Vertebral variation, Hox genes, development, and cancer

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First, a tiny bit of quantitative morphological data you can find in just about any comparative anatomy text:

mammal number of vertebrae
cervical thoracic lumbar sacral caudal
horse 7 18 6 5 15-21
cow 7 13 6 5 18-20
sheep 7 13 6-7 4 16-18
pig 7 14-15 6-7 4 20-23
dog 7 13 7 3 20-23
human 7 12 5 5 3-4

The number of thoracic vertebrae varies quite a bit, from 9 in a species
of whale to 25 in sloths. The numbers of lumbar, sacral, and more caudal vertebrae also show considerable variation. At the same time, there is a surprising amount of invariance in the number of cervical vertebrae in mammals — as every schoolkid knows, even giraffes have exactly the same number of vertebrae in their necks as we do. What makes this particularly striking is that other vertebrates have much more freedom in their number of cervical vertebrae; swans can have 22-25. I was idly wondering why mammals were so limited, and stumbled onto a couple of papers that addressed exactly that question (Galis & Metz, 2003; Galis, 1999). Galis’s explanation is that it is a developmental constraint that may have something to do with the incidence of cancer.

Development is an intricately choreographed process that treads a dangerous line. On one side is stability; but development is in many ways a destabilizing process, in which cells have to change their path and form new tissues, and stability is not compatible with it. On the other side is chaos, unregulated proliferation — cancer. During development, the organism has to foster proliferation and change to a greater degree than it can tolerate later, and that loosening of constraints represents a danger. Galis suggests that one reason we mammals may always have 7 cervical vertebrae is that the regulatory genes that specify the number of vertebrae are coupled to processes that otherwise regulate cell fates, and that modifications to those genes that would cause variation in vertebra number would also lead to unacceptable increases in the frequency of embryonal cancers.

This isn’t at all an improbable idea. Genes exhibit bewilderingly complex patterns of expression, and pleiotropy (the regulation of multiple phenotypic characters by a single gene) is the rule, not the exception. The Hox genes, the particular genes that control the identity of regions along the length of the animal, are known to switch on and off in proliferating mammalian cell lines in culture. Perhaps the Hox genes involved in defining cervical vertebrae are somehow also involved in controlling cell proliferation, making them dangerous targets for evolution to tinker with?

Galis provides several lines of evidence that this is the case. To see whether variation in cervical vertebra number leads to increased incidence of cancer, we need to look for instances of variation in mammalian vertebrae.

There isn’t much variation in cervical vertebra number, though. There is an exception: sometimes, the 7th cervical vertebra is found to undergo a partial homeotic transformation and forms a pair of ribs, which are normally found only on thoracic vertebrae. Humans develop cervical ribs with a frequency of about 0.2%; do they also develop cancers? The answer is yes, with a frequency 125 times greater than the general population.

Another place to look would be in phylogenetic variation — between groups rather than within a population. It turns out that there are two groups of mammals that do have a non-canonical number of cervical vertebrae: one manatee genus and two genera of sloths. No data is available on frequencies of embryonal cancers in either, and Galis reports that manatees at least seem to have a low incidence of cancer. One explanation is that both sloths and manatees have exceptionally slow metabolic rates, which in itself will reduce the frequency of cancer, since it will reduce the rate of oxidation damage; the idea is that this low cancer rate may have made these organisms more tolerant of variation in these genes.

An open question is how birds can have greater variability in the number of cervical vertebrae — they certainly don’t have low metabolic rates. One suggestion is that the coupling between these particular Hox genes and a predilection for cancer is unique to mammals. Another possibility is that birds possess other, unidentified mechanisms that reduce free radical production, reduces oxidative damage, and makes them relatively cancer-free. Galis cites several studies that show that birds do seem to be less severely afflicted with cancers than us mammals.

It’s an interesting idea, but the evidence so far is a collection of correlations. I’d be interested in seeing some direct analyses of the role of patterning genes on carcinogenesis. Still, it’s the first answer I’ve seen to explain why such a peculiar restriction in morphology should be nearly universal within a whole class of animals, when other classes allow so much more diversity.

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Galis, F and JAJ Metz (2003) Anti-cancer selection as a source of developmental and evolutionary constraints. BioEssays 25:1035-1039.

Galis, F (1999) Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. J Exp Zool (Mol Dev Evol) 285:19-26.

The DI can’t get anything right?

A few months ago, after learning that Bill Gates was giving money to the Cascadia branch of the Discovery Institute (which studies transportation issues in the Pacific Northwest), I wondered if the DI was as incompetent and delusional about transportation as evolution. Here’s one answer—not surprisingly, they may again be tools of interests opposed to real advances. I am not by any means an informed expert on these issues, but I do know the Seattle area desperately needs better mass transit—I have seen rush hour on I5, and do not know how people can stand it—yet what the DI offers is a distracting welter of speculative and untried ideas that seem calculated more to muddy the waters and preserve the profitably wasteful status quo than anything else.

A typical right-wing think-tank, in other words.

Castrating trematodes!

Since I mentioned yesterday that penis size mattered, upon stumbling on this article about the horrific effects of a trematode infestation, I thought everyone might enjoy a grim and vivid picture of what trematodes can do to a poor, innocent mollusc.

This is a photo of a trematode, or fluke. Trematodes are parasitic flatworms with very complex life cycles; this particular one is a cercaria, or tailed larva. They swim about and infest various hosts at various stages, proliferating and spreading through tissues, before moving on to infect the next host in their cycle.

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[Read more…]

Vonnegut. And so it goes.

Alas, my daughter and I are big fans of Vonnegut’s writing, but he’s showing signs of losing it. He sounds terribly unhealthy on the radio, and his performance on the Daily Show a while back was depressing. This morning, Vonnegut was on NPR, and said scientists were defending evolution because of “tribalism”, and that “my body and your body are miracles of design”, and that “natural selection couldn’t possibly have produced such machines.” Please, please remind me to stop blogging when my mind deteriorates that far, OK?

To call the body a “miracle of design” is begging the question, while denying the possibility of evolution is the argument from incredulity. Neither is at all persuasive. I would like to know if Vonnegut thinks all those scientists who insist that the Earth is roughly spherical are also arguing for tribal dogma, or whether he suspects that they might actually be relying on this little thing called evidence…and why he thinks biologists fall in the former category and not the latter.

Out of respect for his past writing career, though, I will refrain from cutting him up with a razor here. There were some signs in his interview that he hasn’t drunk deep of the ID kool-ade, but overall it was a sadly muddled exercise in sloppy reasoning, spoken without the sharpness and clarity I’ve expected from Vonnegut.

Carnival of the non-Pascalians

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Carnival of the Godless #32 is available for reading. Once you’ve read through all that, there’s also a somewhat interesting theistic point of view to consider. The author quotes Pascal:

Let the skeptics first learn what religion is before attacking it. If religion boasted that it offered a clear vision of God, and if it asserted that there was ample evidence of his existence, then the skeptic could simply argue that the evidence is not conclusive. But religion says the opposite. It recognizes that people are in darkness, remote from God, that God is hidden from their understanding. Yet it proclaims that God has given signs for those who truly seek him with their hearts. Thus the skeptics could only successfully attack Christianity if they themselves had sincerely sought God, and failed to find any signs.

He takes the claim that God is remote and difficult to know in a curious direction…as an indictment of the so-called ‘spiritual’ leaders who offer simplistic recipes derived from their religious absolutism.

For the fact is, most of those who set themselves up as religious or spiritual authorities in that country [the US], especially in the Christian religion, are just quacks. Fundamentalists are to real spiritual leaders as creationists are to real scientists—in fact, that’s why fundamentalists and creationists overlap so profoundly. They’re a big happy coterie of quackery.

I agree (no one is surprised, I’m sure). But I don’t think the author goes far enough. If gods are murky and nearly unknowable, with no clear evidence for them, why believe in them at all? We shouldn’t trust the charlatans who define Christian behavior so sharply, but why then should we trust any assertion about the nature of any gods, including the claim that there is no “clear vision” of them? Throw out the whole business of god-belief, I say.

As for that concluding bit that says that skeptics who claim to have sought gods and failed to find them had not sought them “sincerely”, well, that’s simply the old No True Scotsman fallacy. Why is it that everything I’ve read of Pascal’s theology suggests that it was painfully simple-minded (I could bring up Pascal’s Wager, the worst argument for gods ever, but I’ll spare you)? The CotG is much more satisfying.

Get ready for the Koufaxes

The Koufax Award nominations are beginning to trickle out. The first ones up are the nominations for Best New Blog of 2005, and oh my, but there is a long, long list. You don’t get to vote yet, though—they are wisely suggesting that you should take a little time to browse and make an informed selection (which is an excellent idea…one of the pitfalls of these weblog award thingies is that they tend to fall towards all the obvious choices, and “most widely known” is treated as a synonym for “best”.)

One unfortunate side effect of our recent jump to scienceblogs.com is that a couple of my colleagues here are listed under their old urls: Aetiology and Adventures in Ethics and Science are in new locations. I hope that doesn’t confuse any of the voters.

It’s also a great category, and there are a whole bunch of my favorites in there: Buridan’s Ass, By Neddie Jingo, Cosmic Variance, Dharma Bums, Evolving Thoughts, firedoglake, I Blame The Patriarchy, The Next Hurrah, Olduvai George…and then there are all those others I have yet to scan. And we’re only supposed to vote for one? Yikes.

Male enhancement works!

I hate those commercials on cable TV for Enz*te, that fake “male enhancement” product that promises a “boost of confidence” for all the guys who take their little pill. I don’t believe it, of course—it’s probably a concoction of sawdust and rat droppings. But the phenomenon of male confidence as a function of the size of their physical attributes might just have some validity.

AL Basolo, who did some well-known work on mate preference in swordtails a few years ago (short answer: lady swordtails prefer males with longer swords), has a couple of new papers on the subject. She has looked at competition between males—the fishy equivalent of checking out the other guy’s equipment in the lockerroom—and found that the length of the sword makes a big difference in the struggle between males, even with no females involved.

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Xiphophorus helleri is a common aquarium fish with a distinctive feature: that long sword on its tail. The males have competitive interactions with each other that are fairly easy to assess: dominant males chase away inferiors, and inferiors avoid the winners, so you just have to record who is chasing who to sort out who thinks they are in charge. Usually, it’s the fish that is bigger overall that wins. The investigators suspected that the size of the sword might also be a deciding factor. Observations of pairs of fish matched for body size, but with natural differences in sword length, did not bear this out, however, showing little correlation. That suggests, as one might expect, that there are multiple factors that influence competitions.

To simplify those factors, they carried out what sounds to me like a very cruel experiment. Pairs of fish matched for body size were anesthetized, their swords chopped off, and replaced with transparent plastic swords of identical size. The difference, though, was that different length swords were painted on the transparent plastic—one lucky fish got a new painted sword roughly the same length as the old one, while the other got a sword half the length.

After recovering from their implant surgery, they were put together in a tank…and the truncated male consistently lost all competitions. I guess size matters, after all.

Without the gross surgical modifications, however, size wasn’t such a clear indicator of victory, so other factors must also play a role. A companion paper looked at stripes on the sword, and how they affected female interests. This work modified the tails digitally; a video recording of a hunky male was made, and then edited to either remove the stripes from its entire length, to remove them from the proximal half or the distal half, or left intact. The video was played back to a female, and the length of time she paid attention to it measured (longer is better).

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Female response to the four male video stimuli (pictured above each bar from left to right: complete sword, distal stripes, proximal stripes, no stripes). Bars with the same colour pattern did not differ significantly.

The lessons are clear. Having a long sword will help you intimidate and beat up your competition, and painting stripes along its length (or at least at the tip) will win you the admiration of females.

If you’re a fish, that is.

There is no necessary expectation that it will help at all if you’re a hairless ape, but if anyone tries it, let me know how it turns out.


Benson KE, Basolo AL (2006) Male–male competition and the sword in male swordtails, Xiphophorus helleri. Animal Behaviour 71(1):129-134.

Trainor BC, Basolo AL (2006) Location, location, location: stripe position effects on female sword preference. Animal Behaviour 71(1):135-140.