Leonie the leopard shark’s switch to asexual reproduction is a world first
By March 2017
There are two tanks at Reef HQ Aquarium in Townsville, Queensland. The Reef Tank holds sharks, more than 100 species of fish, and creatures such as sea stars, feather stars and snails. The Predator Tank houses a tawny nurse shark, blacktip and whitetip reef sharks, and schools of trevally. It also houses Leonie the leopard shark, who became, back in January, the world’s most famous shark.
Leonie is a Stegostoma fasciatum (commonly known as a zebra shark, despite her spots … it’s complicated), and two years ago aquarium staff noticed her doing something unusual. Her regular mate, Leo, had been removed from the tank – he and Leonie had produced 26 pups in seven seasons – and Leonie was still laying eggs. That wasn’t surprising: sharks, like chickens, lay eggs whether fertilised or not. What was extraordinary was that Leonie’s eggs were developing embryos despite not being fertilised by a male. She’d achieved something that, while relatively common in plants and invertebrates, sounds nigh on miraculous in vertebrates: asexual reproduction, or reproduction without mating. The precise term, “parthenogenesis”, comes from the Greek for “virgin birth”.
The Reef HQ staff called Christine Dudgeon, a marine biologist at the University of Queensland who had worked with Reef HQ’s sharks before. Dudgeon suggested two possible explanations: sperm storage (recorded in some sharks for up to four years) or parthenogenesis. “Sperm storage is usually the reason given for this sort of thing,” she says. “I thought, That’s what’s happened; no problem. Then we started seeing evidence of parthenogenesis instead.”
That breeding season (2014–2015), all of Leonie’s prospective hatchlings died. But the next year she laid more eggs with embryos developing inside. And her daughter Lolly (one of Leo and Leonie’s pups who lived in the same tank) laid eggs with embryos as well. She had just reached maturity and had never mated with a male. This time, several hatchlings emerged and stayed alive – three from Leonie’s four embryos, and one from Lolly’s two. Dudgeon examined samples from the pups, and none showed any sign of paternal DNA. Previous examples of parthenogenesis had come from sharks who were just starting to mature, like Lolly. Leonie had a sexually reproductive history, so she’d switched from one kind of reproduction to another. And that made her more rare again.
A paper – with Dudgeon as lead author – was published in Nature’s ‘Scientific Reports’ on 16 January this year. It went global. Dudgeon, on holiday on New South Wales’ North Coast, stood in the one spot with reliable phone reception and did interview after interview. More than 70 news reports were broadcast about Leonie. Mayim Bialik, the neuroscientist who plays Dr Amy Farrah Fowler in The Big Bang Theory, posted the story on Facebook with the comment “Sending this zebra shark all kinds of props right now. Mad respect, sister.” It received more than 13,000 reactions. One German news website ran with the headline ‘Virginal Conception: Shark Lady Leonie Does Not Need a Man’.
Dudgeon had hoped the paper would be published around Christmas – “I was angling for the ‘virgin birth at Christmas’ story” – but the timing didn’t matter. We are, as she says, “obsessed with sharks; we have a fairly innate primordial response to being eaten. But zebra sharks don’t eat you; they eat snails. There are several hundred species of sharks and only a handful of them will bite you – occasionally. Everything else is probably more gentle to approach than a sheep … they’re lovely animals.”
While most people would lump sharks in the same category as fish, Dudgeon puts them closer to humans. “I’m a scientist,” she says. “I know that sharks are more similar to us in terms of their life history strategies – lifespan, how and when they mate, the number of offspring they produce – than they are to bony fish. But do people have a sense of that?”
This was certainly a different kind of shark story. Zebra sharks were once prolific around the Indo-Pacific, but in 2016 they were listed as endangered. Although there are still reasonable populations in western Thailand, the Andaman Sea and along the east coast of Australia down to about 300 kilometres north of Sydney, in the Gulf of Thailand and eastern Indonesia – where they’re heavily fished – they’re rarely sighted anymore.
“We hear a lot about sharks being hunted to death or sharks killing us,” says Dudgeon. “Perhaps people got excited about a ‘good news’ shark story for a change.”
As a scientist who works on population ecology and environment, Dudgeon credits Leonie and her daughter for “throwing a spanner in the works of a lot of conventional thinking about reproduction”. Leonie made the switch from sexual to parthenogenetic reproduction very quickly – in just two years – and such a quick switch, says Dudgeon, raises many questions. “Do they do this in the wild? How often do they do this in the wild? Is this a useful strategy?”
Some snakes and lizards have been known to utilise parthenogenesis – as has poultry, although theirs seems to be triggered by viruses and leads them (as with most reptiles that can reproduce this way) to produce male chicks. “If you don’t have a boy, you make one,” is how Dudgeon abbreviates this. “And that approach makes sense.” But these zebra sharks had female offspring. “Leonie’s approach is, we don’t have any boys so we’ll make some girls as a short-term solution.” It’s an adaptation to isolation, she says, “but I wonder if it’s a default mechanism. If no sperm was present, would more animals be doing this?”
One theory is that animals use parthenogenesis to reproduce their own species so they can colonise new environments. But the downside is that the resulting offspring are inbred, which is a risky option from an evolutionary perspective. “The genes are good enough to keep [a population] going for a while,” says Dudgeon. “For a short-term strategy, it would be effective. Long term, eventually, you need to mix those genes up again.”
Aquaria often bin shark eggs if they’re not running a breeding program, and Dudgeon suggests that’s possibly one of the reasons we don’t know how often this occurs. “Eggs might be thrown out because people assume that since animals haven’t mated, the eggs must be infertile. We also assume that if animals have mated, all their offspring are sexually produced. Perhaps they’re not.” Part of her excitement is that other researchers will now look for the signatures of parthenogenesis in their datasets for other species. “People always ask why we haven’t seen this before,” she says, “and there are two answers. First, we weren’t looking, [and] second, we didn’t have the tools.”
The next question for Lolly and Leonie is whether their pups will reproduce – and if so, how? No one knows if vertebrates’ parthenogenetic offspring can reproduce sexually or not. “There are already other aquaria looking into this, and they might beat us to it,” Dudgeon says pragmatically. “Their sharks have already reached maturity and ours are only a year old. We have to wait six more years.”
Dudgeon will soon head to the Coral Sea to see if the distinct genetic signature of parthenogenesis exists in an isolated population of whitetip reef sharks living there. “We know most of them are girls,” she says. “So that seems a good place to look.”