A Very Special Shark Gives Birth
DEBBIE ELLIOTT, host:
This is ALL THINGS CONSIDERED from NPR News. I'm Debbie Elliott.
You may have heard this week about what scientists are learning from the mysterious birth of a baby shark without a father. We explore that virgin birth called parthenogenesis in this week's Science Out of the Box.
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ELLIOTT: Zookeepers at the Henry Doorly Zoo in Omaha, Nebraska, got a shock a few years back when a baby bonnethead shark mysteriously appeared at a tank with three older females. Their first thought was that it must have been a hoax, that someone must have broken in and left the shark pup in the tank, but there was no evidence of a break-in.
So the puzzled zookeepers turn to shark expert Mahmood Shivji for help. Dr. Shivji is the director of the Guy Harvey Research Institute at Florida's Nova Southeastern University. He worked with an international team of researchers to unravel the mystery of the baby shark.
Dr. MAHMOOD SHIVJI (Director, Guy Harvey Research Institute, Nova Southeastern University, Florida): Other than being a hoax there were three other possibilities. One was that the female had actually stored sperm as a result of copulating in the wild before she was captured. And then she fertilized her eggs after storing the sperm for three years. That was one hypothesis. And actually, it's the most plausible hypothesis scientifically at the time. You and I thought that was the case. The other hypothesis was that the female had potentially mated with male of a different species in the tank. In other words, the bonnethead baby was hybrid, but it didn't look like a hybrid. And a third hypothesis was actually parthenogenesis.
ELLIOTT: Dr. Shivji and his team analyzed the baby shark's DNA and were able to figure out which of the three adult females was its mother.
Dr. SHIVJI: But secondly, and much to our surprise, what we also found was that the baby had no evidence of any paternal DNA. And that nailed down the fact that this was a parthenogenetic birth and not a hybrid or a case of sperm shortage.
ELLIOTT: Sharks aren't the only animals that reproduce through parthenogenesis. A Komodo dragon famously did it this year in a British zoo. And in fact, there are lizards whose entire population is female. For them, parthenogenesis is a way of life. Some birds and certain kinds of fish do it too. But until now, no one had observed it in sharks or their relative's race. Dr. Shivji points out that there are different methods of parthenogenesis. He says the one sharks used is called automictic parthenogenesis.
Dr. SHIVJI: And the way automictic parthenogenesis works is the (unintelligible). When female sharks produce eggs, there are cells called germ cells that have the entire chromosome set of the female, and those germ cells then undergo a couple of different divisions, ultimately, to produce a normal egg, which has half the chromosome numbers, and then a normal egg is then fertilized by a normal sperm.
ELLIOTT: But in automictic parthenogenesis, the egg basically fertilizes itself. When the germ cell divides, it produces smaller cells called polar bodies. Usually, they get reabsorbed into the mother's body.
Dr. SHIVJI: Now in this particular instance, what happened was that one of the polar bodies that should have been absorbed under normal circumstances of sexual reproduction didn't get reabsorbed and actually acted like a sperm and fused with the cell that was actually going to become an egg, and by fusing, it restored the normal chromosome complement but it still only had half the genetic diversity that would have been found in the female.
ELLIOTT: Are you following so far? Without a father contributing DNA, the baby shark isn't quite a clone of its mother. It has the full number of chromosomes as its mother, but only half her genetic diversity. And that's a problem.
Dr. SHIVJI: If this mechanism is actually occurring in the wild under circumstances whether it's paucity or lack of males and the offspring that have been produced have much lower genetic diversity that then resolves in a population of sharks that will be less adaptable to changes in the environment. They will be less evolutionarily or genetically fit.
ELLIOTT: So why does it happen? Dr. Shivji says no one is quite sure why sharks would reproduce through parthenogenesis. But it does seem like the logical response if there are no males around, for instance, because of overfishing.
Dr. SHIVJI: It is well known that many commercially-fish shark species segregate on the basis of sex after mating. In other words, males and females don't do the same thing. They go off in different directions. And if, indeed, there are instances where fishing is actually by chance targeting aggregations of mostly males that then would resolve in a reduction in the male population, making it more difficult for females to find mates and then again triggering parthenogenesis.
ELLIOTT: Shivji says scientists don't know if shark parthenogenesis occurs in the wild because it's difficult, if not impossible to observe. As for the baby bonnethead shark that sparked this storm of scientific inquiry, sadly, it is no longer with us. It was attacked and killed by another fish in the tank just a few hours after its birth.
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