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Digit loss in archosaur evolution

From: Ben Creisler

A new paper in Nature:

Merijn A. G. de Bakker, Donald A. Fowler, Kelly den Oude, Esther M.
Dondorp, M. Carmen Garrido Navas, Jaroslaw O. Horbanczuk, Jean-Yves
Sire, Danuta Szczerbińska & Michael K. Richardson (2013)
Digit loss in archosaur evolution and the interplay between selection
and constraints.
Nature 500: 445–448

Evolution involves interplay between natural selection and
developmental constraints. This is seen, for example, when digits are
lost from the limbs during evolution. Extant archosaurs (crocodiles
and birds) show several instances of digit loss 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. 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.