Fig. 11. A morphogen gradient model accounts for (A) the dorsal-ventral (DV) positioning of the branchiostegal ray (bsr) and opercle (op) of the wild type (WT), and (B) the combined loss of the branchiostegal ray and enlargement of the opercle typical of the edn1 mutant opercle-gain phenotype (e.g. as in Figs 2 and 4). The model was suggested in part from a gradient model of neural crest specification by BMP (Nguyen et al., 1998). The source of the Edn1 gradient is ventral, as we infer from the edn1 mRNA expression pattern in the ventralmost region of the developing pharyngeal arches (Miller et al., 2000; Miller et al., 2003). The sets of positional values (Wolpert, 1971) specifying the branchiostegal ray and opercle are shown along the Y axis in blue and red, respectively. (A) The bones are made in the local zones determined by these values, the branchiostegal ray is ventral to the opercle. (B) Lowering the Edn1 source decreases the slope of the gradient; positional values for the branchiostegal ray are now missing, resulting in loss of the ray, and the region of opercle specification expands, resulting in the opercle-gain phenotype. The model predicts that the enlarged opercle is present more ventrally in the hyoid arch than normal — closer to the Edn1 source. In fact, the position of the opercle seems not to be moved ventrally, rather the ventral part of the arch is missing or reduced in the mutant (Miller et al., 2000), such that the opercle would indeed be closer to a ventral gradient source. If the gradient is lowered more severely (not shown) then the positional values for the opercle would be missing as well, accounting for the opercle-loss phenotype.