The Komodo Dragon's Tale
by Richard Dawkins
An adult Komodo dragon, three yards long and a voracious carnivore, could be mistaken for a decent-sized dinosaur. Komodo dragons are actually giant monitor lizards (Varanus komodoensis) and they have not gone the way of the dinosaurs. Not yet at least, but they were one of the endangered species visited by Douglas Adams and Mark Carwardine in their enchanting Last Chance to See. They are confined to a handful of Indonesian islands, including Komodo itself and also Flores, home of the recently extinct Homo floresiensis on which they, and an even larger species of giant lizard, now extinct, perhaps preyed (if, that is, H.floresiensis was a real species of miniature human at all). Komodo dragons still eat humans when they get the chance. It has been suggested that the Komodo dragon is well named as the real-life origin of all our dragon myths, and it is certainly plausible that Chinese sailors would have brought back awestruck tales of them. They don't breathe fire but their mouths are so riddled with festering bacteria that one bite is fatal. Their preferred method of hunting is to deliver that fatal bite, then follow the prey around until it dies from the resulting bacterial infection, then eat it. Komodo dragons are topical as I write (Christmas 2006) because of an interesting paper in Nature (Vol 444, 28th December 2006) presenting evidence that they can reproduce parthenogenetically ('virgin birth'). This is the subject of my Tale.
For a while, mysterious stories have been surfacing of captive females in zoos producing apparently fatherless offspring. For example, New Scientist reported on 25th April 2006 that Sungai, a female Komodo dragon in the London Zoo, now dead, laid fertile eggs which hatched into male offspring, despite not having seen a male for two and a half years. New Scientist offered two possible explanations. First, Sungai might have stored sperm in the way many insects do, and used it later. Second, Sungai might have cloned herself, producing offspring as genetically identical to her as her left foot is to her right. But isn't cloning ruled out by the fact that the young lizards born by 'virgin birth' were all male? True clones would be genetically identical to their mother, and wouldn't that mean they had to be female like her?
The answer is yes, but it is not quite such an obvious answer as you might think. In most vertebrates, sex is determined genetically, and clonal offspring would have to be of the same sex as their mother. But sex can be determined by non-genetic means, environmentally. In some fish species, males are rare and have harems. The females in the harem sort themselves out into a dominance hierarchy by fighting. If the male of the harem should die, the dominant female changes sex and takes over the harem. All males started out as females and later changed sex in this way.
No reptile does it in that way, but another kind of environmental trigger that can determine sex is temperature. This method of sex determination is quite common among reptiles, including crocodiles, tuataras (a 'living fossil' found only on islands off New Zealand), most tortoises and turtles, and a few snakes and lizards. In some lizard species, eggs that are incubated above a threshold temperature produce males, while colder eggs produce females. Presumably natural selection adjusts the threshold to achieve a 50/50 sex ratio on average, in the normal habitat of the species. One theory for what drove the dinosaurs extinct, now not much favoured, is that global warming led to their producing nothing but male offspring.
If Komodo dragons determined their sex by temperature, females who cloned themselves might produce sons if their eggs happened to be incubated above the threshold temperature. Unfortunately for this theory, however, sex determination in Komodo dragons is not done by temperature as in crocodiles, but by chromosomes, as in birds and mammals. This knocks on the head any suggestion that a female could produce sons by cloning. The clonally offspring of a female dragon would have the same sex chromosomes as her, and would therefore have to be of the same sex.
But New Scientist failed to mention a third possibility. Virgin birth can come about through 'selfing', as is common in plants, and that is not the same thing as cloning. Cloning, for instance in the famous sheep experiment, means making a new sheep (Dolly) from one of the diploid body cells of the old sheep (unnamed and unsung, poor thing). The two sheep in that experiment were equivalent to identical twins but of different ages, and they of course had to be of the same sex. Selfing is different. It is as though an individual mates with it. Two haploid (one set of chromosomes) cells from the same individual, one of them behaving like a sperm and the other like an egg, join together restore the diploid (doubled up) number of chromosomes. It doesn't have to happen like this, but if the two haploid cells that 'mate' have been formed by one haploid cell splitting into two, they will be identical to each other. The diploid baby will have a set of paired chromosomes in which every chromosome is identical to its pair, but not identical to the corresponding chromosomes of other babies in the clutch. The chromosomes of each baby, in other words, will be a set of matched pairs, but its siblings will each have a different collection of matched pairs. To use the genetic jargon, every baby will have 'zero heterozygosis' but in a different way from its siblings. And to put it another way, although the baby is diploid, with doubled up chromosomes, it might as well not 'bother' with the second of each chromosomal pair because it is identical to the first.
Now, what sex would the babies be, if they were the products of selfing? Here is where it gets interesting. If a woman reproduced by selfing, the child would have to be female. Female mammals have two X chromosomes while males have one X and one Y. Every human egg has an X chromosome, while sperms are of two kinds. 50% of sperms have a Y chromosome and the other 50% have an X chromosome. If an X sperm fertilizes an egg (necessarily X) the result is a female (XX). If a Y sperm fertilizes an egg, the result is a male (XY). You can quickly work out why this mechanism produces sons and daughters in equal frequency. And now, to answer the question about selfing. A human female has no Y-chromosomes. Every baby produced by selfing would have to be XX and therefore female.
But with some animals, such as birds and butterflies, the system works the other way around. Females are (the equivalent of) XY and males (the equivalent of) XX. They are actually called W and Z instead of Y and X, but the principle is the same. Now, think what the result would be if a female bird reproduced by selfing. She produces both Z egg cells and W egg cells. If a Z cell split and 'mated' with itself, the result would be ZZ and therefore male. Theoretically a W cell might split and 'mate' with itself to make a WW individual but that combination is unknown in nature (for good reasons which could occupy another Tale) and presumably would not survive. Therefore, if a female bird could reproduce by selfing, the offspring would all be male.
Komodo dragons are like birds in this respect. Males are ZZ and females are ZW. True clones of a female dragon would be ZW and therefore female. But if a female dragon reproduced by selfing, the surviving offspring would all be ZZ and therefore male. This is indeed the result observed. The hypothesis of selfing in Komodo dragons looks good.
The new paper in Nature adds powerful supporting evidence. Watts, Buley, Sanderson, Boardman, Ciofi and Gibson looked at the DNA of two female Komodo dragons, Flora in Chester Zoo and the already mentioned Sungai in London Zoo, and their parthenogenetic (all male) offspring. Exactly as expected, the babies within any one clutch were homozygous at all loci ('matched pairs' of chromosomes). Again as expected, they were not clonally identical to each other or to their mother but, again as expected, they did not have any genes not possessed by their mother. The genome of every baby, in other words, was a proper subset of its mother's genome, but a different subset from its siblings' genomes. The selfing hypothesis is upheld.
It has been suggested that, like some insects, Komodo dragons in the wild use sexual reproduction when they can, but females resort to virgin birth whenever they find themselves without a male. They are good swimmers capable of crossing from island to island. A female who found herself alone on an island could theoretically give asexual birth to sons, then mate with them to produce daughters as well as sons and thus colonize the new island.
The Nature paper's publication in Christmas week has given rise to predictable drollery in the newspapers, and it is probably no accident that Dembski has posted an article about it on his website. The curator of reptiles at Chester zoo and one of the authors of the Nature paper remarked: "Essentially what we have here is a 'virgin birth' and, because the eggs were laid back in May, the incubating eggs could hatch around Christmas time. We will be on the look out for shepherds, wise men and an unusually bright star in the sky over Chester Zoo." Christian apologists for Virgin Birth would be unwise to pin too many hopes on the Komodo dragon, however. The Virgin Mary being a mammal, whether she reproduced by cloning or by selfing, the result could only be a daughter. Jesus either had an earthly father, or Jesus was a woman.
Bizarrely, one evangelical Christian apologist, R J (Sam) Berry, Emeritus Professor of Genetics at University College, London, once suggested another way in which Mary could have given virgin birth to a son. Mary, he speculated, might have been a genetic male, with XY chromosomes. She appeared to be female because of a genetic inability to respond to testosterone. "If this happened, and if the ovum developed parthenogenetically, and if a back-mutation to testosterone sensitivity took place, we would have the situation of an apparently normal woman giving birth without intercourse to a son." Even Berry apparently didn't think much of his own hypothesis: "The mechanisms I have outlined are unlikely, unproven, and involve the implication that either Jesus or Mary (or both) were developmentally abnormal. My purpose in describing them is simply to reduce the assumption of incredibility that seems to dog the doctrine of the Virgin Birth." (Daily Telegraph, 21st November 2001). All a bit unnecessary, given that the entire legend of the Virgin Birth stems, in the first place, from a mistranslation of a Hebrew word meaning 'young woman' into a Greek word meaning 'virgin'. To see how easily this could happen, think of the English words 'maid' or 'maiden'. Berry is an extremely devout man, but his attempt to stretch science to fit scripture, especially mistranslated scripture, seems to be pushing the envelope of apologetics into uncharted waters.
A Merry Christmas to all our readers.
This Tale was specially written for our website at Christmas. It is not in The Ancestor's Tale, but it would have been if the facts reported here had been known two years ago.
