New developmental evidence supports a homeotic frameshift of digit identity in the evolution of the bird wingReport as inadecuate




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Background: The homology of the digits in the bird wing is a high-profile controversy in developmental andevolutionary biology. The embryonic position of the digits cartilages with respect to the primary axis ulnare andulna corresponds to 2, 3, 4, but comparative-evolutionary morphology supports 1, 2, 3. A homeotic frameshift ofdigit 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 bysome other mechanism. In the wing, only the anterior digit lacks expression of HoxD10 and HoxD12, resemblingdigit 1 of other limbs, as predicted by 1, 2, 3. However, upon loss of digit 1 in evolution, the most anterior digit 2could have lost their expression, deceitfully resembling a digit 1. To test this notion, we observed HoxD10 andHoxD12 in a limb where digit 2 is the most anterior digit: The rabbit foot. We also explored whether early inhibitionof Shh signalling in the embryonic wing bud induces an experimental homeotic frameshift, or an experimental axisshift. We tested these hypotheses using DiI injections to study the fate of cells in these experimental wings.Results: We found strong transcription of HoxD10 and HoxD12 was present in the most anterior digit 2 of therabbit foot. Thus, we found no evidence to question the use of HoxD expression as support for 1, 2, 3. When Shhsignalling in early wing buds is inhibited, our fate maps demonstrate that an experimental homeotic frameshiftis induced.Conclusion: Along with comparative morphology, HoxD expression provides strong support for 1, 2, 3 identityof wing digits. As an explanation for the offset 2, 3, 4 embryological position, the homeotic frameshifthypothesis is consistent with known mechanisms of limb development, and further proven to be experimentallypossible. In contrast, the underlying mechanisms and experimental plausibility of an axis shift remain unclear.Nota general

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Author: Salinas Saavedra, Miguel Andrés H.; - González Cabrera, Cristian; - Ossa Fuentes, Luis; - Botelho, Joao F.; - Ruiz Flores, Maca

Source: http://repositorio.uchile.cl/



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