bird teeth (RE: Electronicly published: why don't modern birds have teeth? )

["ekaterina amalitzkaya" <eamalitz@hotmail.com>]

There are two proteins in teeth that are of interest for evolutionary 
biologists amongst other proteins. These are secreted products  with 
hydrophobic signal peptides and are amelogenin and dentin matrix protein-1. 
The issue with these proteins is that they are entirely low complexity 
proteins, that is they are largely composed of serines and prolines in very 
repetitive array. This means that the probability that they are invented de 
novo in course of evolution is very high as it involves just an expansion of 
a few codons as the repeat unit. Thus several low complexity proteins are 
components of structural scaffolds of organisms like silks, hair, cuticle 
and not surprisingly the vertebrate teeth. Hence the origin of vertebrate 
ossification is a very "easy" event in terms of de novo origin of new 
structures as it involved invention of very simple proteins unlike enzymes 
or binding proteins that have complex globular folds. Thus in a sense the 
emergence of ossification was a real side show to the more complex 
organizational issues of the vertebrate body plan.

Coming to birds they have DMP1  for dentin production but not amelogenin for 
enamel production (even turtles lack it). Thus it looks likely that the post 
Jurassic mutations in the neoavian lineage has possibly eliminated this 
gene. DMP1 on the contrary is expressed in chicken development along the 
jaws, suggesting that the bird tooth making system many not entirely be 
disrupted the way claimed in that PNAS paper. However, it is possible they 
are correct and that this expression is related to the development of the 
jaw bones. A comparison of the DMP1 sequences shows that the lizards and the 
mammals retain what I think are the primitive state while the birds show 
regions of extreme divergence that may be related to tooth loss. We will 
need the crocs to be more certain.
-EA
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