2008-10-11

A female blacktip shark in Virginia fertilized her own egg without mating with a male shark, new DNA evidence shows.

This is the second time scientists have used DNA testing to verify shark parthenogenesis—the process that allows females of some species to produce offspring without sperm. (Read about the first time.)

The female shark, dubbed Tidbit, died during a routine physical exam before the pregnancy was identified.

A necropsy—an animal autopsy—after her death revealed she was carrying a near-term pup fetus that was about 12 inches (30 centimeters) in length.

Tidbit was caught in the wild when she was very young and reached sexual maturity in a tank at the Virginia Aquarium in Virginia Beach, where she lived for eight years.

"The interesting thing about that was there were no male blacktip sharks in the tank for the entire time of her captivity," said Demian Chapman, a researcher with the Institute for Ocean Conservation Science at Stony Brook University in New York.

"So the question is, where does this baby come from?" he asked.

Chapman is the lead author of a study on the female blacktip in the latest issue of the Journal of Fish Biology.

DNA fingerprinting

Chapman and his colleagues generated a DNA fingerprint for the mother shark and her pup fetus with a procedure identical to a human paternity test.

Ordinarily, a shark's DNA contains some genetic material from its mother and some from its father. Tidbit's pup, however, was not ordinary.

"Every part of the fingerprint of the embryo comes from the mother," Chapman said. "In other words, there is no genetic material from a father."

"Virgin Births"

All non-mammal vertebrate species are theoretically capable of parthenogenesis, scientists say. Examples have been documented in komodo dragons (read story), pythons, rattlesnakes, chickens, and turkeys.

Parthenogenesis is not possible in humans because if all the genetic material comes from the mother, certain genes will be switched off, and the embryo won't develop.

"For sharks in captivity, [parthenogenesis] has probably occurred more times than has been documented," says Robert Hueter, director of the Center for Shark Research at Mote Marine Laboratory in Sarasota, Florida.

The question then becomes, is parthenogenesis a type of developmental anomaly or a response to the female shark not having a mate in captivity?

"The fact that only one shark embryo was formed may suggest that this is more a case of an egg developmental aberration rather than a physiological response to the lack of a mate," said Hueter, who was not involved with the study.

No Variation

Normally, an embryo is formed when an egg containing half its chromosomes is fertilized by a sperm containing the other half.

When an egg cell is formed, a plant or female animal also produces three other cells called polar bodies. In the type of parthenogenesis observed in sharks, one of those cells behaves like a sperm and fertilizes the egg.

"But that cell is genetically identical to the egg," Chapman said. "So that's where you lose a lot of genetic variation."

Offspring produced by parthenogenesis are not exact clones of their mothers, however, because the genetic material is mixed differently.

Still, researchers believe the risk of congenital defects increases in animals whose DNA lack genetic variation.

"There's an increased risk of having a weakened immune system and there's a risk of reproductive abnormalities," Chapman said. "But in some cases, they'll be able to survive."

The scientists have not ruled out the possibility that increased stress from the abnormal pregnancy contributed to Tidbit's death.

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