Key Concepts

Figure 1. Expression pattern of 5¢ Hoxa/d genes in the anterior paired appendages of chick embryos. Expression of Hoxa10, Hoxa11 and Hoxa13 (10, 11 and 13) in the chick wing bud (left). In the early chick wing bud, Hoxa10–13 genes are expressed in a posteriorly nested manner (top), and by the late stage Hoxa13 is expressed in the autopodal region and Hoxa11 is expressed in the zeugopodal region. Expression of Hoxd10, Hoxd11, Hoxd12 and Hoxd13 (10, 11, 12 and 13) in the chick wing bud (right). During the early phase of limb development, 5¢ HoxD genes exhibit posteriorly nested patterns in chick wing bud (top). At the later stage, there is inverted collinear expression of 5¢ HoxD in the autopodal region of the chick wing bud. Expression patterns of 5¢ HoxA/D genes according to Yokouchi et al. (1991). AEF, apical ectodermal fold.

Figure 2. Hoxd gene expression patterns (blue) at early and late phases of paired appendage development. The late phase of inverted Hoxd expression is seen in the paired fins of chondrichtyes (S. canicula) and of chondrostei (Polydon spathula). This suggests that the late phase of Hoxd expression was acquired in the paired fins of the common ancestor of chondrichtyes and osteichthyes. Paired fins of zebrafish seem to have lost reverse colinearity in distal hoxd expression. In mouse limb buds, 5¢ Hoxd expression at later phases was expanded. Expression patterns according to Dolle et al. (1989), Ahn and Ho (2008), Davis et al. (2007) and Freitas et al. (2007); the model is described by Davis et al. (2007), Freitas et al. (2007) and Ahn and Ho (2008).

Figure 3. Models for the evolution of paired appendages in vertebrates. (a) The ‘lateral fin fold theory’, in which two paired appendages evolved from a continuous lateral fin, was proposed by Thacher (1877), Mivart (1879) and Balfour (1881). (b) The ‘pelvic before pectoral fin’ model suggested that the co-option of collinear expression of Hox (green bars) from the body axis in paired appendages originated in the pelvic appendage. In this model, pelvic Hox expression was subsequently co-opted for the development of the pectoral appendage (Tabin and Laufer, 1993). (c) The ‘pectoral before pelvic fin’ model was advocated based on fossil records and on the general anterior–posterior gradient of development (Coates, 1993; Thorogood and Ferretti, 1993). (d) Two alternative models for the evolution of paired appendages proposed by Ruvinsky and Gibson-Brown (2000) highlighted the coevolution of the T-box gene with Pitx1. Initially, an ancestral jawless vertebrate acquired a novel expression domain of Tbx4/5 (purple) in the lateral plate mesoderm at the pectoral level, and this led to the formation of the first pair of fins (top). Then two alternative scenarios were considered: the T-box cluster underwent duplication and Tbx4 (red) and Tbx5 (blue) were coexpressed in a single pair of fins (left middle). Subsequently Tbx4 (red) acquired the novel expression domain in the body wall at the pelvic level (bottom). Alternatively, Tbx4/5 (purple) acquired a novel expression domain in the posterior part of the lateral plate mesoderm (right middle), and then Tbx4/5 underwent duplication and gave rise to pectoral (blue) and pelvic fins (red) (bottom). According to this model, posteriorly expressed Pitx1 (yellow) modified the identity of the posterior/pelvic appendages together with Tbx4. (e) The molecular mechanisms of fin development in paired appendages have been proposed to be adopted from the median fins (Freitas et al., 2006). This view was based on the observation that Hoxd genes (green bars) were expressed in a nested manner in the developing shark median fin, as seen in the paired appendages. Models in panels a–e were proposed by Thacher (1877), Mivart (1879) and Balfour (1881); Tabin and Laufer (1993); Coates (1993) and Thorogood and Ferretti (1993); Ruvinsky and Gibson-Brown (2000) and Freitas et al. (2006), respectively.

Figure 4. Skeletal patterns of sarcopterygian pectoral fins/limbs. Distal radials of sarcopterygian fishes have been proposed as digit homologues (Shubin et al., 2006; Johanson et al., 2007; Boisvert et al., 2008). Figures were redrawn and modified from Latimeria, Eusthenopteron, Acanthostega (Johanson et al., 2007), Panderichthys (Boisvert et al., 2008) and Tiktaalik (Shubin et al., 2006). Reproduced by permission of Wiley (Johanson et al., 2007) and Nature Publishing Group (Boisvert et al., 2008; Shubin et al., 2006).