Parthenogenesis in Komodo Dragons

_{December 21, 2006

Intelligent Design}

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I blogged about the possibility of parthenogenesis in mammals a month or so ago. Here’s another newly discovered case of it in a higher animal.

Virgin Dragon Prepares to Give Birth
Dec 20, 1:36 PM (ET)
By MARIA CHENG

CHESTER, England (AP) – In an evolutionary twist, Flora the Komodo dragon has managed to become pregnant all on her own without any male help. She is carrying seven baby Komodo dragons.

“We were blown away when we realized what she’d done,” said Kevin Buley, a reptile expert at Flora’s home at the Chester Zoo in this town in northern England. “But we certainly won’t be naming any of the hatchlings Jesus.”

Other reptile species reproduce asexually in a process known as parthenogenesis. But Flora’s virginal conception, and that of another Komodo dragon earlier this year at the London Zoo, are the first time it has been documented in a Komodo dragon.

The reptiles, renowned for their intelligence, are native to Indonesia. They are the world’s largest lizards and have no natural predators – making them on par with sharks and lions at the pinnacle of the animal kingdom.

The cases of Flora and the London lizard, Sungai, are described in a paper published Thursday in Nature.

Parthenogenesis is a process in which eggs become embryos without male fertilization. It has been seen in about 70 species, including snakes and lizards. Scientists are unsure whether female Komodo dragons have always had this latent ability to reproduce or if this is a new evolutionary development.

Comments

BTW, doesn't this simply underline the saying that "all living things come from an egg"?PaV_{December 22, 2006
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Fross: "being on top of the food chain doesnÃ¢â‚¬â„¢t mean they still donÃ¢â‚¬â„¢t face natural challenges. If males become scarce, itÃ¢â‚¬â„¢s a good ability to make babies without them." As I posted, I was sure that a Darwinist would come up with that explanation--the only one possible. A shark kept in an aquarium is supposedly given birth via parthenogenesis. But then the question becomes: how did it evolve? When it evolves, sure, it's better; but HOW did it evolve? What's the mechanism? As to "topping the top", how does an organism that is the "fittest" become "more fit"? But lest you suggest that parthenogensis is such an example, please first explain the mechanism?PaV_{December 22, 2006
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Thus, sex may be favored. Well that's sure a fine theory, b. But if that's your story and you're sticking with it you're stuck explaining why, in the fossil record, species with obligatory sexual reproduction appear abruptly, hang around unchanged for an average of 10 million years, and then 999 times out of 1000 go extinct without splitting off any new species. Sexual reproduction allows deleterious mutations to accumulate in individuals heterozygous for the mutation. That's fine in a large population where the mutations aren't likely to match up. But if the environment becomes adverse and reduces the population then all of a sudden the effects of inbreeding show up. Deleterious mutations start becoming homozygous and the small population quickly dies off. Parthenogenesis resulting in individuals homozygous at all loci would thus be rare; a last ditch effort at survival because in an old species it would bring out all the deleterious mutations previously hidden by heterogeneity. Parthenogenesis is worse than doing it with a cousin, it's doing it with yourself, inbreeding on steroids so to speak. But hey, that's just my theory. Don't let its ability to explain the facts cause you to abandon any explanation that can't. NDE couldn't pass the giggle test if it had to fit the facts. DaveScot_{December 22, 2006
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DaveScot, I wasn't precise. What I meant is regarding two non-allelic mutations (not copies). In other words, say the deleterious selection coefficients were designated s. The fitness of an individual, in the scenario I mentioned, with 1 mutation would be (1-s). If the fitness of an individual with 2 mutations were less than (1-s)^2, then the individual with two mutations would be disporportionally less fit than individuals with just one mutation. Thus, sex may be favored. Of course whether selection acts like this is not known.bdelloid_{December 21, 2006
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bdelloid Another one of them is that harmful mutations are more than double harmful when in two copies. That's the wrong POV. Only one copy in a diploid organism is half as harmful as one copy in a haploid organism. Thus harmful mutations can accumulate until they eventually drive the species into extinction. Allelic mutation allows rapid adaptation but the eventual cost is extinction.DaveScot_{December 21, 2006
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Atom, As you point out, the question of why sex is maintained is a big question. As Bob points out, there are many theories. Another one of them is that harmful mutations are more than double harmful when in two copies. Thus, when brought together by recombination (ie, sex) they are purged much faster in the population. Thus, sex may allow populations to avoid the slow decline of deleterious mutations due to Mullers Ratchet. This argument has many complications however - one being, as you point out, selection doesn't act strongly on the "benefit to the population". However, it has been shown to be true that very few asexual lineages have lasted a long time. If one makes a phylogeny, one will see that most asexuals are at the "tips" of the tree - there are not a lot of deeply diverging asexual clades. One major exception to this - the bdelloid rotifers (thus my handle !).bdelloid_{December 21, 2006
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Fross, granted in this case. But I only use this specific case to bring up the general question of sexual vs. asexual reproduction.Atom_{December 21, 2006
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hey Atom, in the case of these dragons, they can only give birth to males when reproducing asexually. So if she wants a male to mate with, she's going to have to give birth to one first. ewwwwwwFross_{December 21, 2006
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Thank for you feedback Bob. I appreciate your points. My question is, absent of such catastrophic changes in environment (which may or may not provide temporary greater than 50% increases in fitness) why don't the animals with the ability to reproduce in both modes (such as these dragons) return to favoring asexual reproduction? Again, if fractional gene passing can explain altruism in Kin Selection theory, why does it not apply here, when we see an obvious "selfish gene" benefit?Atom_{December 21, 2006
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Just to update my previous comment about males, at Pharyngula, PZ Myers points out that the offspring are homozygous at the all loci screened, so they'll all be male. Go on, follow the link, you know you want to :-) BobBob OH_{December 21, 2006
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Atom - point 1 is why I mentioned the two levels of selection. If the Red Queen idea is right, populations that went over to asexualreproduction would go extinct as they couldn't keep up with the environment. I think there's some evidence for this. You're right, the 50% cost looks huge, but I think it can be overcome: the effects of a new pathogen, for example, can be large enough. I haven't been following the literature, but I think there's some empirical evidence from snails that the benfits can be large enough. I suspect that the problem hasn't been nailed yet, but there are several interesting lines of research being followed. BobBob OH_{December 21, 2006
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Addendum: by "form" I meant from. Also, "forward selection" (selecting traits for future usefulness rather than immediate utility) is a property indicative of Telic processes.Atom_{December 21, 2006
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Bob, thank you for the response. ReMine mentions the Red Queen Hypothesis in his book as well as most other attempts to explain (or explain away) the sex issue. The main argument against such hypotheses is two-fold: 1) They rely on deferred success. Basically, they say the organisms will benefit in the not too distant or maybe distant future form this. This will not work because the advantag of asexual reproduction is immediate and extremely high, thus overpowering any deferred fitness improvements. 2) The new benefits would have to increase fitnes by 50% or more. Evolutionists usually discuss fitness increases in the low 1% range, so a net 50% increase from merely increasing variation (or from any other benefit) is very unlikely. Unless an exact benefit is found and can be shown to increase fitness the necessary amount, the problem is still a problem. Just-so isn't enough in this case. I realize that you're probably not making the argument yourself, but I just wanted to discuss these issues. They have intrigued me since I read about them first in The Biotic Message. It really got my attention because other thinkers such as Elisabeth Elliot have suggested that sex itself is a symbol, representing a message.Atom_{December 21, 2006
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antg - according to this site, the female is heterogametic (i.e. the sex chromosomes are different), so the answer will be "yes". Atom - the problem of the cost of sex predates ReMine's interest: it was first pointed out by John Maynard Smith in the 60s. There are now a few theories kicking around that could explain it: one possibility comes from the Red Queen's hypothesis (i.e. that species are evolving to keep up with their environment. The idea is that sex helps variation to be maintained). Essentially, there is selection acting at several levels: between genes, against sex, and between populations/species, for sex. I almost got involved in this research, but ended up working on fungicide resistance instead. BobBob OH_{December 21, 2006
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hey Pav, being on top of the food chain doesn't mean they still don't face natural challenges. If males become scarce, it's a good ability to make babies without them. I don't know what you mean by topping the top so you may have to explain that one to me.Fross_{December 21, 2006
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It's interesting to put these two comments found in the article side-by-side: "Scientists are unsure whether female Komodo dragons have always had this latent ability to reproduce or if this is a new evolutionary development. " "[The Komodo Dragons] are the worldÃ¢â‚¬â„¢s largest lizards and have no natural predators - making them on par with sharks and lions at the pinnacle of the animal kingdom." If they have no predators, if they're at the top, then, well, how do you top the top? And.....why? Any Darwinists like to answer?PaV_{December 21, 2006
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We'll find out. (They're expected to hatch in January, according to the article.)Atom_{December 21, 2006
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Are any of the offspring males?antg_{December 21, 2006
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Since it is now on-topic, I will reproduce my comment form the earlier thread here, for discussion...(sorry mods if reposting comments is bad form...) =========== Now here is what I wonder. It has been brought up by Walter ReMine that sexual organisms shouldn't exist, since changing from asexual to sexual reproduction results in a 50% reduction in "Selfish gene"™ propagation. By numbers, asexual organisms put twice as many of their genes into the gene pool per reproduction, and so this 50% increase in "fitness" should, by natural selection, cause them to take over the gene pool. (Think of any other trait that would give a 50% increase in fitness!) Yet sexual organisms exist in abundance. Now, we see that some organisms have the ability to reproduce sexually or asexually. Why hasn't the magic wand of RM+NS caused these to switch to a strictly (50% more efficient) asexual mode of reproduction? If the fractional genes possesed by kin can be used to explain altruism in Kin Selection theory, then why does it somehow not apply here? Finally, yes, sexual selection does have LONG TERM benefits. But NS has no foresight. It cannot preserve based on future, long term benefits to the gene pool…especially when the short term prize is so hefty. (A 50% increase in fitness)Atom_{December 21, 2006
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Good post Dave. I was wondering if anyone had noticed that story...Atom_{December 21, 2006
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