Evolution involves interplay between natural selection and developmental constraints1,2,3. This is seen, for example, when digits are lost from the limbs during evolution1,3,4. Extant archosaurs (crocodiles and birds) show several instances of digit loss3,5,6 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 vestiges7,8,9,10. 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 Sox9- 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 Hoxd11 and 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 (ZPA11). 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.
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New sequences of probes used here are deposited in GenBank under accession numbers KC667071 (chicken Gli3), JF317542 (chicken HoxD11), JF317555 (chicken HoxD12), JQ717196 (Nile crocodile Sox9) and JQ717195 (emu Sox9).
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We thank M. C. M. Welten for technical advice; W. Bruins for help with collecting the emu embryos; D. van der Marel for taking the radiographs; H. J. Meijer for advice on the phylogeny and for guiding D.A.F. through the the collections at the Smithsonian Institution; the Delaware Museum of Natural History for allowing D.A.F. to measure their specimens; and P. den Hartog, H. Koolmoes and S. de Schaaf-Timmerman for collecting the zebra finch and Barbary dove eggs.
The authors declare no competing financial interests.
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de Bakker, M., Fowler, D., Oude, K. et al. Digit loss in archosaur evolution and the interplay between selection and constraints. Nature 500, 445–448 (2013). https://doi.org/10.1038/nature12336
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