Queensland leopard shark makes first recorded sexual transition
WHAT DO YOU do when you have a biological urge to reproduce but you don’t have a partner?
If you’re Leonie, resident leopard or zebra shark (Stegostoma fasciatum) at the Reef HQ Aquarium in Townsville, then you make babies anyway.
Leonie is the first documented case of a shark switching from regular sexual reproduction to the ‘virgin birth’ method of parthenogenesis.
Researchers were surprised when Leonie hatched three eggs in April 2016, three years after being separated from her mating partner for space reasons. She had previously produced pups the usual way with the male leopard shark.
“When aquarium staff noticed the embryos forming, we thought Leonie had stored sperm,” said lead researcher Dr Christine Dudgeon, from the University of Queensland.
Sperm storage – where a female stocks up on sperm from a mating event for later fertilisation – is documented in many animals including sharks. One record-holding shark held onto sperm for nearly four years before producing a pup.
To figure out what had happened in this case, researchers screened genetic markers in Leonie, the possible dad, and the new embryos.
“It’s essentially the same as DNA fingerprinting for people,” explained Christine. “There are usually two different copies of each marker – one from each parent.”
But that’s not what they found. Instead, the baby sharks had only one copy of each marker, indicating they had been produced asexually via parthenogenesis. The offspring are not clones of Leonie, Christine explained. “They have half her genetic diversity, equivalent to one of her egg cells.”
The phenomenon was observed in a leopard shark kept in captivity at Reef HQ in Townsville, Queensland, after her mate was relocated for space reasons. (Image courtesy University of Queensland)
Parthenogenesis has been documented in several shark species, according to Professor Colin Simpfendorfer, a shark expert from James Cook University and co-chair of the IUCN’s Shark Specialist Group. “These findings confirm what we are seeing in other species of sharks and rays – when they cannot find a mate, they can produce young asexually through parthenogenesis.”
However, Leonie’s switch from regular baby-making to the more unconventional parthenogenesis is a transition that has never been seen in sharks before. “For the first time we have direct evidence that individual sharks can switch between sexual and asexual modes of reproduction,” said Colin, who was not involved in the research.
The speed of Leonie’s transition was particularly curious. “It’s surprising she can switch so quickly, after just two years,” said Christine, “It raises questions about the molecular processes underlying this type of reproduction.”
The big question for Christine now is whether the asexually produced offspring can engage in sexual reproduction. She’ll have to wait seven years for the pups to reach sexual maturity before she can find out. “We assume that these offspring can mate sexually, but it hasn’t been demonstrated,” she said. “Parthenogenesis is not a long-term strategy, because of the decreased genetic diversity.”
Christine also hopes to investigate whether this reproductive strategy switch occurs in wild sharks – and if so, how often it happens. “A population of the critically endangered smalltooth sawfish contained five individuals with genetic signatures of parthenogenesis,” she said. “Screening of leopard sharks across their wild range has yet to turn up genetic signatures of parthenogenesis, but maybe it’s restricted to populations that are particularly exploited or isolated.”
The findings have important implications for shark conservation – especially for endangered species such as leopard sharks. The adaptation could help sharks survive when breeding is difficult, by allowing them to extend their lineage without the input of a mate. “Sharks and rays are one of the most threatened groups of vertebrates, and many are rare anyway,” said Christine. “It’s really interesting that they have this capacity, which could be used more in extreme conditions of exploitation.”
But Colin stressed that we shouldn’t rely on parthenogenesis to save these apex predators. “Without addressing the causes of population declines, this amazing reproductive trait alone will not be enough to save them,” he said.