| dc.description.abstract | Background: Comparative morphology identifies the digits of the wing of birds as 1,2 and 3, but they develop at
embryological positions that become digits 2, 3 and 4 in other amniotes. A hypothesis to explain this is that a homeotic
frame shift of digital identity occurred in the evolution of the bird wing, such that digits 1,2 and 3 are developing from
embryological positions 2, 3 and 4. Digit 1 of the mouse is the only digit that shows no late expression of HoxD-11. This is
also true for the anterior digit of the bird wing, suggesting this digit is actually a digit 1. If this is the case, we can expect
closer relatives of birds to show no HoxD-11 expression only in digit 1. To test this prediction we investigate HoxD-11
expression in crocodilians, the closest living relatives of birds.
Methodology/Principal Findings: Using degenerate primers we cloned a 606 nucleotide fragment of exon 1 of the alligator
HoxD-11 gene and used it for whole-mount in-situ detection in alligator embryos. We found that in the pentadactyl
forelimbs of alligator, as in the mouse, late expression of HoxD-11 is absent only in digit 1.
Conclusions/Significance: The ancestral condition for amniotes is that late-phase HoxD-11 expression is absent only in digit
1. The biphalangeal morphology and lack of HoxD-11 expression of the anterior digit of the wing is like digit 1 of alligator
and mouse, but its embryological position as digit 2 is derived. HoxD-11 expression in alligator is consistent with the
hypothesis that both digit morphology as well as HoxD-11 expression are shifted towards posterior in the bird wing. | |
