The Komodo dragon (Varanus komodoensis) is one of the most endangered species of lizard on the planet. With only 350 breeding females left in existence1 it is placed on the IUCN Red List of Threatened species and considered vulnerable2. These beasts are endangered mainly because of human factors like hunting and habitat loss1 and their predicament is made even worse by the fact that they live as largely solitary creatures and rarely meet appropriate mates.
However, female Komodo dragons are strong, confident lizards who don’t need any male – they can produce a clutch of eggs without being fertilised by a male3.
If a female has not encountered a male in a while she can self-fertilise her eggs and give viable offspring that are not simply clones of the mother3. The way she does this is very clever: in regular sexual reproduction, the genomes of the male and the female are combined to give offspring that are genetically different from both of the parents. This is a characteristic which is vital to natural selection as it enriches the gene pool. So how does a self-fertilising female produce genetically different children?
Well what she does is a complex shuffling of the DNA present in her egg cells which gives each one a double set of chromosomes which have mutated such that each one has a different genotype. We know that the female has definitely not mated with a male as if we consider the gene pool in her clutch alone, it displays the exact genotype of their mother3. This is called parthenogenesis.
What we see in these organisms is that eggs left unfertilised (i.e. untouched by male member) develop in to male insects. We see this in all the hymenoptera, the winged insects; so that means fruit flies (the geneticist’s favourite), dragonflies and honey bees. This occurs because these insects, somewhat unusually, can develop and function with only a single set of chromosomes, unlike humans which must have two (and only two) sets. Hymenopterans have their sex determined by their number of chromosomes, in essence6. A single set of chromosomes makes a male and any more makes a female. Thus an egg left unfertilised will ultimately become male offspring. This is observed as an important part of the population dynamic we see in honey bees.
But what about humans doing parthenogenesis? Parthenogenesis presents a promising opportunity for the production of human stem cells. As you may know stem cells hold incredible potential in medicine, they are essentially the most basic form of cell and still hold the potential to multiply and develop in to any type of cell, or tissue or even a new organ like a heart. It is for this reason that they may in the future be used for organ transplants.
Currently, stem cells obtained for research purposes are obtained from aborted foetuses. This is a matter of some controversy. Parthenogenetic development of stem cells from a woman’s egg cells may hold the key to a more ethical method of acquiring stem cells7.
This may be done by taking the eggs from a consenting female donor and essentially tricking them into thinking that they had been fertilised so that they developed in to a ball of cells called the blastocyst7, the next stage in the development of the foetus. This may be seen to be more ethical as the cells are not becoming anything relatively near a developed baby, both in shape and in size.
But all we are doing in this endeavour is attempting to replicate a mechanism already seen in nature. As usual, nature is at least one step ahead of us; our technology is already rendered obsolete by nature and we are barely managing to catch up with it, and the Komodo dragon or the bumble bee’s parthenogenetic ability is only one example of that.
- Federation, W. W. Komodo Dragon, King of the Lizards, URL:http://wwf.panda.org/about_our_earth/teacher_resources/best_place_species/current_top_10/komodo_dragon.cfm, 2012
- List, I. R. Varanus komodoensis, URL:http://www.iucnredlist.org/details/22884/0 (2012).
- Watts, P., C., Buley, Kevin, R., Sanderson, Stephanie, Boardman, Wayne, Ciofi, Claudio, Gibson, Rischard,. Parthenogenesis in Komodo Dragons. Nature 444, 1021-1022, doi:http://dx.doi.org/10.1038/4441021a (2006).
- White, M. J. D. Animal Cytology and Evolution. (Cambridge University Press, 1973).
- W. Booth, L. M., R. G. Reynolds, G. M. Burghardt, E. L. Vargo, C. Schal, A. C. Tzika, G. W. Schuett. Consecutive virgin births in the new world boid snake, the Colombian rainbow boa, Epicrates maurus. Journal of Heredity 102, 759-763 (2011).
- D. P. Cowan, J. K. S. Functionally reproductive diploid and haploid males in an inbreeding hymenopteran with complementary sex determination. Proceedings of the National Academy of Sciences of the United States of America 101, 10374-10379 (2004).
- Ester Polak de Fried, M. D., Pablo Ross, M.Sc., Gisela Zang, M.Sc., Andrea Divita, M.D., & Kerrianne Cunniff, M. S., Flavia Denaday, M.D., Daniel Salamone, M.Sc., Ann Kiessling, Ph.D., and Josie Cibelli, Ph.D. Parthenogenetic blastocysts from frozen oocytes. Fertility and Sterility 89, 943-947 (2008).