Heterotopic shift of epithelial-mesenchymal interactions in vertebrate jaw evolution.

Genes involved in late specification of the mandibular arch, the source of the vertebrate jaw, are expressed with similar patterns in the oral regions of chick and lamprey embryos. However, morphological comparisons indicate that apparently orthologous homeobox genes were expressed in different subdivisions of the ectomesenchyme in the two species. Therefore, the homology and gene expression of the oral region are uncoupled during the transition from agnathan to gnathostome; we conclude that a heterotopic shift of tissue interaction was involved in the evolution of the jaw.