The Questions of Developmental Biology

The posterior region expresses all the Hoxd genes from Hoxd-9 to Hoxd-13, while only
Hoxd-9 is expressed anteriorly. In the third phase of limb development, when the autopod is
forming, there is a further redeployment of Hox gene products. Hoxd-9 is no longer expressed.
Rather, Hoxa-13 is expressed in the anterior tip of the limb bud and in a band marking the
boundary of the autopod. Hoxd-13 products join those of Hoxa-13 in the anterior region of the
limb bud, while Hoxa-12,Hoxa-11, and Hoxd-10 12 are expressed throughout the posterior twothirds
of the limb bud.
*When referring to the hand, one has an orderly set of names to specify each digit (digitus pollicis, d. indicis, d.
medius, d. annularis, and d. minimus, respectively, from thumb to little finger). No such nomenclature exists for the
pedal digits, but the plan proposed by Phillips (1991) has much merit. The pedal digits, from hallux to small toe, would
be named porcellus fori, p. domi, p. carnivorus, p. non voratus, and p. plorans domi, respectively
Hox Genes and the Evolution of the Tetrapod Limb
Macroevolution, the generation of morphological novelties in the evolution of new
species and higher taxa, results from alterations of development. One of the most obvious
macroevolutionary changes is that from the fish fin to the amphibian leg. As Richard Owen
(1849) pointed out, there is considerable homology between the bones of the fish fin and the
tetrapod limb, the pectoral and pelvic fins of the fish being homologous to the tetrapod forelimb
and hindlimb, respectively. While specific homologies were able to be made between the
proximal elements of the fin and the limb, the homologies proposed between the autopod of the
limb (the hand or foot at the distal end) and the rays of the fins "did not hold water." This was
true even when one compared the tetrapod limb with the fins of the crossopterygian (lobe-finned)
fishes thought to have been closely related to the ancestors of the amphibians (see Coates 1994;
Hinchliffe 1994). While there seems to be homology for the proximal and central elements of the
limb, the autopod seems to be something new what evolutionary biologists call a neomorphic
structure.
Recent studies have strongly suggested that the expression of the 5´ genes of the Hoxd
group may be crucial in the change from fin to limb. Tetrapods and fishes share the first two
phases of the Hox expression pattern in their appendages. Thus, both groups form stylopods and
zeugopods. However, the phase III pattern of Hox gene expression is unique to tetrapods and is
not found in fishes. Moreover, this change in Hox gene expression is mediated by a single
enhancer element that is not found in fishes (Gerard et al. 1993; van der Hoeven et al. 1996). This
phase III change represents an inversion of gene expression, placing the most 5´ Hox gene
products in the anterior of the limb bud. Instead of being restricted to the posterior of the limb
bud, the expression of the 5´ Hox genes sweeps across the distal mesenchyme, just beneath the
AER. This band of expression is coincident with the "digital arch" from which the digits form
(Figure 16.16; Morgan and Tabin 1994; Sordino et al. 1995; Nelson et al. 1996).