A chick wingless mutation causes abnormality in maintenance of Fgf8 expression in the wing apical ridge, resulting in loss of the dorsoventral boundary

We analyzed a Japanese chick wingless mutant (Jwg) to know a molecular mechanism underlying wing development. We observed expression patterns of eleven marker genes to characterize the mutant. Expressions of dorsoventral (DV) and mesenchymal marker genes were intact in nascent Jwg limb buds. However, expression of Fgf8, a marker gene for the apical ectodermal ridge (AER), was delayed and shortly disappeared in the wing regressing AER. Later on, ventral expression of dorsal marker genes of Wnt7a and Lmx1 indicated that the wing bud without the AER became bi-dorsal. In addition, the posterior mesoderm became defective, as deduced from the impaired expression patterns of Sonic hedgehog (Shh), Msx1, and Prx1. We attempted to rescue a wing by implanting Fgf8-expressing cells into the Jwg wing bud. We found that FGF8 can rescue outgrowth of the wing bud by maintaining Shh expression. Thus, the Jwg gene seems to be involved in maintenance of the Fgf8 expression in the wing bud. Further, it is suggested that the AER is required for maintenance of the DV boundary and the polarizing activity of the established wing bud.