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Shark conceived without male contact
By FIONA MACRAE -
More by this author » Last updated at 22:00pm on 22nd May 2007
It is the stuff of science fiction - fearsome females becoming pregnant without any contact with males.
In movie blockbuster Jurassic Park, scientists thought they could control the dinosaurs by only rearing females. However, the cloned creatures ran amok after perfecting the art of reproducing without any male input.
Now, scientists have shown that sharks are capable of performing such a trick of nature. Genetic analysis of a bonnethead shark pup has revealed it is the first example of a virgin birth in the sharkworld.
The female baby - which died from a stingray bite just hours after being born - did not contain a single strand of male genetic material.
Instead, she had inherited all of her genes from her mother.
Her birth, in an aquarium, stunned scientists, who had thought that, like humans, sharks always required a sperm and an egg to become pregnant.
Now, it seems, that when starved of male attention, female sharks are capable of activating an ancient survival mechanism that allows them to reproduce without any sexual contact.
In this particular case, the pup's mother had had not been near another male bonnethead shark for at least three years.
Her chief companions in a tank at the Henry Doorly Zoo in Nebraska, USA, were two other female bonnetheads - so called because of their large, rounded snouts.
While a male leopard shark did share the same tank, scientists thought it was nearly impossible the two would have mated - as the difference in size between the two would have made the feat comparable to a Chihuahua impregnating a Saint Bernard.
Another possibility was that the bonnethead was impregnated while still in the wild and that the sperm had lain in her body for several years before finally fertilising the egg.
However, while female sharks are known to sometimes 'store' sperm for future use, there was no evidence that this particular one had ever had sex.
Now, six years after the shark's birth, genetic analysis has solved the mystery.
Analysis of the baby shark's DNA has shown it reproduced by parthenogenesis - a process in which eggs develop into embryos without being fertilised by sperm.
Although such virgin births are common in the insect world, and have been known to occur among lizards and some snakes, fish and birds, they had never been documented among sharks.
Researcher Dr Paulo Prodhol, of Queens University Belfast, said: "The findings were really surprising because as far as anyone knew, all sharks reproduced only sexually by a male and female mating, requiring the embryo to get DNA from both parents for full development, just like in mammals.
"The discovery that sharks can reproduce asexually by parthenogenesis now changes this paradigm, leaving mammals as the only major vertebrate group where this form of reproduction has not been seen."
It is thought that sharks use parthenogenesis as a survival strategy that allows them to breed when males are few and far between, the Royal Society journal Biology
Letters reports.
However, while the process may aid short-term survival of the species, it does little to help the creatures' long-term prospects.
Lack of male DNA means that young produced by virgin births are less genetically diverse than those produced through the traditional route.
Over time, this lack of variety could cause major problems, by making it harder for the creatures to adapt to threats such as new diseases and changes in their environment.
