May 24, 2013
By Medical Discovery News
What do the New Mexico whiptail lizard, the water flea, the marbled crayfish, the boa constrictor and the bonnethead shark have in common? All these animals can create offspring without a fertilized egg. This means a diverse group of animals can produce young by “virgin births,” without sexual contact. This is not a new concept but we are now recognizing the wide diversity of species capable of virgin births.
This nonsexual process of reproduction in animals is called parthenogenesis, from the Greek word for virgin birth. In the blockbuster movie “Jurassic Park” one of the precautions taken was that only female dinosaurs were produced by recombinant DNA methods, to prevent them from reproducing uncontrollably. But when paleontologist Alan Grant (played by actor Sam Neal) and the children escaped from T. rex, they saw hatched dinosaur eggs. This showed that the female dinosaurs in the park were reproducing by the process of parthenogenesis.
Basic biology teaches that the process of reproduction has a strict requirement: a sperm and an egg must unite to create a fertilized egg that will develop into an embryo. During this process, a re-assortment of the male and female chromosomes occurs, which are then reduced to the normal level. This process of chromosome reduction is meiosis. In parthenogenesis, because there is only the set of female chromosomes, meiosis does not occur. And since all the chromosomes come from the mother, all the offspring are females, identical clones of the mother.
Parthenogenesis is risky to a species because it does not allow for increasing genetic diversity, a hallmark of sexual reproduction. Without genetic diversity, mutations can be passed down to new generations, which can accumulate and have damaging effects on the species. One clever mechanism for parthenogenic organisms to achieve genetic diversity is used by bdelloid rotifers, microscopic freshwater animals. They have evolved the ability to “grab” DNA from their environment and incorporate it into their own genome, handing down this new genetic information to the next generation. This must be successful, since this species has not had a male member (or any sex) in at least 40 million years!
Over the past decade, several new animals were found to be capable of parthenogenic reproduction in captivity. For example, female Komodo dragons that had not been in the presence of a male produced offspring that were genetically identical to the mothers. In nature, these lizards were thought to reproduce only sexually, so this was a surprise. In addition, a bonnethead shark and a boa constrictor were also found to produce genetically identical female offspring without sexual contact.
In all these cases, scientists do not know what triggers this nonsexual mechanism of reproduction. It seems certain that the process can sustain the species when no males are available. In itself, this may represent an evolutionary adaptation to preserve a species.
Science faces a frontier in terms of understanding parthenogenesis in the animal world. Understanding the signals that trigger this process could help save endangered species on Earth. As far as whether this is possible in humans may be a future concern for medical ethics and society.
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Pamela K. Bond 