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PERSPECTIVES: EVOLUTION
In his poem about the lonely "Maldive Shark," Herman Melville describes the daunting jaws of a serious meat-eater, which serve as an asylum for the sleek little pilot fish, azure and slim, hiding in his "jaws of the Fates." The "jaws of the Fates" may act rather unpredictably in the uncharted oblivion of the Indian Ocean, but Depew et al. (1) report a remarkable catch-the genes that dictate the fates of jaws-on page 381 of this issue. These authors turn lower jaws into upper jaws by simultaneously inactivating the homeobox genes Dlx5 and Dlx6 of mice. Such a spectacular transformation of "jaw identity" unveils a family of genes that are crucial for directing formation of the vertebrate face. This gene family may have been subject to profound modifications during vertebrate evolution and in certain human congenital diseases. The Dlx5 and Dlx6 genes are now implicated in the elaboration of vertebrate lower jaws, from the ferocious feeding machinery of the great white shark to the sophisticated hearing system of mammals.
A complex series of cellular and molecular interactions underlies the assembly of the vertebrate face. Most structures are formed by the neural crest, a tissue that emanates from the early embryonic brain and populates the socalled bronchial arches (2). Bronchial arches are a segmental series of bulges in the embryonic head and are predecessors of all facial elements. Within the first (mandibular) bronchial arch, jaw elements develop from three bulges: the mandibular, maxillary, and frontonasal processes that are filled by neural crest cells from different origins (midbrain and hindbrain) (3). Widespread mixing between them supports the notion that jaw neural crest is exposed to instructive signals from its enironment that establish a proximodistal axis to the jaw-forming branchial arch (3). Such external cues are translated into a code of neural crest proximodistal "identity," leading to the precisely orchestrated formation of skeletal and muscular elements. Much evidence implicates the Hox homeobox genes as the encoders of "rostrocaudal identity" in all...