Figure 5.

Scheme showing the relationships between the function of the HoxA and HoxD complexes in distal limbs and the dactyly. Hox complexes are shown either in light gray, when not functional, or bold boxes when functional. In the absence of HoxA and HoxD expression, a complete adactyly is observed. This situation may reflect an ancestral step in which neither complexes had been recruited in distal limbs. Activation of the HoxA complex genes in distal limb, in absence of HoxD function (pathway on the top) coincides with the appearance of a series of truncated digit-like bony elements with a clear polydactyly. The subsequent recruitment of Hoxd genes in the digit domain could lengthen all the digits while reducing their number to the pentadactyl formula. Alternatively, activation of the Hoxd genes first (pathway in the bottom) would have generated an oligodactyl limb, with potentially long digits, and the subsequent activation of the Hoxa genes would have shortened digits and extended their number to five. Paleonthological and developmental evidence support the first pathway (see the text).