Imagine for a minute that you are a zookeeper and that one day while inspecting your prized lizard exhibit you make a surprising discovery: your rare female Komodo dragon has produced a clutch of viable eggs despite having no contact with a male dragon! “What’s going on here?” you wonder. Well, the staff at two different European zoos encountered this scenario recently, and their findings have led to the discovery that the Komodo dragon, the largest of the world’s lizards, and an endangered species, is capable of reproducing asexually, making it the largest vertebrate animal known to reproduce in this way.
Vertebrate animals usually reproduce sexually, generating offspring through the fusion of male and female gametes (the egg and the sperm). However, some species have evolved an alternative to sexual reproduction. In fact, females from over 70 vertebrate species, including some snakes, birds, and small lizards, are capable of producing eggs which can become viable embryos without male fertilization. This phenomenon is known as parthenogenesis (from the Greek for “virgin birth”). Parthenogenesis seems to fly in the face of what is commonly assumed about reproduction, but it turns out that the ability to reproduce asexually is more prevalent in the animal kingdom than was previously believed. Interestingly, the biology behind parthenogenesis might inform scientific fields as diverse as reproductive biology, conservation biology, and even stem cell biology.
The Virgin Dragons
According to the article published last month in Nature, the only two sexually mature female Komodo dragons (Varanus komodoensis) in Europe, laid clutches of viable eggs within the past year, despite having no contact with males. One of the dragons, Flora, who is now 8 years old, was born in captivity and lives at the Chester Zoo in the UK. Although Flora’s eggs are not expected to hatch until later this month, researchers were able to analyze the genetic make-up of the embryos inside after a few of the eggs collapsed prematurely. They used a technique called genetic fingerprinting, the same technique that is used in human paternity tests and by forensic scientists to analyze biological samples at a crime scene.
The logic behind genetic fingerprinting is that by comparing samples of genetic material, or DNA, one can determine the degree of relatedness between different individuals. For Komodo dragons, as for humans, the majority of DNA (99% in humans) is identical between individuals, but the 1% that differs can be very informative when one wants to resolve a case of unknown paternity. The differences in the DNA are due to random variations in the DNA code in so-called “junk regions” of DNA, those regions that are not known to contain genes or other important regulatory elements. Variations in “junk regions” can persist and accumulate through the generations, because variations in these regions do not affect the survival of the animal. Scientists can assess the degree of relatedness by comparing the variable genetic code at particular sites in the genome. In the case of the Komodo dragons, a few blood samples were all that were required for researchers to confirm that the embryos had no father.
Does Parthenogenesis Produce Clones?
Although the offspring that result from parthenogenesis, known as parthenogens, have only one parent, they are not clones of their mother. This is because initially the unfertilized egg contains only half the number of chromosomes as the mother (the other half would usually come from the sperm). However, for reasons still not understood, the egg’s half-set of chromosomes can double to supply a full complement of genetic material.
Although Flora’s eggs are not due to hatch until later this month, it is already known that all of her new offspring will be male. In contrast to humans, in which females have a matching pair of sex chromosomes (designated XX), and males have a mismatched pair (designated XY), it is the male Komodo dragon who has a matched pair (designated ZZ) and the female who has the mismatched pair (designated WZ). Because each of the offspring produced as a result of parthenogenesis will be the product of one egg (which contained just one sex chromosome), the doubling of the egg’s genetic material to achieve a full complement of chromosomes can result only in progeny of ZZ (male) or WW (unviable) but never WZ (female)
Conservation of the Endangered Dragons
Komodo dragons are native to Indonesia and are increasingly under threat as wild populations become smaller and more fragmented. It is estimated that only 4000 Komodo dragons remain in the wild, of which perhaps fewer than 1000 are mature females. Fragmented populations make matings more difficult and decrease genetic diversity because the dragons have fewer potential partners, and the potential partners they do encounter might very well be closely related.
On the face of it, one might think that the ability to reproduce by parthenogenesis would be an advantage to an endangered species in need of bolstering its numbers. After all, if females can lay viable eggs without mating, that should speed along reproduction and produce more animals, right? But it turns out that in reality the situation is much more complex. For one thing, all of the offspring produced by Komodo dragon parthenogenesis are males, and a breeding population skewed toward males is not all that helpful when you want to boost the birth rate.
There is another reason that parthenogenesis could actually hamper conservation efforts: parthenogenesis decreases the genetic diversity of the population, making it more vulnerable to infection, disease, and other stressors. Although parthenogens are not clones of their mother, they are not as diverse as sexually produced offspring that carry two differing sets of chromosomes. The ability to combine the genes of two different individuals is one of the main advantages of sexual reproduction, and one of the reasons that it has been such a successful strategy for so many organisms.
Parthenogenesis and Stem Cells
Although 0.1% of vertebrate species are known to be capable of reproducing by parthenogenesis, no mammals are known to reproduce in this way. However, mammalian eggs, including those of human females, can spontaneously activate. When this occurs, the egg divides as if it had been fertilized, but the incipient embryo fails to progress past a very early stage of development. Scientists have determined how to activate unfertilized eggs in the lab and the hope is that they might be able to generate stem cells from these dividing eggs. Because there is no evidence that an unfertilized mammalian egg can become a viable embryo, some people see the development of stem cell technology based on the activation of unfertilized eggs as a way to develop stem cell therapies without becoming ensnared in the ethical tangles typically associated with embryonic stem cells. As this research progresses, researchers envision that, for example, a young woman with type 2 diabetes would be able to use one of her own unfertilized eggs to generate compatible stem cells capable of regenerating the insulin producing cells in her pancreas. This technology is still a long way off, but it provides an exciting area of research which could someday lead to great clinical benefits. In the meantime, after an 8-9 month gestation, Flora’s baby Komodo dragons are about to emerge from their eggs — and in the process, give new meaning to the term “single mom”.
— Kelly Dakin, Harvard Medical School
For More Information:
Scientific American article: Strange but True: Komodo Dragons Show that “Virgin Births” Are Possible
NOVA interview with a researcher who is studying parthenogenesis as a method of producing stem cells from unfertilized eggs and thereby side-stepping the ethical tangles encountered by using fertilized eggs. <http://www.pbs.org/wgbh/nova/sciencenow/3209/04-alternative.html>