My Herald on Sunday column (not online yet but published below)
It’s taken six years to find out, but the zookeepers at Henry Doorly Zoo in Nebraska finally know how the female hammerhead shark that was in their care managed to get pregnant on her own.
Scientists revealed last week that DNA profiling showed the shark’s baby contained no paternal DNA. That means no dad and the first recorded example of a shark reproducing on its own.
(Graphic: Phil Welch Herald on Sunday)
Such an occurrence is known as parthenogenesis, virtually translated from Greek as ’virgin birth’ and is reasonably common in nature. A number of species are able to reproduce without fertilization by a male. Several species of insects, bony fish, reptiles like the whiptail gecko and the Komodo dragon can reproduce asexually.
It is virtually unknown in mammals however in 2004 a team at the Tokyo University of Agriculture, were able to produce a mouse that was the daughter of two females.
Kaguya the mouse, as she became known, was created from the genetic material of two egg cells — not a male sperm in sight. Scientists have baulked at the idea of applying the method to humans. There’s no guarantee it would work anyway as Kaguya was pretty much a fluke, the only success in hundreds of delicate attempts to reconstruct eggs. But the experiment has proven valuable in researching fertility techniques for normal conception in female humans.
While parthenogenesis helps several species reproduce, it doesn’t allow for as great genetic diversity as when a male impregnates a female.
Bees are a good example of this. While the queen bee is the only bee that gives birth, replenishing the entire population of the hive, female bees will often resort to laying their own eggs if their queen happens to die. This is a ’non-viable’ version of parthenogenesis, because the female worker bees can only produce male ’drone’ bees which in turn can only mate with the queen. With no queen in the hive, the population starts to die off.
It is for this reason that confirmation of parthenogenesis in the hammer head shark has been met with dismay from some quarters. For many scientists, it’s a sign that the world shark population is adapting to meet its own population shortage, one caused by over-fishing. Female sharks may be resorting to parthenogenesis when they can’t find a mate. If more of this is to happen, the genetic diversity of sharks will be diluted, lessening the species’ ability to adapt to environmental changes.
That’s a bad thing given the importance of sharks to the marine food chain. At least now we know what got the hammerhead pregnant and can start to look at whether the same process is happening among sharks in the wild. Such research is essential. It’s the only way we can really gauge the impact on procreation the world’s environmental changes are having.