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Evolution involves interplay between natural selection and developmental constraints (1-3). This is seen, for example, when digits are lost from the limbs during evolution (1,3,4). Extant archosaurs (crocodiles and birds) show several instances of digit loss (3,5,6) under different selective regimes, and show limbs with one, two, three, four or the ancestral number of five digits. The 'lost' digits sometimes persist for millions of years as developmental vestiges (7-10). Here we examine digit loss in the Nile crocodile and five birds, using markers of three successive stages of digit development. In two independent lineages under different selection, wing digit I and all its markers disappear. In contrast, hindlimb digit V persists in all species sampled, both as cartilage, and as Soxx9- expressing precartilage domains, 250 million years after the adult digit disappeared. There is therefore a mismatch between evolution of the embryonic and adult phenotypes. All limbs, regardless of digit number, showed similar expression of sonic hedgehog (Shh). Even in the one-fingered emu wing, expression of posterior genes Hoxd11and Hoxd12 was conserved, whereas expression of anterior genes Gli3 and Alx4 was not. We suggest that the persistence of digit V in the embryo may reflect constraints, particularly the conserved posterior gene networks associated with the zone of polarizing activity (ZPA (11)). The more rapid and complete disappearance of digit I may reflect its ZPA-independent specification, and hence, weaker developmental constraints. Interacting with these constraints are selection pressures for limb functions such as flying and perching. This model may help to explain the diverse patterns of digit loss in tetrapods. Our study may also help to understand how selection on adults leads to changes in development., Digit loss is defined as the complete loss of all phalanges and the metapodial bone; it should be distinguished from digit reduction, in which only phalanges are lost (3). Digit [...]

In humans and mice, the early development of [alpha][beta] T cells is controlled by the pre--T-cell receptor [alpha] chain (pT[alpha]) that is covalently associated with the T-cell receptor [beta] (TCR[beta]) chain to form the pre--T-cell receptor (pre-TCR) at the thymocyte surface. Pre-TCR functions in a ligand-independent manner through self-oligomerization mediated by pT[alpha]. Using in silico and gene synteny-based approaches, we identified the pT[alpha] gene (PTCRA) in four sauropsid (three birds and one reptile) genomes. We also identified 25 mammalian PTCRA sequences now covering all mammalian lineages. Gene synteny around PTCRA is remarkably conserved in mammals but differences upstream of PTCRA in sauropsids suggest chromosomal rearrangements. PTCRA organization is highly similar in sauropsids and mammals. However, comparative analyses of the pT[alpha] functional domains indicate that sauropsids, monotremes, marsupials, and lagomorphs display a short pT[alpha] cytoplasmic tail and lack most residues shown to be critical for human and murine pre-TCR self-oligomerization. Chicken PTCRA transcripts similar to those in mammals were detected in immature double-negative and double-positive thymocytes. These findings give clues about the evolution of this key molecule in amniotes and suggest that the ancestral function of pT[alpha] was exclusively to enable expression of the TCR[beta] chain at the thymocyte surface and to allow binding of pre-TCR to the CD3 complex. Together, our data provide arguments for revisiting the current model of pT[alpha] signaling. doi/ 10.1073/pnas.1010166107

Abstract Background The ability to form teeth was lost in an ancestor of all modern birds, approximately 100-80 million years ago. However, experiments in chicken have revealed that the oral epithelium can respond to inductive signals from mouse mesenchyme, leading to reactivation of the odontogenic pathway. Recently, tooth germs similar to crocodile rudimentary teeth were found in a chicken mutant. These "chicken teeth" did not develop further, but the question remains whether functional teeth with enamel cap would have been obtained if the experiments had been carried out over a longer time period or if the chicken mutants had survived. The next odontogenetic step would have been tooth differentiation, involving deposition of dental proteins. Results Using bioinformatics, we assessed the fate of the four dental proteins thought to be specific to enamel (amelogenin, AMEL; ameloblastin, AMBN; enamelin, ENAM) and to dentin (dentin sialophosphoprotein, DSPP) in the chicken genome. Conservation of gene synteny in amniotes allowed definition of target DNA regions in which we searched for sequence similarity. We found the full-length chicken AMEL and the only N-terminal region of DSPP, and both are invalidated genes. AMBN and ENAM disappeared after chromosomal rearrangements occurred in the candidate region in a bird ancestor. Conclusion These findings not only imply that functional teeth with enamel covering, as present in ancestral Aves, will never be obtained in birds, but they also indicate that these four protein genes were dental specific, at least in the last toothed ancestor of modern birds, a specificity which has been questioned in recent years.

The evolutionary trends among the osteichthyan and chondrichthyan fish suggest that there is affinities between odontodes, teeth and denticles, between the ganoid scales of polypterids and the elasmoid scales of teleosts. Furthermore, the tissues of the elasmoid scale originated from dental tissues.