Genetics of virgin birth in the Komodo dragon


I’ve just read the article on the parthenogenetic Komodo dragons in Nature, and it’s very cool. They’ve analyzed the genetics of the eggs that have failed to develop (the remainder are expected to hatch in January) and determined that they were definitely produced without the aid of a male.

We analysed the parentage of the eggs and offspring by genetic fingerprinting. In the clutches of both females, we found that all offspring produced in the absence of males were parthenogens: the overall combined clutch genotype reconstructed that of their mother exactly. Although all offspring were homozygous at all loci, they were not identical clones. Parthenogenesis was therefore confirmed by exclusion (clutches had different alleles from potential fathers) and by the fact that the probability of obtaining a clutch of homozygous individuals after sexual reproduction was very low (P<<0.0001). Sungai’s resumption of sexual reproduction confirmed that parthenogenesis was not a fixed reproductive trait (that is, it is facultative) and that asexual reproduction is likely to occur only when necessary.

That line about “all offspring were homozygous at all loci, they were not identical clones” might need a little more explanation. Mama Dragon is heterozygous at some loci, but the meiotic mechanism that produces a diploid egg means that one cleavage (most likely the second meiotic cleavage) was suppressed, so both homologous chromosomes in the resultant ovum were derived from the same replicated DNA strand. They are not clones of the mother, because they are all homozygous while she was heterozygous; they are not identical, because which of each of the paired homologous chromosomes was passed on to an individual is random.

(I’m a little confused by the statement that they offspring are homozygous at all loci, though; that would imply that there was no crossing over at all in meiosis I, which doesn’t sound right. There ought to be reduced heterozygosity but not complete homozygosity, unless reptiles are weirder than I thought.)

The other useful snippet of information is that sex determination in these reptiles is of the WW/WZ type, where the females are the heterogametic sex. Since all of the progeny of parthenogenesis are homozygous, they are all of the homogametic genotype, and therefore male.

Parthenogenesis can also bias the sex ratio: in Varanus species, females have dissimilar chromosomes (Z and W), whereas the combination ZZ produces males10, so the parthenogenetic mechanism can produce only homozygous (ZZ or WW) individuals and therefore no females.

This has theological implications, obviously. We can now understand how a female could give rise to a male by parthenogenesis: Mary Mother of God must have been a heterogametic reptoid. David Icke will be so pleased.

Watts PC, Buley KR, Sanderson S, Boardman W, Ciofi C, Gibson R (2006) Parthenogenesis in Komodo dragons. Nature 444:1021-1022.


  1. says

    Thanks, PZ–when I saw this story yesterday, I was completely confused as to how the offspring could be homozygous @ all loci but not clones of each other or of the mother. Appreciate your shedding some light on it for us biology-illiterate types!

  2. Steve_C says

    Who knew the return of christ would be in the form of a Komodo Dragon?

    I deny the existence of the Komodo Christ Dragon. And the Holy Komodo Dragon Spirit.

  3. says

    I’m thinking the authors must have meant offspring were homozygous at all observed loci; otherwise, it just doesn’t work for me.

    This is nagging at me, you can tell.

  4. Phoenician in a time of Romans says

    We will note that the creationists are already using this to show… well, I’m not too sure, but it may have something to do with Mary and Jesus.

    Consider this exchange:

    Creationist: “These dragons produced without males!”
    Normal Person: “How do you know?”
    C: “DNA fingerprinting proved it!”
    NP: “Doesn’t DNA fingerprinting also prove common descent of related species?”
    C: “… Why are you so anti-Christian?”

  5. says

    Anybody else having trouble using trackbacks on this blog? I keep getting error messages saying that the provided trackback link is invalid.

  6. says

    Oops, right – the babies are homozygous at all the loci that were genotyped.

    And most likely homozygous at the others, too – but, there’s no physical proof.

  7. Steve_C says

    Anyone seen the documentary about these goofballs who manage to sneak into one of those renaissance weekends… there’s a whole funny ceremony and they tape it from the woods and act as if they’re terrified and that there’s going to be a human sacrifice. Many powerful people go on these retreats. For some reason I thought Icke was associated with it.

  8. John Burns says

    Are there examples of parthenogenesis in organisms that are heterzygous for the male where the sex of parthenogenetic reproduction for all offspring being female?

  9. says

    Paul, I think the homozygosity is not surprising if I remember my genetics correctly. (It’s been a few years since I taught biology, so I may be a little hazy.) I seem to recall a herpetologist mentioning that parthenogenesis in reptiles usually occurs via recapture of a second polar body.

    Since the second polar body is formed by the second, nonreductive, meiotic division (separating the two strands of the haploid chromosomes), and since crossing over normally involves exchange of double-stranded fragments between homologues, the polar body and the egg nucleus should be essentially genetically identical.

  10. Stephen Wells says

    I think parthenogenetic whiptail lizards are all-female- though they do display courtship behaviours, crazy lesbian lizard orgy ahoy!- and parthenogenetic reproduction in aphids is all-female too. I don’t think any mammal has displayed this behaviour (assuming we’re not taking the gospels as Gospel).

  11. Steve_C says

    The retreat is called Bohemian Grove. And Icke claims that children are sacrificed there.
    It’s funny. I think Harry Shearer at one point is doing the narration. Obviously a reptilian.

    Apparently they don’t understand metaphor. Burning of “care”.

  12. says

    Recapture of the second polar body will still produce some heterozygosity. Sister chromatids begin as identical, but recombination exchanges genetic material between homologs, causing sisters to differ.

    As for the lack of heterozygosity, is it possible that crossing over is suppressed in females? In some taxa, recombination is lower in the heterogametic sex than the homogametic sex (ie, Drosophila, humans). In fact, Drosophila males totally lack recombination.

  13. says

    A couple of things (but off the top of my head because I’m at home and about to go to bed)…

    Parthenogenesis is a surprisingly common phenomenon; it’s also a loose name for a bunch of different phenomena. For starters, as PZ noted in his comment, parthenogenesis doesn’t have to be STRICTLY asexual, because one of the benefits of sex comes from chromosome shuffling at meiosis I, and parthenogenesis allows that. This means that parthenogenetic offspring can have new gene combinations which weren’t present in the parent and so can get some ‘sexual’ benefit, just without the messy tentacle sex.

    On the other hand, as RPM notes, in some cases you can have egg generation through weird apo-meiosis, where crossing over doesn’t occur at meiosis I. This could certainly be one explanation of the ‘homozygous at all loci’ statement, or their English may just be a bit lax.

    As for where it happens, yes in whiptail lizards, although there it’s associated with polyploidy (as it often is), yes in frogs, yes in birds (and possibly dinosaurs?), yes in earthworms, yes in all sorts of insects, but generally no to mammals because of something called imprinting, where genes in the egg pronucleus are turned off. If you tweak the genes, you can get parthenogenetic mammals (as was done with mouse eggs a couple of years ago). However, there are papers dating back to the ’50s which detail experimentally induced parthenogenesis in rabbits. You can get parthenogenetic egg activation in humans, but development is whack and it causes ovarian tumours (although technically technically I guess proper embryonic development is at least a possibility).

    Finally, to answer John Burns, there are lots of cases where homozygous females give rise to parthenogenetic daughters. Rabbits, as I’ve mentioned, but many insects use an XX/XO system (XX=female) so a lot of them produce daughter parthenotes (which is known in the trade as THELYTOKY, as opposed to parth males, which is ARRHENOTOKY. I’ll let you do the Greek…)

  14. Jim in STL says

    8 Holy Komodo Sons (Daughters?)
    8 Holy Komodo Fathers
    1 Holy Komodo Mothers
    8 Holy Komodo Ghosts

    I look forward to the holidays.

    Merry Komodomas (in advance) to Dr. Myers and all. And a Happy New Year.

  15. says

    Sandy, your explanation doesn’t address my objection. Crossing over will scramble a small subset of the genes, so that the two replicated strands aren’t exactly identical — RPM has it right.

    This is going to be a small proportion of the total, though, so maybe it’s just that none of their assayed genes exhibited this effect. If they’d found one or a few loci that were het in the offspring, it would not mean they couldn’t be the products of parthenogenesis. It’s just that I wouldn’t say in a Nature letter that all loci were heterozygous as they did.

    Oh, and resorption of the second polar body would be identical in effect to suppressing the second meiotic division.

  16. says

    The BBC article that I read mentioned that some Japanese scientists had been successful in developing a way for mice to reproduce parthenogenically and that in time they could adapt the process for humans. Anyone who isn’t wowed by the 21st century isn’t paying attention.

  17. says

    If this sounds like splitting hairs, I apologize, but crossing over would not cause heterozygosity if parthenogenesis occurred by either of the methods discussed. What would cause heterozygosity is a different, although similar, process called sister chromatid exchange. This is a somewhat rarer even to begin with, but it is also possible to conceive of one process occurring without the other.

    I would also point out that recapture of a polar body is entirely different from suppression of the second meiotic division in that the result of the former would be diploid while that of the latter would be haploid, unless some special even occurs that allows replication of the centromeres. Normally this would happen at meiosis II, but if that process is suppressed, some other special process would have to account for it. If such a process were available, one would expect to see polyploidy occurring randomly all over the place. I think polar body recapture is a much more reasonable mechanism in a species that is capable of both sexual and asexual reproduction, as it only involves substituting a polar body for a sperm nucleus.

  18. Scott Hatfield says

    PZ: (in a playful tone of voice) And I thought Jim Morrison was the Lizard King.

    Hey, let’s mine this well a little deeper. Perhaps, as the offspring of a parthenogentic Komodo dragon, Jesus would’ve also acquired its extremely virulent microflora, and thus its highly toxic saliva.

    Thus, much of the alleged “miracles” credited to our Savior are merely side-effects, hallucinations triggered by inhaling small amounts of the Nazarene’s airborne saliva.


  19. hoody says

    Ths hs thlgcl mplctns, bvsly. W cn nw ndrstnd hw fml cld gv rs t ml by prthngnss: Mry Mthr f Gd mst hv bn htrgmtc rptd.

    n thrws ntrstng nd chrnt pst f yrs scrmbld by yr nblty t cntn yr nt-Chrstn bgtry.

    nd s prdctbl. Y’r flggng n yr dtg, PZ.

  20. says

    Please explain to me how sister chromatid exchange would result in heterozygosity. Both supression of meiosis II and reabsorption of the second polar body would result in a pseudo-diploid cell made up of pairs of sisters, rather than pairs of homologs. Sisters are identical following the replication that precedes the first meiotic division. Exchange between sisters will not shuffle genetic material because there is nearly no variation between sisters prior to recombination between homologs.

    The end result of suppression of a second meiotic division and reabsorption of the second polar body is the pseudodiploid described above. While the mechanisms are drastically different, they have very similar effects.

  21. William says

    Quick question: are WW and ZZ both males, or is one of them (like a YY in humans) nonviable?

  22. says

    Right, PZ, I see your point.

    I commented on evolgen, once I thought about this problem a bit more.

    The authors weren’t looking for recombinants, they were looking for parthenogenesis. So, I don’t think they surveyed enough sites (they only did 7!) or enough offspring (only 3!), to see any.

  23. CCP says

    Parthenogenic whiptail lizards result from hybridization between species, and some, but not all, are polyploid. They’re all female because they have XX/XY (male heterogametic) sex determination. Interestingly, all such lineages are believed to be only hundreds of years old.