And then there were four… Here’s the scenario: three sharks are in a tank, all three are female and all were captured when they were sexually immature babies. They spend three years in the tank together without ever coming in contact with a male. Then, one day, a baby shark pops up.
The sharks are hammerheads, living in an aquarium at Omaha’s Henry Doorly Zoo, in the US. The pup was born on 14 December 2001, and triggered a great deal of confusion, which has only now finally been cleared up: the pup was the result of a “virgin birth”.
For many years, different theories were argued over. Perhaps one of the females had been inseminated by a shark from another species? Or maybe she had been inseminated before she was captured? Female sharks do have an organ that allows them to store sperm, but a three-year storage would have been unprecedented.
What is more, sex between sharks tends to be rather rough and females are usually left with marks as a result of this. But none of the three females from Florida Keys had any marks on them when they were captured.
Still, the insemination theory was considered “because it was even more difficult to imagine asexual reproduction in a shark,” says Paulo Prodöhl of Queen’s University Belfast in Northern Ireland.
Prodöhl is part of a team of researchers which has determined “beyond doubt” that one of the females did reproduce asexually to produce the mystery pup.
The team, led by Demian Chapman of the Pew Institute for Ocean Science in the US, used a DNA fingerprinting technique similar to that is used in human paternity tests. Initially, this was to determine which of the three females was the mother.
In offspring that are produced sexually, half the DNA comes from the mother and the other half from the father.
So, seeking evidence of the father, they then subtracted the mother’s contribution from the offspring’s DNA. “In this particular case, after we subtracted the mother’s DNA, there was nothing left,” says Prodöhl. “It was fantastic!”
The researchers were forced to conclude that the pup had no father, making it the first documented case of asexual reproduction in cartilaginous fish. Sadly, the remarkable specimen later died, apparently killed by another fish in the aquarium.
The discovery leaves mammals as the only vertebrates not known to be able to reproduce asexually outside of the lab.
The researchers believe the hammerhead shark reproduced by a type of asexual reproduction called automictic parthenogenesis, whereby an unfertilised egg is activated to behave as a normal fertilised egg by a small, nearly genetically identical cell known as the sister polar body.
Because the unfertilised egg and the polar body both contain only half of the mother’s genes – and the same half – not only did the pup not get any genes from a father, it also only got half of its mother’s genetic diversity.
The researchers believe this restriction of genetic diversity could be detrimental to the survival of endangered shark species if female hammerhead sharks switch to asexual reproduction when they are having trouble finding a mate.
Less genetically diverse populations are less able to adapt to changes, such as disease or the changes to their habitat brought about by global warming.
“In some regions shark populations are declining by more than 90%,” says Prodöhl. “Over time, if that continues and increases the incidence of asexual reproduction, then that might worsen the decline of the sharks.”
Journal reference: Biology Letters (DOI: 10.1098/rsbl.2007.0189)
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