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The origin of jaws

The development of jaws was a critical event in vertebrate evolution, marking a transition to a predatory lifestyle. How this innovation came about has been a mystery – one that a recent Brief Communication in Nature could go some way to solving.

In the embryos of jawed vertebrates (gnathostomes), the jaw cartilage develops from the mandibular arch, where none of the Hox patterning genes is expressed. If the Hox genes are expressed artificially, jaw development is inhibited.

Martin J. Cohn of the University of Reading, UK, has discovered that in the lamprey, a primitive jawless (agnathan) fish that is a sister group to the gnathostomes, a Hox gene is expressed in the mandibular arch of developing embryos. This suggests that loss of Hox expression from the mandibular arch of gnathostomes may have facilitated the evolution of jaws.

Martin Cohn may be contacted by e-mail.

Cohn, M.J. (2002) Evolutionary biology: Lamprey Hox genes and the origin of jaws.
Nature 416, 386-387 DOI: 10.1038/416386a

Nature (pp386-387)
28 March 2002

Veterinary Sciences Tomorrow invited Dr. Juncal Gonzalez-Soriano of the Faculty of Veterinary Medicine, University of Complutense, Madrid, Spain, to comment on this story. Read below what he has to say:

“To know the mechanisms that control the development of the cephalic neural crest is one of the most important issues in developmental biology. In jawed vertebrates (gnathostomes), the first branchial arch (the one which contributes to the formation of the lower jaw), is colonised by cells of the neural crests coming from the caudal mesencephalon and from rhombomeres 1 and 2. Hox genes are expressed in cells of these cephalic neural crests as well as in these rhombomeres, with the exception of the cells which will be directly involved in the lower jaw differenciation. Martin J. Chon (University of Reading, UK) has discovered that there is a Hox gene which is expressed in the first branchial arch of developing embryos of an agnathan primitive fish. His results suggest that the loss of a Hox gene expression is critical in the course of gnastome evolution.”