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

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.
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