Hox genes and axial specification in vertebrates

The colinear, anterior to posterior expression domains of the Hox genes in vertebrate embryos is strongly correlated with regional changes in vertebral morphology. The limbs of tetrapods are consistently aligned with specific areas of the vertebral column. However, control of limb development is apparently situated in the lateral plate mesoderm, and has been experimentally shown to be independent of an axial Hox code (Cohn et aL, 1997, Nature 387:97-101). We have used experimental manipulation of chick embryos to test the causal role of Hox genes in patterning derivatives of the paraxial mesoderm. Hox expression in heterotopically transplanted segmental plate responds in a manner consistent with a patterning role for these genes in the morphological behavior of the transplants. Expression is maintained in dorsal paraxial regions where patterning is also intrinsic to the donor site of the graft. However, expression is apparently lost in somite cells that migrate into the host lateral plate environment and form appropriate host-level muscles. This arrangement could enable increased plasticity in the evolution of transpositional variation in the vertebrate body plan.