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Virgin Birth: Shark has daughter without a dad

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10:01am, May 23, 2007
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Geneticists have confirmed a case of birth without mating in a bonnethead shark, one of the smaller hammerhead species. That makes sharks the fifth major vertebrate lineage with documented virgin births.

The mother bonnethead (Sphyrna tiburo) lives at the Henry Doorly Zoo in Omaha, Neb., far from any males of her kind. Her infant, born nearly 6 years ago, was killed by other fish in the zoo's aquarium. DNA analysis of its preserved tissues revealed no evidence of genes other than its mother's, researchers report online in the Aug. 22 Biology Letters.

Sharks belong to an ancient vertebrate lineage, notes Mahmood Shivji of Nova Southeastern University's Guy Harvey Research Institute in Dania Beach, Fla. As one of the team confirming the shark's sex-free reproduction, he says that the finding "means this ability evolved very early on in vertebrate evolution."

This kind of reproduction, parthenogenesis, "is probably more widespread" than biologists have realized, comments Ed Heist of Southern Illinois University in Carbondale. Parthenogenesis may seem weird to people, but DNA analysis has confirmed some form of it among bony fish, amphibians, reptiles, and birds. "I think mammals are the odd ones," Heist says.

The bonnethead birth came as a surprise. On Dec. 14, 2001, zookeepers in Omaha discovered a newborn along with their usual three bonnetheads, all adult females.

The females, captured near the Florida Keys, arrived at the zoo before they had matured sexually. They had lived in all-female bonnethead groups for about 3 years.

That isolation didn't prove virgin birth. But other scenarios also had problems. For example, bonnetheads can store sperm, but no one had documented such storage for more than 5 months, or that juveniles would mate in the first place. Alternatively, the mother could have uncharacteristically mated with a male of another shark species at the zoo.

At a shark-biology meeting in 2004, zoo interns described their mystery to Demian Chapman, now at the Pew Institute for Ocean Science at the University of Miami. Chapman gave a talk about genetic markers that can identify individual bonnetheads.

He and his colleagues agreed to check the infant tissue and the three possible mothers to figure out the infant's origin. "Ninety percent or more of shark biologists, including myself, thought [the answer] was going to be sperm storage," he says.

Instead, the team found that the infant shark carried a portion of the genes of one Omaha female but no contribution from a male.

That finding suggested a particular kind of virgin birth called automictic parthenogenesis, previously reported in a komodo dragon (SN: 12/23 & 30/06, p. 403) and some snakes. As in sexual reproduction, youngsters acquire one set of chromosomes when the mother's chromosomes split during egg making. But instead of uniting with similarly split chromosomes from sperm, that set somehow pairs with a copy of itself. The resulting baby "isn't a clone," says Chapman. "It's half a clone."

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