Fins to limbs paired appendages and the tetrapod hand

The adaptive evolution of vertebrates capable of surviving in aquatic, terrestrial, and aerial environments involved the acquisition and modification of paired pectoral and pelvic appendages. These limbs boosted vertebrates' maneuverability and speed in water and later were used as the primary means of locomotion on land. The early history of paired

Tetrapod Limb

Figure 6.10

Evolution of vertebrate paired appendages

Vertebrate paired appendages evolved in a series of steps. Paired appendages are first encountered in the fossil record as elongate fins extending laterally along the body wall or as paired pectoral fins in jawless fish (shown on basal branches). Jawed fish are characterized by multiple sets of paired appendages that often have a defined fin axis (shown in blue; top left). Tetrapods (top right) have a distal autopod, with digits branching from a bend in the limb axis. One step in the evolution of serially homologous paired appendages was the co-option of nested patterns of posterior Hox genes expression in both sets of paired appendages (lower right). Source: Modified from Shubin N, Tabin C, Carroll S. Nature 1997; 388: 639-648.

Figure 6.10

Evolution of vertebrate paired appendages

Vertebrate paired appendages evolved in a series of steps. Paired appendages are first encountered in the fossil record as elongate fins extending laterally along the body wall or as paired pectoral fins in jawless fish (shown on basal branches). Jawed fish are characterized by multiple sets of paired appendages that often have a defined fin axis (shown in blue; top left). Tetrapods (top right) have a distal autopod, with digits branching from a bend in the limb axis. One step in the evolution of serially homologous paired appendages was the co-option of nested patterns of posterior Hox genes expression in both sets of paired appendages (lower right). Source: Modified from Shubin N, Tabin C, Carroll S. Nature 1997; 388: 639-648.

appendages remains uncertain, however. Diverse fin morphologies are present in Ordovician fish (463-439 Ma). Some of these ancient fish possessed median fins running the length of the body, some had paired fins near the head, and others had no fins at all. Based on the fossil record, median fins appear to have preceded the appearance of paired fins, and paired pectoral fins preceded the evolution of pelvic fins (Fig. 6.10).

The group of bony fishes that later gave rise to the tetrapod lineage included animals with similarly patterned pectoral and pelvic fins. This similarity may reflect the deployment of the same regulatory genes in each limb pair. Modern tetrapods and bony fish exhibit similar patterns of Hox gene expression during the development of pelvic and pectoral appendages. The posterior genes of both the HoxA and HoxD complexes (Hox9-13) are expressed in complex and dynamic patterns during tetrapod limb development (see Chapter 3). In unpaired fish fins, however, these Hox genes are not deployed. Consequently, primitive vertebrate appendages may not have been patterned by Hox genes, and Hox genes may have been recruited later to pattern paired fins.

The evolution of posterior HoxA and HoxD gene expression in both pectoral and pelvic limbs may have occurred in two discrete steps (Fig. 6.10). First, axial patterns of posterior Hox gene expression may have been recruited during the development of the adjacent paired pelvic limbs. Later, the pectoral limbs may have co-opted the pelvic pattern of Hox deployment. The nested domains of Hox gene expression found along the primary body axis and within developing limbs suggest that the ancestral colinearity of Hox expression was maintained as these genes assumed novel roles in limb development.

Another novel feature of posterior Hox gene regulation is evident in the distal element of tetrapod limbs. The autopod, which consists of the hand (or foot) and digits, is unique to tetrapods. During autopod development, a late phase of Hox gene expression (phase III, see Chapter 3) appears that is not present in teleost fish (Fig. 6.11). This phase is characterized

Paired Appendages

Figure 6.11

The origin of tetrapod digits

The evolution of the autopod is characterized by branching of digits from a bend in the limb axis and by the appearance of a third phase of Hox gene expression in the distal limb. A fin axis is present in the paired fins of bony fish (gray; first four fins on left). The distal tetrapod limb has a 90° bend in the equivalent limb axis (gray; top right). (box) Proximal elements of the vertebrate limb branch to the posterior from the limb axis, whereas elements of the autopod branch to the anterior (red). The reversal in branching direction is also reflected in a reversal in the nested domains of phase III Hox gene expression (light blue, Hoxd11; dark blue, Hoxd13). Source: Modified from Shubin N, Tabin C, Carroll S. Nature 1997; 388: 639-648.

Figure 6.11

The origin of tetrapod digits

The evolution of the autopod is characterized by branching of digits from a bend in the limb axis and by the appearance of a third phase of Hox gene expression in the distal limb. A fin axis is present in the paired fins of bony fish (gray; first four fins on left). The distal tetrapod limb has a 90° bend in the equivalent limb axis (gray; top right). (box) Proximal elements of the vertebrate limb branch to the posterior from the limb axis, whereas elements of the autopod branch to the anterior (red). The reversal in branching direction is also reflected in a reversal in the nested domains of phase III Hox gene expression (light blue, Hoxd11; dark blue, Hoxd13). Source: Modified from Shubin N, Tabin C, Carroll S. Nature 1997; 388: 639-648.

by a reversal in the anteroposterior order of Hox gene expression in the autopod compared with the earlier, more proximal expression domains in other developing limb elements. A single cis-regulatory element drives phase III expression of all of the posterior HoxD genes, indicating that several HoxD genes in the autopod may have been recruited simultaneously through the evolution of this element.

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  • segan
    What are paired appendages?
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    Where is paired appendages found?
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    Are all appendages in fins of fish paired?
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