May 23, 2007 in Nation/World

Shark reproduces asexually

Juliet Eilperin Washington Post
 

A team of American and Irish researchers have discovered that some female sharks can reproduce without having sex, the first time scientists have found the unusual capacity in such an ancient vertebrate species.

Their report that sharks can produce asexually through the process known as parthenogenesis is being published online today in the British journal Biology Letters. Researchers have observed parthenogenesis in certain species of birds, reptiles, amphibians and bony fishes, but the new finding suggests that vertebrates’ ability to reproduce without sex evolved much earlier than scientists had thought.

Scientists began their investigation after a female hammerhead shark was mysteriously born at Omaha’s Henry Doorly Zoo in December 2001, in a tank that held three adult female hammerheads but no males. The seven-inch long baby was killed within a day its birth, apparently because another fish in the tank, most likely a sting ray, attacked it.

Though the three females had been caught before they reached sexual maturity and held in captivity for more than three years, researchers initially thought one of them had stored sperm from a male shark before fertilizing an egg. But the team – which included scientists at Nova Southeastern University in Florida, Queen’s University Belfast and the zoo – determined that the baby shark’s genetic makeup perfectly matched one of the females in the tank, with no sign of a male parent.

Nova Southeastern’s Guy Harvey Research Institute Director Mahmood Shivji – one of the paper’s authors – said he and his colleagues determined that a byproduct formed when sharks produce eggs, known as a sister polar body, had fused with an unfertilized egg to produce the baby shark, whose DNA had only half as much genetic variability as the mother’s.

“Yes, indeed this is a virgin birth,” Shivji said in an interview, adding that this could help explain why other sharks have suddenly been born in captivity.

“We have now demonstrated that sharks are actually able to use an alternative, previously unknown reproductive pathway, which is parthenogensis. The problem here is that this alternative reproductive pathway results in offspring that have much lower genetic diversity,” he said.

The paper’s lead author, Demian Chapman, said the virgin birth does serve as a testament to sharks’ resourcefulness. Mammals cannot reproduce asexually.

“It just goes to show, life will find a way,” he said, adding that during his research in Belfast he bet several other scientists the answer to the Omaha shark mystery would turn out to be something other than parthenogenesis. “I lost so many pints of Guinness over that one,” he said.


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