A zebra shark in an aquarium
[Photo by Fred Bavendam, Minden Pictures, National Geographic]
Virgin birth, or parthenogenesis, is a rare phenomenon in which reproduction occurs without fertilization. Generally, a sperm fertilizes an egg, which then results in the formation of an embryo.
A captive shark has been found to have laid fertilized eggs, in spite of having no male companionship for three years. Aquarium keepers at the Reef HQ Great Barrier Reef Aquarium were surprised to find that a zebra shark laid a total of 41 eggs, three of which hatched and produced healthy female pups.
The shark, called Leonie, had already mated with a male shark before being moved to a different tank with no males around. After moving to her new tank, Leonie kept laying unfertilized eggs, just like a chicken does.
Actual parthenogenesis is quite rare, and of course, quite surprising to witness. Still, many species exhibit parthenogenesis, even those that are not meant to do so. A number of shark species has exhibited parthenogenesis, particularly individuals that live in aquariums.
Zebra shark egg cases
[Photo by Akhilesh K.V]
Scientists initially thought that female sharks had been storing sperm from sharks they had previously mated with. The stored sperm, allegedly, fertilized the eggs of female sharks, which led to the females laying viable eggs. However, females that had never mated at all before also began laying viable eggs.
"I think it happens when they get pushed into an evolutionary corner, like in an aquarium with no males around," says shark expert George Burgess from the Florida Museum of Natural History. This may be possible, but the fact is that scientists aren't sure why species that don't reproduce through virgin births do so in certain circumstances. For one thing, reproduction with two parents ensures genetic diversity within the species. Parthenogenesis, however, does nothing for the species's gene pool. On the other hand, it can ensure that the mother passes her genes on, albeit in less-than-ideal circumstances.
It was the lack of genetic diversity in Leonie's pups that convinced scientists that it really was a case of parthenogenesis. Leonie had already mated before, which led scientists to guess that she had simply stored sperm. However, when scientists tested Leonie's pups, they discovered that the lack of genetic diversity in the pups indicated that they had only one parent, not two.
Researchers have also written a paper exploring Leonie's parthenogenesis.
A juvenile zebra shark
[Photo by Line1]
Typically, having offspring with low genetic diversity isn't the best option. This can lead to the offspring having weaker immunity against a number of different threats. However, it can work as a last-ditch effort to ensure that the species will survive when it finds itself in a difficult spot. Low population density, for example, can be solved without having to wait for a suitable male to mate with.
So far, scientists have yet to discover a virgin birth or two among sharks in the wild. However, they believe that it's possible. It's also possible that parthenogenesis isn't actually all that rare in sharks, and that it can happen sooner or later without any triggers. Scientists also say that parthenogenesis does not occur among mammals, so you don't have to worry about sudden virgin births any time soon.
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