TBX GENES AND LIMB IDENTITY IN CHICK-EMBRYO DEVELOPMENT

Tbx-2, Tbx-3, Tbx-4 and Tbx-5 chick genes have been isolated and, like the mouse homologues, are expressed in the limb regions. Tbx-2 and Tb x-3 are expressed in anterior and posterior domains in wings and legs, as well as throughout the flank. Of particular interest, however, are Tbx-5, which is expressed in wing and flank but not leg, and Tbx-4, w hich is expressed very strongly in leg but not wing. Grafts of leg tis sue to wing and wing tissue to leg give rise to toe-like or wing-like digits in wing and leg respectively. Expression of Tbx-4 is stable whe n leg tissue is grafted to wing, and Tbx-5 expression is stable when w ing tissue is grafted to leg. Induction of either extra wings or legs from the flank by applying FGF-2 in different positions alters the exp ression of Tbx-4 and Tbx-5 in such a way that suggests that the amount of Tbx-4 that is expressed in the limb determines the type that will form. The ectopic limb always displays a limb-like Tbx-3 expression. T hus Tbx-4 and Tbx-5 are strong candidates for encoding 'wingness' and 'legness'.