Emerging shark conservation efforts
December 14, 2008 - Newsday
By Demian D. Chapman
Over the past few years I have been fortunate enough to have been given the opportunity to solve a real biological mystery: How do female sharks kept in captivity without males become pregnant?
Since 2000 there have been almost a dozen cases of this occurring in aquaria in the United States and Europe, each one triggering a flurry of media interest and speculation. Were these cases hoaxes? Had the females mated a long time ago and then stored the sperm? Were the baby sharks hybrids between shark species? Or, several reporters suggested half jokingly, were they oceanic immaculate conceptions?
As a marine biologist specializing in shark genetics, I hoped to shed some light on the origins of these baby sharks by looking at their DNA.
The first of the cases I worked on involved an unnamed female hammerhead shark in Omaha that had delivered a live baby in December 2002. The second case involved a female blacktip shark named Tidbit in Virginia, who lived in a tank with no male blacktips for over eight years. She died in 2007, but left an intriguing legacy when she was found to be carrying an embryo at the time of her death.
How did DNA solve these cases? Researchers at the aquaria were able to provide me with tissue samples from the shark mothers and their babies. I analyzed these samples in the laboratory using DNA fingerprinting technology that is almost exactly the same as that used in human paternity testing. Like humans, sharks reproduce by internal fertilization and get half of their DNA (which is packed into chromosomes) from their mother and half from their father.
When I looked at the DNA fingerprints of the mother sharks and compared it to their offspring I saw something very strange. Neither baby shark had any DNA from a father. In other words, these young sharks had no father. They were not hoaxes, hybrids or the products of long term sperm storage. What was going on?
The answer is a process known as parthenogenesis - derived from Greek words meaning "virgin birth." Eggs and sperm contain one copy of every chromosome and when they come together they provide the two copies of every chromosome needed for development. For each egg cell that a female produces there are three by-product cells called "polar bodies," which also contain one copy of each chromosome. Ordinarily, these cells do not participate in fertilization. In the kind of parthenogenesis observed in sharks, one of these polar bodies injects its chromosomes into the egg, providing all of the chromosomes needed for an embryo to develop. As always, nature finds a way.
Most people would be surprised to find out that parthenogenesis has been documented in a wide variety of vertebrates (animals with backbones), including bony fish, lizards, snakes and birds like chickens and turkeys. Now we know that female sharks can reproduce by parthenogenesis too. This is especially significant because sharks were among the first of the vertebrates to appear, which suggests something new about the evolution of reproduction: occasional parthenogenesis may well be a fundamental ability of all vertebrates.
Of course, this begs the question why don't female mammals, including humans, conceive in this way? As it turns out, mammal genomes are processed in such a way that a mother's chromosomes and father's chromosomes are "packaged" differently and it is necessary to combine one of each for the successful development of an embryo.
Parthenogenesis doesn't work in mammals because combining two copies of the mother's genetic material is literally a "dead end" for the embryo very early on in development. As a result, human males need not worry about reproductive redundancy just yet.
A leading shark researcher and a mentor to me, Samuel H. Gruber, predicted in a landmark 1990 paper that many sharks are headed for extinction. Almost 20 years later, this prediction is coming true, especially in the U.S. Atlantic, where sharks have severely declined due to overfishing. Does the newly discovered parthenogenesis mean that sharks have a reproductive edge that could make them resilient to overfishing by humans? In my opinion, parthenogenesis is probably not a great way for dwindling shark populations to rebuild themselves. Among other things, sexually reproducing sharks usually have multiple births - from two to 300 babies at a time depending on the species. Producing one baby by occasional parthenogenesis is pitifully inefficient in comparison.
Despite this, I hold great hope for the recovery of sharks in our oceans. While parthenogenesis probably won't save sharks, perhaps we will. The tremendous worldwide interest in the shark virgin birth mystery underscores our primal fascination with these mysterious ocean predators, which I suspect may inspire broad public support for emerging shark conservation efforts.
The shark parthenogenesis research is a collaboration with Nova Southeastern University/Guy Harvey Research Institite, Queens University of Belfast, Henry Doorly Zoo and the Virginia Beach Aquarium and Marine Science Center.