A leopard shark in an Australian aquarium has reproduced asexually after being separated from her mate.
It is the first reported case of a shark switching from sexual to asexual or parthenogenetic reproduction and only the third reported case among all vertebrate species.
The leopard shark, Leonie, was captured in the wild in 1999 and introduced to a male shark at the Reef HQ aquarium in Townsville, Queensland, in 2006. Leopard sharks are also known as zebra sharks.
She began laying eggs in 2008 and had several litters of viable, sexually produced offspring before being separated from her mate in 2012, because the aquarium decided to scale back its breeding program.
One of the offspring was Lolly, a female shark who has shared a tank with her mother since 2013. Lolly has not shared a tank with a male shark since she reached sexual maturity.
In 2014, both Leonie and Lolly laid eggs.
According to Dr Christine Dudgeon, a biologist with the University of Queensland who has studied the two sharks and published a report on Leonie’s unusual reproductive shift in Scientific Reports this week, laying eggs without a male around was not in itself unusual.
“Much like a chicken, they will lay eggs if the conditions are good, whether they are fertile or infertile,” Dudgeon told Guardian Australia.
However, staff at the aquarium noticed that some of Leonie’s eggs contained embryos. They attempted to incubate them but none of the eggs hatched.
The next year, both Leonie and Lolly produced eggs that contained embryos. Together they produced five live hatchlings, of which two, Leonie’s hatchling Cleo and Lolly’s hatchling Kitkat, remain on display at the aquarium.
Genetic testing of Leonie’s hatchlings revealed elevated homozygosity, indicating they were the result of asexual reproduction and not the result of stored sperm. Female sharks have been known to store sperm for up to four years.
Dudgeon said asexual reproduction among female sharks that had never reproduced sexually, like Lolly, was not unusual, but Leonie’s case was a world first.
“The onset of asexual reproduction with the onset of maturity has been documented before with sharks, and rays, and particularly with reptiles, but what we have shown for the first time is the switch,” she said.
Genetically, she said, it is like “a severe case of inbreeding.” The offspring are not clones of the mother and only have half of her genetic material, which makes them less hardy.
“The idea behind sexual reproduction is you keep mixing up the gene pool to give these animals the biggest set of traits to combat pathogens and other environmental factors that change very rapidly,” she said.
Dudgeon said it was not clear what triggered the switch, but said it appeared to be a short-term evolutionary response to extend the reproductive life of a female in response to a scarcity of mates.
However it’s success as a long-term strategy is unclear. There have been no reported cases of an offspring that was the product of asexual reproduction going on to reproduce sexually.
Dudgeon will be monitoring the reproductive habits of Cleo and Kitkat, when they reach sexual maturity at the age of seven.
Zebra sharks, or stegostoma fasciatum, are named for the zebra-like stripes they display as babies. Mature zebra sharks have spots, meaning they are commonly also known as leopard sharks, not to be confused with the different species of leopard sharks off the coast of California.
They are listed as endangered on the International Union for Conservation of Nature red list.
Hamish Tristram, a senior aquarist with Reef HQ, said he hoped the unusual case of Leonie would bring greater attention to the struggling species.
Tristram said Leonie and Lolly’s ability to reproduce asexually had somewhat stymied the aquarium’s attempts to slow down its breeding program.
“We really were trying to scale back our breeding program and as a result we have divested from having any male sharks,” he said. “That’s the irony in the fact that they have gone on to reproduce anyway.”
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