In 2016, Leonie, a zebra shark at Australia's Reef HQ Aquarium in Queensland, hatched three eggs, usually an unremarkable event. But there was something a little different about Leonie's new offspring: they don't have a father.
Sharks have been known to reproduce asexually in the past, but Leonie is the first known zebra shark to switch from more traditional sexual reproduction to parthenogenesis, as it is sometimes known.
An increasing number of vertebrates have been observed to resort to asexual reproduction under the right conditions, but parthenogenesis remains a poorly understood and rare phenomenon for most species. Leonie's species, Stegostoma fasciatum, has only seen one other known case of parthenogenesis, in Dubai. Australian researchers plan to follow Leonie and her offspring closely over the next several years to better understand how asexual reproduction works and why the switch took place.
Leonie was captured in the wild in 1999. In 2006, she was introduced to a male shark, and began laying eggs fertilized by him two years later. But in 2012, the two sharks were separated as the aquarium breeding program was scaled back. Ever since, Leonie has shared her tank with Lolly, one of her female offspring.
In 2016, both sharks produced eggs – not, in itself, a shocking occurrence, said Christine Dudgeon, a biologist with the University of Queensland and lead author of an article about it in the journal Nature.
"Much like a chicken, they will lay eggs if the conditions are good, whether they are fertile or infertile," she told The Guardian.
But this time, some of those eggs held baby sharks.
Sharks have been known to produce young asexually if there has been no prior sexual reproduction, so Lolly's offspring were not entirely surprising. Leonie was a different story.
"We thought [Leonie] could be storing sperm, but when we tested the pups and the possible parent sharks using DNA fingerprinting, we found they only had cells from Leonie," Dr. Dudgeon said in a statement.
Parthenogenesis is common in many insect species, but is rare among vertebrates, and completely unknown in mammal species, as Lucy Schouten previously reported for The Christian Science Monitor:
Parthenogenesis was discovered in the 1800s with domesticated birds, according to an article by Royal Society Publishing. The first studies involved turkeys. Many parthenogenetic offspring were born with abnormalities, but some could live and reproduce normally.
On a cellular level, parthenogenesis occurs when smaller cells produced alongside an egg fertilize the egg rather than decaying normally.
Experts once thought parthenogenesis was an adaptation of animals in captivity, since that is where it had been observed, but in 2012, researchers found cases of single-parent fish in the wild. Andrew Fields of Stony Brook University and his colleagues were sequencing the genome of 190 endangered sawfish, and they found seven whose "parents" were genetically identical – meaning both parents were the same fish.
So how unusual is Leonie?
"Leonie adapted to her circumstances, and we believe she switched because she lost her mate," said Dudgeon. "What we want to know now is could this occur in the wild and, if so, how often does it?"
This study comes after the IUCN Red List of Threatened Species identified the zebra shark as endangered last year. But while the ability to reproduce asexually under stress might seem like a good idea for an endangered species, Dudgeon says it's not quite that simple. Leonie's offspring are not clones; they contain only half their mother's genetic material. As a result, they might be unable to reproduce themselves, or have other genetic problems.
"Having the capacity to extend the life of that egg into maturity when that female can find a mate to reproduce may just be enough to enable those populations to hold on for a little bit longer," Dudgeon told the Brisbane Times. "However, because of the reduced diversity, it is not likely this is a long-term strategy and so as far as conservation actions go, we still have the responsibility to try and preserve and manage these populations."