REAL chicken teeth

[Richard.Zander at MOBOT.ORG]

This costs $30 to get a copy from the Web, so the complete abstract, for
those like me without access to Current Biology, is:

"Modern birds do not have teeth. Rather, they develop a specialized
keratinized structure, called the rhamphotheca, that covers the mandible,
maxillae, and premaxillae. Although recombination studies have shown that
the avian epidermis can respond to tooth-inductive cues from mouse or lizard
oral mesenchyme and participate in tooth formation 1 and 2, attempts to
initiate tooth development de novo in birds have failed. Here, we describe
the formation of teeth in the talpid2 chicken mutant, including the
developmental processes and early molecular changes associated with the
formation of teeth. Additionally, we show recapitulation of the early events
seen in talpid2 after in vivo activation of β-catenin in wild-type embryos.
We compare the formation of teeth in the talpid2 mutant with that in the
alligator and show the formation of decidedly archosaurian (crocodilian)
first-generation teeth in an avian embryo. The formation of teeth in the
mutant is coupled with alterations in the specification of the oral/aboral
boundary of the jaw. We propose an epigenetic model of the developmental
modification of dentition in avian evolution; in this model, changes in the
relative position of a lateral signaling center over competent odontogenic
mesenchyme led to loss of teeth in avians while maintaining tooth
developmental potential."
 
I think but am not sure that genuine teeth were found by the above authors.
In a previous article:

Chen, YiPing, Yanding Zhang, Timg-Xing Jiang, A. J. Barlow, T. R. St. Amand,
Yueping Hu, S. Heaney, P. Francis-West, Cheng-Ming Chuong & R. Maas. 2000.
Conservation of early odontogenic signaling pathways in Aves. PNAS 97:
10044--10049. The authors said: "Although latent, the early signaling
pathways involved in odontogeneises remain inducible in Aves and suggest
that loss of odontogenic Bmp4 expression may be responsible for the early
arrest of tooth development in living birds." "It does not, however,
demonstrate the formation in Aves of structures that can be called teeth."
"...it seems likely that unless expressed in other contexts, many of the
genes that characterize the hard mineralized dentition of toothed
vertebrates would have sustained inactivating mutations and therefore
represent pseudogenes in birds." 

Therefore, one would need to read the first article closely to see how much
of the teeth were actually expressed, and how much were lost to, say,
gradual degradation (into pseudogenes or impaired genes) over eons of the
gene complex coding for teeth in Aves.

______________________
Richard H. Zander
Bryology Group, Missouri Botanical Garden
PO Box 299, St. Louis, MO 63166-0299 USA
richard.zander at mobot.org <mailto:richard.zander at mobot.org> 
Voice: 314-577-5180;  Fax: 314-577-0828
Websites
Bryophyte Volumes of Flora of North America:
http://www.mobot.org/plantscience/bfna/bfnamenu.htm
Res Botanica:
http://www.mobot.org/plantscience/resbot/index.htm
Shipping address for UPS, etc.:
Missouri Botanical Garden
4344 Shaw Blvd.
St. Louis, MO 63110 USA

-----Original Message-----
From: Ken Kinman [mailto:kinman2 at YAHOO.COM] 
Sent: Wednesday, February 22, 2006 12:40 PM
To: TAXACOM at LISTSERV.NHM.KU.EDU
Subject: [TAXACOM] REAL chicken teeth

Dear All,
       Speaking of silenced genes, this week's publication on teeth in a
chicken mutant is highly interesting.  These are real bird teeth, not
laboratory-induced teeth from other vertebates (mice, etc.).  The
developmental potential for teeth is still in the chicken genome itself.

       Minor mutations can occasionally cause major morphological change,
sometimes even a cascade of related changes (although not always in the same
part of the body).  While I appreciate John Grehan's efforts to warn us of
the problems with reading too much into some gene sequences, one can also be
led astray by attaching too much significance to certain morphological data
sets as well.  I still believe his orang-human similarities are
developmentally related, and claiming that they are independent does not
make them so.  I've had similar conversations with theropod workers who
claim their phylogenies have more synapomorphies than mine (as though
quantity should trump quality).  It's not just a numbers game, and doubly so
for dinosaurs because we don't have molecular data as an added test to
detect homoplasy.  Clearly, we need to be more picky with BOTH our
morphological data AND our molecular data!!!   One way to be able to do that
more effectively in the future is to tra
 in a lot more developmental biologists.
   ----Ken Kinman
----------------------------------------------------------
Anyway, here's the citation to the chicken mutant paper.  Luckily, these
mutants don't survive to adulthood (although that might be an interesting
plot for a modern sequel to "The Birds").   ;-)

M.P. Harris, S.M. Hasso, M.W. J. Ferguson, and J.F. Fallon (February 2006).
The Development of Archosaurian First-Generation Teeth in a Chicken Mutant.
Current Biology, 16:371-377.

>From the summary:
 "The formation of teeth in the mutant is coupled with alterations in the
specification of the oral/aboral boundary of the jaw.  We propose an
epigenetic model of the developmental modification of dentition in avian
evolution; in this model, changes in the relative position of a lateral
signaling center over competent odontogenic mesenchyme led to loss of teeth
in avians while maintaining tooth developmental potential."