Ct

)(5 6 100's of thoracic vertebrae

\Hoxc6 \Hoxc8

Figure 5.6

Hox genes and the evolution of tetrapod axial identities

Figure 5.6

Hox genes and the evolution of tetrapod axial identities

Differences in the axial organization of tetrapods are reflected in shifts in Hox gene expression domains between animals. (a) The anterior boundaries of expression of several Hox genes in the paraxial mesoderm are shown beneath the somites (circles) and vertebrae (squares) of the mouse, chick, goose, and python body plans. In mammals and birds, which have distinct cervical (green) and thoracic (purple) axial regions, the anterior boundary of the Hoxc6 gene lies at the cervical-thoracic transition, even though the axial position (somite number) of this transition falls at a different position in each organism. Similarly, the anterior boundary of Hoxc8 expression lies within the thoracic region of chicks and mice; the Hoxa9, Hoxb9, and Hoxc9 boundaries lie near the thoracic-lumbar transition, and the Hoxd9 boundary lies near the lumbar-sacral transition. In the python, the Hoxc6 and Hoxc8 genes have a more anterior expression boundary, reflecting the expanded thoracic vertebral identities of the snake body plan. (b) Expression of the mouse Hoxc8 gene in the thoracic region (the extent of high levels of expression is indicated by arrows). (c) Expression of the snake Hoxc8 gene extends through the anterior of the axial skeleton (indicated by arrows and arrowheads).

Sources: Part a modified from Burke AC, Nelson CE, Morgan BA, Tabin C. Development 1995; 121: 333-346; parts b and c from Cohn MJ, Tickle C. Nature 1999; 399: 474-479.

Hoxc8 extends far to the anterior, up to the cranial region (Fig. 5.6). The posterior boundaries lie at the level of the (vestigial) hindlimb, as in other tetrapods. Within the domain of Hoxc6 and Hoxc8 expression, all of the python vertebrae bear ribs, indicating thoracic identity. Interestingly, a subset of cervical characters is present on the most anterior rib-bearing vertebrae of pythons, suggesting that thoracic identities overlie the ancestral cervical identities near the head. Thus the loss of the snake's neck and the expansion of its rib-bearing vertebrae are correlated with the anterior shift in the expression of Hoxc6 and Hoxc8.

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