New developmental evidence supports a homeotic frameshift of digit identity in the evolution of the bird wing

Miguel Salinas-Saavedra, Cristian Gonzalez-Cabrera, Luis Ossa-Fuentes, Joao F Botelho, Macarena Ruiz-Flores, Alexander O Vargas

The homology of the digits in the bird wing is a high-profile controversy in developmental and evolutionary biology. The embryonic position of the digits cartilages with respect to the primary axis (ulnare and ulna) corresponds to 2, 3, 4, but comparative-evolutionary morphology supports 1, 2, 3. A homeotic frameshift of digit identity in evolution could explain how cells in embryonic positions 2, 3, 4 began developing morphologies 1, 2, 3. Another alternative is that no re-patterning of cell fates occurred, and the primary axis shifted its position by some other mechanism. In the wing, only the anterior digit lacks expression of HoxD10 and HoxD12, resembling digit 1 of other limbs, as predicted by 1, 2, 3. However, upon loss of digit 1 in evolution, the most anterior digit 2 could have lost their expression, deceitfully resembling a digit 1. To test this notion, we observed HoxD10 and HoxD12 in a limb where digit 2 is the most anterior digit: The rabbit foot. We also explored whether early inhibition of Shh signalling in the embryonic wing bud induces an experimental homeotic frameshift, or an experimental axis shift. We tested these hypotheses using DiI injections to study the fate of cells in these experimental wings.