Frame-shifts of digit identity in bird evolution and Cyclopamine-treated wings.

A highly conserved spatio-temporal pattern of cartilage formation reveals that the digits of the bird wing develop from positions that become digits 2, 3, and 4 in other amniotes. However, the morphology of the digits of early birds like Archaeopteryx corresponds to that of digits 1, 2, and 3 of other archosaurs. A hypothesis is that a homeotic "frame-shift" occurred, such that in the bird wing, digits 1, 2, and 3 develop from the embryological positions of digits 2, 3, and 4. Experimental homeotic transformations of single digits are well-documented, but frame-shifts of more than one digit are not. We investigated the pattern of cartilage formation in the development of Cyclopamine-treated wings. When Cyclopamine was applied between stages 18 and 21, morphologies that normally develop from positions 2 and 3 developed from positions 3 and 4. The serial shift of digit identity toward posterior confirms a mechanistic possibility that was previously inferred from the evolutionary history of birds.