Friday, 15 July 2011

Of Hens Teeth and IDiots.

The literature published by the Discovery Institute often confuses me. I'm never quite sure if it should frustrate me or amuse me. Their constant mangling of science combined with their propensity for telling half-truths and distorting reality both makes me laugh (hah! They really think they have science on their side?) and makes me embittered (How dare they twist science to deceive and miseducate?). I guess this recent article by Discovery Institute crony Casey Luskin should be of no surprise, then. In the piece, titled Of Hen's Teeth and Neutral Mutations, Luskin attempts to dismantle a claim made by Stephen Jay Gould about hen's teeth (or the lack thereof):
"Evolutionists often cite an experiment which purportedly induced tooth growth in chickens, supposedly confirming that birds have genes for teeth and are descended from toothed reptilian ancestors. For example, in his book Hen's Teeth and Horse's Toes, Stephen Jay Gould discusses this experiment...But there's a problem with Gould's argument: as Sean Carroll explains, neo-Darwinism has a 'use-it-or-lose-it' rule. According to neo-Darwinism, if a trait is not used then the DNA which encodes it will accumulate neutral mutations, and eventually the trait will be lost forever. If supposed chicken genes for producing teeth haven't been used for 60+ million years, then that would strongly suggest that neutral mutations should have long-since destroyed their ability to function."
For those of you who might be unfamiliar with the experiment in question - and with Gould's discussion of it - it would be worth the while to go into detail.  In 1980, Gould published the book Hen's Teeth and Horse's Toes, a collection of articles he had written for various magazines (primarily for Natural History). Included was an article of the same name where he discussed atavisms - apparent reversions in individuals to an ancestral phenotype. Gould claimed that atavisms are a shining example of the evolutionary past of a species coming to the surface. He illustrated his point with two examples: polydactyl horses and chickens with teeth. It is this second example towards which Luskin has aimed his bow and launched forth a volley of ignorance.

On p.1821, Gould explains a curious experiment performed by E.J. Kollar and C. Fisher: they devised a way to prompt chickens to develop teeth. If it's been a while since you took a good look in the mouth of your local avian friends, then it might interest you to know that birds don't have teeth. The most recent known fossil of toothed birds dates to around 80 million years ago, so somewhere in the intervening time, birds lost the ability to produce teeth. Odontogenesis in vertebrates is a complex process (then again, developmental programs always are!). It requires two different tissue types to occur: epithelial tissue and mesenchyme. The outer enamel layer of a tooth is formed by the epithelial tissue, while the inside dentin later of your tooth grows from the mesenchyme. But there's a catch: the mesenchyme cannot produce dentin by itself, it needs to be in contact with epithelium for dentin production to begin - that is to say, epithelium induces the production of dentin. This dentin, in turn, induces the production of enamel in the epithelium. Birds don't produce dentin, nor, consequently, enamel, so birds are born toothless. Kollar and Fisher's idea was brilliant but simple: what happens if you graft chick epithelium with murine (mouse) mesenchyne? Mice most definitely have teeth, so we know their mesenchyne is capable for producing dentin if prompted by epithelial tissue. What they found was astounding: when mouse mesenchyne was grafted to chick epithelium, teeth (dentin and all) were produced. This meant that avian epithelial tissue - despite the fact that birds have no teeth, and have not had teeth for as long as 80 million years - is still able to induce dentin production in the appropriate mesenchyne. Gould mused that this experimental result displayed the evolutionary history of birds. Why else would avian epithelial tissue have the latent ability to induce dentin production unless they had descended from toothed ancestors?

Luskin, however, thinks that Gould was completely wrong. Luskin argues that, if birds lost the ability to produce teeth 80 million years ago, then the genes for tooth production would have accumulated so many mutations that it would be impossible to revert back to the original toothed phenotype. The tooth production genes, he claims, would have since been destroyed beyond the ability to function. He bases this argument in something called Dollo's Law. Dollo's Law, put simply, states that evolution cannot reverse itself, and that genes which escape selection pressure will degrade fast enough that reverting to the original phenotype is tantamount to impossible. According to Luskin, the example of toothed hens is not the resurrection of a lost developmental pathway but the result of an experimental mistake.

Luskin cites a paper from Marshall, Raff and Raff2 that seemingly supports his argument. In the paper, the authors devise an equation that determines the probability of a silenced gene's reversion as a function of time passed. They concluded that, for a gene that has been silenced for 10 million years, there is a near-zero probability for reactivation. How do they account for Kollar and Fisher's results? They state that "the classic example of the resurrection of "hen's teeth" is most likely an experimental artifact". Well, that settles it, right?

Well, no. Marshall, Raff and Raff's paper was published in 1994, and despite what Luskin might think, science has progressed in the two decades since. Perhaps if he had read through more recent literature he would have realized some problems with his argument and with Marshall et al's conclusion.

First, let's tackle the "experimental artifact" claim. When Kollar and Fisher's original paper was published, there was some skepticism about their results. There was controversy over whether or not the mouse mesenchyne they used was contaminated with mouse epithelial tissue. If this was the case, then their results would be invalid: it would be impossible to tell whether or not the formation of dentin was prompted by the chick epithelium or the mouse epithelium. Despite the experiment being repeated by other researchers, the possibility of contamination meant that many people wrote off their result as an "experimental artifact". This debate was put to rest, however, by an paper published by Cai et al in 20093. In their paper, the team repeated the tissue graft experiment using mesenchyne from transgenic mice expressing the LacZ gene (LacZ is used in molecular biology as a reporter gene, because it produces a dark blue pigment when supplied the proper substrate). Like Kollar and Fisher, Cai et al's results showed the induction of dentin by chick epithelium. To prove that there was no contamination by mouse epithelium, they took cross sections of the graft and stained them. The transgenic mouse tissue, expressing the LacZ gene, stained a dark blue while the chick tissue remained unstained. What they found was that the entire epidermal tissue remained unstained, while only the mesenchyne stained blue, ruling out the possibility of contamination. Kollar and Fisher's original results, then, are still valid.

So if Kollar and Fisher were correct all along, then don't their findings go against Dollo's Law? Shouldn't the genes for tooth production, being free from selective pressures, have accumulated many mutations that would prevent the pathway from functioning at all? The answer, again, is no. Perhaps if Luskin had read the Marshall et al paper more closely (if, indeed, he had read it at all, since he only quotes the abstract) he would have gotten a hint. The authors mention in their discussion that "[r]eversals of long-lost structures do occur but evidently result from the cooption of genes that continue to survive in other roles". In other words, genes involved in traits no longer expressed can avoid the fate of accumulating mutations if they have other roles in development. The genes for tooth production most certainly fit this description. Work by West et al in 19984 found that many of the genes required for odontogenesis are still expressed in the developing chick embryo, indicating that they still play important roles. BMP4, for example, plays important roles in muscle development and bone development as well as in the development of teeth. Members of the  Hedgehog family of proteins are involved in a whole slew of developmental processes, only one of which is odontogenesis. Toyosawa et al5, in 1999, looked at one protein in particular, Dentin Matrix Protein 1, or DMP1. Since birds don't produce dentin, what use would they have for such a gene? Toyosawa et al not only found that birds have this gene but found it was being expressed in the jaws of chickens. The case of hen's teeth escapes Dollo's Law because many of them are not silenced, and many of them have other functions in the developing embryo.

If you think about it, this really should come as no surprise. Dollo's Law describes what happens to single genes that control single phenotypes when they become silenced. Dollow's Law makes no claims about what happens to genes involved in complex developmental pathways. In order for Luskin to be correct, then it would require all the genes in a developmental pathway to have become silenced. Given the interconnected nature of developmental pathways, this simply is not a reality. One or two genes in the pathway may be lost, but the rest remain due to their involvement in other roles. If the missing genes are supplied, then the original, ancestral pathway is reconstructed and the ancestral phenotype is "resurrected". This is precisely what is going on in the example of hen's teeth. The tooth development pathway remains largely intact since many of the genes are involved in other roles. The genes in chick mesenchyne that respond to signals from the epithelial tissue have been lost, which is why birds do not develop teeth. But if you supply these genes in the form of mesenchyne from mice, then the lost pathway is reconstructed and teeth develop. This in no way violates Dollo's Law.

As for Dollo's Law itself, there is mounting evidence that would indicate apparent exceptions to Dollo's Law might be the rule. In the last ten years, many examples of exceptions to Dollo's Law have been noted, including the evolution of  wings in stick insects6, the larval stage in salamanders7, digit loss in some lizards8, egg laying in sand boas9, teeth in frogs10 (which, by the way, have been toothless for 200 million years, more than twice as long as birds), shell coiling in limpets11, and even the re-evolution of sexuality in Oribatid mites12. As noted by Collin and Miglietta13:
"with the growing number of phylogenetic studies showing patterns consistent with re-evolution of characters, and genetic data showing that developmental pathways can be maintained for tens of millions of years, is it time to give up Dollo’s Law? Perhaps."
So what remains of Luskin's argument but smouldering rubble? Kollar and Fisher's experimental results were not due to experimental error, their results don't violate Dollo's Law, and Dollo's Law itself is on shaky ground. Gould was perfectly correct in referring to hen's teeth as an atavism hearkening back to a bygone day of toothed birds.

Once again, an argument put forth by the ID crowd has failed. Are they incapable of delivering a good argument? It sure seems hard to find one that is the least bit compelling. You might even say they're as scarce as hen's teeth.


1. Stephen Jay Gould . Of Hen's Teeth and Horse's Toes. 1980

2  C. Marshall, E. Raff and R. Raff . Dollo's law and the death and resurrection of genes. Proceedings of the National Academy of Sciences. 1994. 91:12283-12287

3. Cai J, Cho S-W, Ishiyama M, Mikami M, Hosoya A, Kozawa Y, Ohshima H, Jung H-S. Chick tooth induction revisited. 2009. J. Exp. Zool. (Mol. Dev.Evol.) 312B:465–472.

4. Philippa Francis-West, Raj Ladher, Amanda Barlow, Ann Graveson, Signalling interactions during facial development. 1998. Mechanisms of Development. 75(1-2):3-28, DOI: 10.1016/S0925-4773(98)00082-3.

5. Satoru Toyosawa, Akie Sato, Colm O'hUigin, Herbert Tichy and Jan Klein. Expression of the Dentin Matrix Protein 1 Gene in Birds. 1999. Journal of Molecular Evolution. 50(1), 31-38, DOI: 10.1007/s002399910004

6. Whiting MF, Bradler S, and Maxwell T. Loss and recovery of wings in stick insects. Nature. 2003 421(6920):264-7.

7. Chippindale PT, Bonett RM, Baldwin AS, and Wiens JJ. Phylogenetic evidence for a major reversal of life-history evolution in plethodontid salamanders. 2004.  Evolution. 58(12):2809-22.

8. Kohlsdorf T, Wagner GP. Evidence for the reversibility of digit loss: a phylogenetic study of limb evolution in Bachia (Gymnophthalmidae: Squamata). 2006. Evolution. 60(9):1896-912

9.V. Lynch and G. Wagner. Did egg-laying boas break Dollo's Law? Phylogenetic evidence for reversal to oviparity in sand boas. 2010. Evolution. 64(1):207-216

10.Wiens JJ. Re-evolution of lost mandibular teeth in frogs after more than 200 million years, and re-evaluating Dollo's law. 2011 . Evolution. 65(5):1283-96.  doi: 10.1111/j.1558-5646.2011.01221.x

11. Collin R, and Cipriani R. Dollo's law and the re-evolution of shell coiling. 2002. Proceedings of the National Academy of Sciences. 270(1533):2551-5

12. Domes K, Norton RA, Maraun M, and Scheu S. Reevolution of sexuality breaks Dollo's law. 2007 . PNAS . 104(17):7139-44

13. Collin R, and Miglietta MP. Reversing opinions on Dollo's Law. 2008. Trends Ecol Evol. 23(11):602-9

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