproduction,” says Russell Bonduriansky at the University of New South Wales in Sydney. “However, it’s much less common to observe switches in the other direction.”

In sharks, asexual reproduction can occur when a female’s egg is fertilised by an adjacent cell known as a polar body, Dudgeon says. This also contains the female’s genetic material, leading to “extreme inbreeding”, she says. “It’s not a strategy for surviving many generations because it reduces genetic diversity and adaptability.”

Nevertheless, it may be necessary at times when males are scarce. “It might be a holding-on mechanism,” Dudgeon says. “Mum’s genes get passed down from female to female until there are males available to mate with.”

It’s possible that the switch from sexual to asexual reproduction is not that unusual; we just haven’t known to look for it, Dudgeon says.

Bonduriansky agrees. “It would seem to be highly advantageous,” he says. “It could be much more common than we currently realise.”

Source: New Scientist