Fig. 6. Hox expression in agnathans and gnathostomes. (A) Correlations between Hox expression and jaw development in chordates. The phylum chordata can be subdivided into two groups: jawed gnathostomes (green) and jawless agnathans (red). Some organisms in both groups, including the jawless lamprey and the jawed teleost fish, possess neural crest (blue) that can be acted on by Hox genes. Recent experiments (Ferrier et al., 2000; Cohn, 2002) have demonstrated that Hox expression exists as anterior as the first pharyngeal arch (PA1) in agnathan lampreys and amphioxus. Conversely, in most gnathostome vertebrates, Hox expression is evident only up to the second pharyngeal arch (PA2), and no Hox expression is seen in PA1. As such, loss of Hox expression in PA1 can be correlated with the development of jaws in vertebrates.