Pharyngula

Chicken, archosaur…same difference

i-19dd121ee06032b71d40a09c5b870d12-talpid2.jpg

My daughter is learning about evolution in high school right now, and the problem isn’t with the instructor, who is fine, but her peers, who complain that they don’t see the connections. She mentioned specifically yesterday that the teacher had shown a cladogram of the relationships between crocodilians, birds, and mammals, and that a number of students insisted that there was no similarity between a bird and an alligator.

I may have to send this news article to school with her: investigators have found that a mutation in chickens causes them to develop teeth—and the teeth resemble those of the common ancestor of alligators and chickens, an archosaur.

The mutant chickens Harris studied bear a recessive trait dubbed talpid2. This trait is lethal, meaning that such mutants are never born, but some incubate in eggs as long as 18 days. During that time, the same two tissues from which teeth develop in mammals come together in the jaw of the mutant chicken–and this leads to nascent teeth, a structure birds have lacked for at least 70 million years. “They don’t make a molar,” explains development biologist John Fallon, who oversaw Harris’s work. “What they make is this conical, saber-shaped structure that is clearly a tooth. The other animal that has a tooth like that is an alligator.”

Previous efforts to produce teeth in chickens had relied on introducing genetic information from mice, resulting in chickens growing mammalian molars. But a chicken’s underlying ability to grow teeth derives from a common ancestor with alligators–archosaurs–that is more recent than the one linking birds and mammals. Nevertheless, the underlying genetic mechanism that produces teeth in mice, alligators and mutant chickens remains the same.

i-e0baac349d197e62c0d8d650595672df-talpid_domains.gif
Schematic of gene expression seen in early development of the
wild-type and ta2 mutant jaw showing coordinated changes in
fgf8, bmp4, and shh expression in outlining the boundary of the
oral/aboral boundary.
The following abbreviations are used: mxp, maxillary; mdp, mandib-
ular; fnp, frontonasal processes; and orl, oral cavity.

The mutant is interesting: it’s not one that was expected to directly affect teeth, but it does illustrate how genes have many downstream effects. What the authors propose is that birds retain a tooth signaling pathway and center, but that the way they’ve turned off tooth expression is by shifting its domain out of the oral mesoderm and into aboral mesoderm, where the overlying ectoderm is not competent to respond. The effect of the talpid2 mutant is to disrupt various crucial genes, shh, fgf8, and bmp4, which will lead to the embryo’s death, but they also have the side effect of shifting other domains of gene expression, sliding that tooth signaling center into contact with oral ectoderm, and as a side effect exposing a hidden developmental program. In the diagram to the right you can see how the colored domains of gene expression (especially bmp4, in blue) are shifted near the maxillary and mandibular processes (mxp and mdp), where the teeth form.

This, I think, is a productive explanation that explains many phenomena.

We hypothesize that the loss of teeth in birds was due to the loss of direct apposition between an epithelial signaling center at the oral/aboral boundary and the underlying mesenchyme of the oral cavity competent to form integumentary appendages. Our model provides a unique developmental mechanism for understanding how specific structures are lost and reinitiated and goes beyond contemporary models of selective gene loss or loss of signaling capability during tooth ontogeny in evolution. Importantly, the control of this inductive event in different facial prominences during development would permit the regional, or modular, loss of teeth as seen in many nonavialan dinosaurs and avialans while allowing them to retain the ability to form teeth on separate regions of the jaw derived from different facial prominences.

It’s telling us something about how birds lost their teeth in evolution. It wasn’t by throwing away the parts of the toolkit responsible for generating teeth, but by changing the patterns of expression of those genes in time and space so that they no longer interact properly. That makes sense; the genes involved, shh, fgf8, and bmp4, are major players in many developmental processes, so their loss would represent a sweeping and destructive change…but delaying one at one point in development so that it doesn’t influence one tissue at one particular time is feasible.


Harris MP, Hasso SM, Ferguson MWJ, Fallon JF (2006) The Development of Archosaurian First-Generation Teeth in a Chicken Mutant. Current Biology 16(4):371-377

Comments

  1. #1 plunge
    February 22, 2006

    What’s that? Creatures showing atavisms most often of their purported recent ancestors, but NEVER atavistic traits that were developed in a different ancestral line and are thus not in their genetic past? Why, that must just be a coincidence! That couldn’t possibly be more evidence for common descent could it?

    And look: even human designers can transfer genes latterally, putting molars in chickens. And yet, this simple pattern, which would be such an obvious potential marker for an intelligent designer at least as intelligent as us, simply doesn’t show up anywhere. It’s absent from the fossil and genetic record. THAT too is just a coincidence! The designer stuck to the restrictions that specifically characterize evolutionary mechanisms because… uh… well it’s just another coincidence!

  2. #2 Jim H
    February 22, 2006

    I’m an admitted biology novice, but I found this part of the article interesting.

    Exactly how the mutation causes the chickens to sprout teeth is unknown, Fallon notes, but a similar effect can be produced in normal chickens. Harris proved this by engineering a virus to mimic the molecular signals of the mutation and caused normal chickens to briefly develop teeth

    Is there another evolutionary clue here? Could a virus affect one generation of a species in such a way that it would become a permenant mutation of the species for later generations that do not possess the virus, or could a virus cause the mutation in the reproductive cell, allowing for the mutation to pass from generation to generation?

  3. #3 Kristine
    February 22, 2006

    Maybe that teacher needs to give the class a pop quiz at the end of the semester in which she flashes the images on the screen without text and asks the students to identify which is the alligator, which is the chicken, etc.

  4. #4 Sarahkm
    February 22, 2006

    “Could a virus affect one generation of a species in such a way that it would become a permenant mutation of the species for later generations that do not possess the virus, or could a virus cause the mutation in the reproductive cell, allowing for the mutation to pass from generation to generation?”

    Highly unlikley. As you say, it would have to affect a reproductive cell, and would probably make it inviable rather than causing a mutation in a specific protein.

    This work is extremely cool! Gould would be so proud.

  5. #5 Paul W.
    February 22, 2006

    Was this an adaptive response to some major event 70 million years ago, like a sudden drop in the dentist population due to an asteriod?

    Is dentistry an atavism that has only recently reappeared?

  6. #6 wamba
    February 22, 2006

    Could a virus affect one generation of a species in such a way that it would become a permenant mutation of the species for later generations that do not possess the virus, or could a virus cause the mutation in the reproductive cell, allowing for the mutation to pass from generation to generation?

    Unlikely in any individual, especially because of the need to hit the reproductive cells. But considering the number of organisms on the planet, and the depth of time, it almost certainly occurs from time to time.

    Things to research, if you have the druthers:
    1) Horizontal Gene Transfer

    2) Viral role in evolution, particularly early evolution:

    New evolutionary frontiers from unusual virus genomes
    Christopher Desjardins, Jonathan A Eisen and Vishvanath Nene
    Genome Biology 2005, 6:212

    The two ages of the RNA world, and the transition to the DNA world: a story of viruses and cells
    Patrick Forterre
    Biochimie
    Volume 87, Issues 9-10 , September-October 2005, Pages 793-803

  7. #7 QrazyQat
    February 22, 2006

    I think there’s two basic issues at the core of this problem (of people not seeing the connections). One is active disinformation from many well-funded groups (like DI); the other is a general lack of education about thinking and how to use an analogy.

    I ran into this when my (adult) step-daughter asked a variation of the “why are there still monkeys” question, one that added in the idea that non-human primates haven’t evolved while we have (itself a variation of the idea that everything and everyone should, through evolution, progress toward something that’s as much like “us” as possible). Any analogies, like why are there still Italians if her family is now Canadian, were met with “but that’s not the same thing”. Well, no, it is the same thing — it’s an analogy. But these are too often, for several reasons, people who are far too literal in their thinking. They don’t see subtleties and they don’t see connections. These things just don’t exist for them.

    (For similar reasons she thinks that at least some of Criss Angel’s Mindfreak series shows actual magic as opposed to tricks done well.)

    BTW, my 7-year-old granddaughter now says she “doesn’t believe in dinosaurs” becuase of the Bible. Sounds like someone needs a dinosaur book and a trip to a museum. Anyone know where a good museum for dinosaur stuff is in the Pac NW, Washington or Oregon?

  8. #8 Camilla
    February 22, 2006

    I watched my peers in high school biology really struggle with stuff like osmosis (we spent almost a term on it); it seemed so obvious to me that I couldn’t understand how they failed to get it.

    Since most of them were not stupid, and many of them excelled in chemistry and physics, and we had good teachers, I don’t think it’s all a “kids these days can’t think” issue. (I can’t tell you prescisely what was wrong, though.)

    My best guess is that they came into biology class expecting to be asked to memorise (and indeed, labels on an anatomy diagram or identifying the parts of a cell went fine), and were simply unable to shift to a “reason it out” mindset.

  9. #9 RavenT
    February 22, 2006

    QrazyQat, the Burke Museum at the University of Washington in Seattle has some quite nice displays about dinosaurs.

  10. #10 Diego
    February 22, 2006

    I think it might help if the teacher had the students explore some of the pertinent synapomorphies. Heck, then the students could be given the task of creating the tree themselves without simply having it handed to them. Afterword they might understand better just how similar archosaurs really are.

  11. #11 greensmile
    February 22, 2006

    I’d cut people a little slack…as long as they will at least take facts as evidence. Even evolutionary biologists have been a little off track in assessing how “connected” [last common ancestor] some branches are in the tree[s?] of life. Today’s correction is from PNAS:
    Sox9 is NOT a baseball score but it has helped keep score on how skeletons evolved…looks like ALL vertebates started with a collegen matrix [If I weren’t a software engineer, I’d try to think up a post for the Carnival of the Spinefull.
    Oh! oops, never mind]

  12. #12 greensmile
    February 22, 2006

    I botched the PNAS link on the above comment, sorry.

  13. #13 Randy
    February 22, 2006

    Golly Gee PZ, don’t you know that this experiment is evidence of design? PaV says so over on Dembski’s group blog.

  14. #14 megan
    February 22, 2006

    QrazyQat, you should tell your granddaughter that even the Creationsts believe in Dinosaurs. According to them, they were in the Garden of Eden.

    http://shop5.gospelcom.net/epages/AIGUS.storefront/43fcd96b0735dd9e271d45579e7906f1/Product/View/10&2D1&2D166

    I keep thinking I should buy this just to see how they try to explain all of this…

  15. #15 Carl Buell(OGeorge)
    February 22, 2006

    I’m sorry I didn’t do birds and alligators yesterday PZ.

    “…and that a number of students insisted that there was no similarity between a bird and an alligator.”

    An easy similarity to “see”, and easier to understand than gene expression or teeth, are the skeletal features of both. They have just about everything in common; you can throw in my dingo and thylacine too.

  16. #16 QrazyQat
    February 22, 2006

    Thanks for the heads up about the Burke in Seattle, RavenT.

  17. #17 Dr Steve B
    February 22, 2006

    Even puttting aside all the skelatal simlarities and after-the fact obvious comparative anatory… Just look at the lower legs and feet of living birds (i.e. the scales and claws)… the real question is how did we miss the conncetion of birds to dinos for so long. Nothing like a mindset to prevent one from seeing the obviosus.

  18. #18 t. comfyshoes
    February 22, 2006

    If you’re willing to go a bit further north, the Tyrrell Museum in Drumheller, Alberta http://www.tyrrellmuseum.com/home/ is dedicated to everything paleontology. But that’s probably a big family road trip as opposed to a quick jaunt. Worth it though.

  19. #19 RavenT
    February 22, 2006

    You’re welcome, QQ. Hey, I see that Dino Day is coming right up on March 4, fyi.

  20. #20 Thomas R. Holtz, Jr.
    February 22, 2006

    Another set of similarities between the two living groups of archosaurs (crocodylians and birds) are behavioral traits.

    * Both groups use vegetation in making their nests (with a rare exception in each clade), whereas lizards & snakes, tuataras, turtles, and monotremes typically do not.

    * Both groups vocalize with the young in late stages immediately before hatching.

    * Both groups exhibit parental care for a period of days-weeks after hatching (in other sauroposids, this is not the case).

    * Both groups vocalize during courtship.

    Since these traits seem to be absent (basally) in the outgroups, they were likely found in the common ancestor of all archosaurs.

  21. #21 KiwiInOz
    February 22, 2006

    I just need to look my chickens in the eye to see their reptilian precedents! And the way they run up my backyard for food – like a pack of velociraptors.

  22. #22 Jessica Theodor
    February 22, 2006

    There’s also a new visitor’s center with good displays at John Day Fossil Beds National Monument in Oregon. It focuses on the Cenozoic, since that’s the local record, but the bonus there is you can go out to the outcrop and see the geology, too.

  23. #23 Ick of the East
    February 22, 2006

    the real question is how did we miss the conncetion of birds to dinos for so long.

    Or not so long, when you consider that Huxley championed that very idea.

  24. #24 Henry
    February 22, 2006

    It’s such a pity that the Enantiornithes are all dead. Imagine the fun of examining this in a taxa containing both toothed and toothless birds. Damn asteroids.

    Another interesting tanget would be trying to induce proper digit formation somehow. The South American hoatzin chicks have 2 claws that they use to move through dense branches, so I’d think whatever genes or genetic interactions that are responsible wouldn’t be terribly degraded if there’s a modern example.

  25. #25 Bro. Bartleby
    February 23, 2006

    We too at the monastery face this same problem, some of the novice that come to the dining table to debate life seem to lack teeth, their minds filled with Sunday school dogma that calcified in their brains at about the age of seven, all further learning and understanding thus prevented. So it may take years, and much toil in the vineyards, before the budding of milk teeth take form in them. So I understand your quest, alas, from somewhat a differing origin than ours, and stupid in and of itself is usually benign and singular, but when said stupid is raised to the position of belief and this ‘belief’ becomes indoctrination in our schools, then it must be exposed to the light of day, so thank you for exposing it to the light of day, but in your zeal please be civil to those of us that are too seeking TRUTH where ever it leads us, and who may have an entirely different understanding of God than those who sully smart with stupid.

    Shalom,
    Bro. Bartleby

  26. #26 hoover
    February 23, 2006

    Fascinating, but does no one find a hint of humor in the ‘hens teeth’? perhaps this mutation occurring, however infrequently, is the source of the expression ‘rare as hens teeth’. It needn’t be, but the irony that the expression not simply be hyperbole is appealing.

  27. #27 Limulus
    February 24, 2006

    “My daughter is learning about evolution in high school right now, and the problem isn’t with the instructor, who is fine, but her peers, who complain that they don’t see the connections. She mentioned specifically yesterday that the teacher had shown a cladogram of the relationships between crocodilians, birds, and mammals, and that a number of students insisted that there was no similarity between a bird and an alligator.”

    The trick is not to look at the faces, but to look at the feet 🙂

    http://www.jimhayes.com/photo/raven.jpg
    http://www.photohome.com/pictures/animal-pictures/wildlife/alligator-1a.jpg

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