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