Re: Embryological evidence that bird fingers are I, II, III

[David Marjanovic <david.marjanovic@gmx.at>]

From the Discussion:

"The finding that the chick wing polarizing region only gives rise to 
soft tissues along the posterior margin of the most-posterior digit, but 
not the cartilage, is strong evidence that the bird wing digits are 1, 2 
and 3, and not 2, 3 and 4. It is striking that the contribution the 
mouse forelimb polarizing region makes to digit 3 is also predominantly 
to soft tissues along its posterior margin^8. Therefore, because a 
primary axis of cartilage condensation running through the digit 4 
position is not conserved in all vertebrate limbs, the primary axis 
cannot be used to assign digit identity, thus eliminating the 
requirement of a digit frame-shift in bird wing evolution (Fig. 1d,e). 
Our results support the fossil record, indicating that birds evolved 
from theropod dinosaurs that lost digits 4 and 5 (Fig. 1a) and is 
consistent with the overlooked 'axis-shift' hypothesis that proposes the 
primary axis runs through the digit 3 position in bird wings^19–21 (Figs 
1f and g). We speculate that the inability of the chick wing polarizing 
region to form a digit in the wing, but not in the leg, is associated 
with cell death that is higher along the posterior margin of the wing 
bud compared with the leg bud^22. This cell death could account for the 
progressive decrease in the width of the stripe of cells derived from 
the chick wing polarizing region going from proximal to distal (Fig. 
2d). We further speculate that this pattern of cell death evolved around 
200 million years ago and eliminated posterior digits of the theropod 
hand/bird wing (Fig. 1a)."

"Our findings lead us to suggest that the digit 4 position in all 
vertebrate limbs is within the polarizing region, and this may be 
because three is the maximum number of different digits that can be 
specified by a concentration gradient of paracrine Shh signalling. This 
implies that additional digit types will always be derived from the 
polarizing region and specified by autocrine Shh signalling."