Figure 4. The expression of HoxD-11 in alligator and chicken limbs.

A) The expression of HoxD-11 in the developing forelimb and hind limb of the crocodilian Alligator mississippiensis (staging is according to Ferguson [29]) B) The expression of HoxD-11 in the chicken Gallus gallus (staging according to Hamburger-Hamilton [30]). Alligator forelimb: Early stages 12–14 show extension of HoxD-11 along the limb border, including anterior regions (black arrows). At stage 16 and onwards, there is no detectable expression in the anterior-most digit 1 region. At stage 16 posterior expression is temporarily down-regulated in the region of digits 4 and 5 (black arrow) but is re-expressed by stage 17. At stage 17, only very low expression is detectable in the interdigit between digit 1 and 2 (black arrow). Strong anterior and posterior expression in the wrist and forearm region at stage 14 continues up to stage 17 (white arrows). Alligator hind limb: Stage 12 shows some expression along the anterior margin (white arrow) but this is undetectable in the anterior-most digit 1 region from stage 14 onwards. At stage 16, only very low expression is detectable in the interdigit between digits 1 and 2 (black arrow). Expression in the foreleg is restricted to posterior (stages 12 and 14, black arrow) and absent in the ankle region. Stage 17 and stage 18 show a sharp anterior limit of HoxD-11 expression along the posterior margin of digit 2 (white arrows). Chicken wing: Stage 24 presents anterior expression (black arrow). Expression is undetectable in digit 1 in stage 26 and subsequent stages. No low expression is detectable between interdigits 1 and 2. In stages 26–34, expression of HoxD-11 extends to the anterior border of digit 2 (black arrows). Chicken hind limb: Early stage 24 shows no anterior expression. No expression is ever detected in digit 1 precursor cells or the interdigit between digit 1 and digit 2. In stages 29–34 HoxD-11 shows a sharp anterior limit along the posterior border of digit 2 (black arrows in stage 17–18 of alligator and stages 31–34 of chicken; stage 34 is shown in ventral view). Strong expression is found in the interdigit between digits 2 and 3 (stage 29, black arrow), as in forelimbs. HoxD-11 expression in alligator limbs is consistent with the notion that, as in chicken, normal digit 1 determination occurs at late stages with absent or very low HoxD-11 expression.