Evolution of the vertebrate jaw: homology and developmental constraints

In embryonic development of the vertebrate head, neural crest-derived ectomesenchyme contributes to a wide range of tissue types including oro-pharyngeal and ethmoidal cartilages. The evolution of the jaw, therefore, can be viewed as a change of developmental program for specification of the crest cells. Along the anteroposterior axis of the neural crest of amniote embryos, a series of homeobox genes are expressed in a nested pattern, and the jaw-forming mandibular arch receives crest cells expressing no Hox genes and midbrain-derived crest cells that express Otx2. Cognates of these regulatory genes are present in the lamprey, and are expressed in the comparable cell lineages of the embryo. Evolution of the jaw cannot be explained from such shared developmental mechanisms, but rather noncomparable elements have to be sought, if the jaw is truly an evolutionary novelty. By precise comparative morphology and gene expression analyses, a possibility was inferred that ammocoete lips may not be identical to gnathostome jaws.