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Fatherless Sharks

Fatherless Sharks Juvenile brownbanded bamboo shark (Chiloscyllium punctatum)
Photo Credit: By Steve Childs [CC-BY-2.0], via Wikimedia Commons

Virgin sharks can produce multiple offspring that survive to adulthood.


Transcript

Fatherless sharks. I'm Bob Hirshon and this is Science Update.

In recent years, scientists have learned that sharks can have virgin births, called parthenogenesis. But it wasn't clear that the fatherless sharks could survive to adulthood. Now, Doug Sweet of the Bell Isle Aquarium in Detroit, and Kevin Feldheim of the Field Museum in Chicago, have shown that they can. Feldheim's team analyzed the DNA of two bamboo sharks born at the aquarium in 2002. Their birth made headlines at the time, because their mother had had no male contact in captivity.

Feldheim:
We came along with the genetic work to show that it was in fact parthenogenesis because one possibility is many shark species can store sperm, so it's possible that sperm storage could explain it. 

Feldheim's group ruled that out. Next, they'd like to find out if fatherless sharks can reproduce the old-fashioned way. I'm Bob Hirshon for AAAS, the Science Society.


Making Sense of the Research

A human virgin birth—or parthenogenesis, as scientists call it—would truly be a miracle. But for many animals, including some insects, reptiles, birds, and fish, parthenogenesis is indeed possible, and in some cases not all that uncommon. (Natural parthenogenesis has never been documented in any mammal, although it has been artificially induced in mouse and monkey egg cells. However, only a tiny fraction of the mouse cells, and none of the monkey cells, developed into mature animals.) 

Parthenogenesis was not confirmed in sharks until 2007, when a female hammerhead shark at a Nebraska zoo gave birth with no male help. Another confirmed case, in 2008, involved a blacktip shark. But the baby hammerhead was killed by a stingray, while the blacktip mother shark died just before she would have given birth. Therefore, scientists didn't know if the offspring of virgin sharks were really healthy and viable.

The Belle Isle Aquarium bamboo sharks made headlines when they were born in 2002, since it appeared to be a virgin birth: their mother was sexually immature when she arrived at the aquarium, and there had been no male shark in her tank since then. The possibility that the mother had, in fact, mated before she arrived and stored the sperm seemed to be an extreme longshot—but then again, so was parthenogenesis, which had never been documented in sharks at the time.

Feldheim's team performed a genetic analysis of the sharks and confirmed that they were only their mother's offspring. They aren't exact clones of her, though, nor are they identical twins. That's because of the way parthenogenesis happens in sharks. Instead of a sperm, the mother shark's egg cell is fertilized by the mother's own polar body—a DNA-packed particle that her body sheds during her  reproductive cycle. Normally, the mother's system simply reabsorbs these polar bodies. But in the case of these virgin births, the polar body actually fuses with the egg and somehow triggers the creation of a new shark. Since every polar body contains somewhat different DNA, the resulting offspring are mostly, but not entirely, genetically similar to the mother herself. 

It's not known how often this happens in nature, if at all, and with so many sharks in the sea, it would be nearly impossible to find out. It's also unclear whether parthenogenesis in sharks simply happens by accident, or if certain conditions—like the inability to find a mate—can create biological triggers that make it more likely. What scientists can learn, however, is whether the offspring of virgin births can reproduce sexually themselves. If so, perhaps the right question is not why parthenogenesis happens, but why it doesn't happen more often.

Now try and answer these questions:

  1. What is parthenogenesis?
  2. Why did Feldheim's team need to confirm the case of parthenogenesis genetically?
  3. Why is it significant that the fatherless sharks can survive to adulthood? What would it mean if they couldn't?
  4. In any species, parthenogenesis produces only female offspring. Why?
  5. Why aren't the fatherless sharks exact clones of the mother?

You may want to check out the February 5, 2010 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: DNA analysis confirms virgin birth in sharks, bonobos are forever young, elephant genes fight disease, and mussel glue inspires medical sealants. 


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