UPDATE: Second 'Virgin Birth' Documented in Shark

SEE: http://thesteadydrip.blogspot.com/2008/10/scientists-virginia-sharks-pup-virgin.html

Second 'Virgin Birth' Documented in Shark
Jennifer Viegas, Discovery News

Oct. 10, 2008 -- The first documented virgin shark birth seemed like an odd miracle, but now a second female has become pregnant sans daddy, adding to the evidence that female sharks don't always need a male to conceive, according to a new study.

Virgin birth, scientifically known as parthenogenesis, has previously been proven in certain amphibians, reptiles, birds and bony fish. It's now suspected that most, if not all, female sharks possess the ability.

"Parthenogenesis may not have evolved in sharks," Demian Chapman, who led the research, told Discovery News. "It may just be an occasional mistake that sometimes occurs when eggs are left unfertilized."

Chapman, a shark scientist with the Institute for Ocean Conservation Science at Stony Brook University, explained that during egg production, a female shark produces four cells. Only one of these becomes the egg. Another of the four is called "the sister polar body," which is a close genetic match to the egg.

During parthenogenesis, according to Chapman, "the sister polar body fuses with the egg and injects its chromosomes into it."

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"Therefore, it acts like a sperm and triggers the development of an embryo," he said.

The discovery in this case is bittersweet, since the virgin shark in question, a female blacktip shark named "Tidbit," died after living for eight years without a male companion at the Virginia Aquarium.

An autopsy upon her death revealed that she was pregnant with a single offspring. DNA testing of the unborn offspring showed that it contained no genetic material from a father.

A paper outlining the findings appears in the latest issue of the Journal of Fish Biology.

Chapman and his team made a similar discovery just last May, when a hammerhead residing at an Omaha, Neb., zoo became pregnant after not being in contact with male sharks for at least three years. DNA fingerprinting techniques were used in that case as they were for Tidbit.

The method is identical to how researchers handle human paternity testing.

Female sharks can store sperm for long periods of time, which is why the scientists needed to confirm, through the DNA analysis, the lack of paternal input.

Despite the novelty of the phenomenon, virgin births are inferior to regular shark births in two major respects.

First, the female appears to only be able to produce a single offspring in this manner. Females that mate can have litters ranging from a few to more than 100 shark pups, depending on the species.

Second, the offspring of a virgin birth has "reduced genetic diversity when compared to its mother or any other shark produced by sex," Chapman said, adding that the process is "no great white hope for depleted shark populations."

He hopes that future conservation efforts will focus on "robust shark populations with balanced gender ratios, rather than hoping for a bailout from some interesting quirk of reproduction."

Robert Hueter, director of the Center for Shark Research at the Mote Marine Laboratory in Florida, told Discovery News that he supports the new findings since "the research methods are sound and the conclusion of a parthenogenetic birth in this shark species is proven."

Hueter said it's still possible that virgin births are more of "an egg developmental aberration rather than a physiological response to a lack of a mate," but he predicts "time will tell as more research is focused on this interesting subject."

Such revelations may come sooner rather than later, as Chapman is now analyzing the DNA of an offspring from yet another suspected virgin shark birth.
http://dsc.discovery.com/news/2008/10/10/virgin-birth-shark-print.html

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