A leopard shark in Australia has perplexed researchers by reproducing live hatchings asexually after being separated from her mate in 2012.
According to researchers, this is the first reported case of a shark switching from sexual to asexual or parthenogenetic reproduction.
Leopard sharks are also known as zebra sharks (stegostoma fasciatum) and get their name due to zebra-like stripes in babies. When these sharks get matured, they develop spots and are then commonly called leopard sharks.
The zebra sharks are found throughout the tropical Indo-Pacific. They have a cylindrical body on which five longitudinal ridges can be found. A low caudal fin comprises about half the total length. A unique feature of these sharks is a pattern of dark spots on a pale background. In comparison, young zebra sharks feature a different pattern that comprises of light vertical stripes on a brown background. Zebra sharks can grow up to 2.5 m in size. They are nocturnal creatures and most of their day time is spent by resting motionless on the sea floor. Female zebra sharks produce several dozen large egg capsules, and then use adhesive tendrils to anchor these capsules to underwater structures.
Leonie was captured in 1999 and was kept at the Reef HQ aquarium in Townsville, Queensland. In 2006, she was introduced to a male shark, and after two years, she started laying eggs through sexual reproduction. The pair produced over two dozen offspring in next six years. In 2012, Leonie was separated from her mate after aquarium decided to cut back its breeding program.
Leonie reproduced many offspring through sexual reproduction, and one among them was Lolly—a female shark. Lolly was sharing a tank with Leonie since 2013, and was never introduced to a male shark since she attained sexual maturity.
In 2014, both Lolly and Leonie laid eggs, and an unusual thing about this was that some of Leonie’s eggs had embryos, as noticed by the aquarium staff. The staff also tried to incubate these eggs, none of which hatched.
Leonie and Lolly laid eggs next year and these eggs also contained embryos. In total, they produced five live hatchlings, and of these, Lolly’s hatchling Kitkat and Leonie’s hatchling Cleo are on display at the aquarium.
In their study, researchers explored the possibility that Leonie was storing sperms from her ex mate and using them to fertilize her eggs. But, genetic testing revealed that Leonie’s hatchlings had elevated homozygosity, meaning they were reproduced asexually instead of being the result of stored sperm.
Dr Christine Dudgeon, a biologist with the University of Queensland, who has been studying both sharks for the past four years, says laying eggs without a male around is nothing unusual for sharks. Some vertebrate species, including some sharks, Komodo dragons, turkeys, rays and snakes, can reproduce asexually even though they normally reproduce sexually. However, asexual reproduction occurs mostly in females who never met any male partner.
“Much like a chicken, they will lay eggs if the conditions are good, whether they are fertile or infertile,” Dudgeon told Guardian Australia.
However, Leonie’s case was different and world’s first as a female shark had shifted from sexual reproduction to asexual reproduction.
“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.
According to Dr Christine, this is like “a severe case of inbreeding.”
So far, there are only two documented cases in vertebrates where a female switched from sexual to asexual reproduction. These cases include an eagle ray and a boa constrictor.
Dudgeon thinks this switching could be a short-term evolutionary response to extend the reproductive life of a female shark in absence of males. However, the success of this evolutionary response in long term is unclear.
Dr Dudgeon will continue to monitor these sharks as well as Cleo and Kitkat, when they reach the age of sexual maturity.
The detailed findings of Dr Christine Dudgeon study on Leonie’s reproductive shift have been published in Scientific Reports.
You must be logged in to post a comment.