CORRELATION OF WING-LEG IDENTITY IN ECTOPIC FGF-INDUCED CHIMERIC LIMBS WITH THE DIFFERENTIAL EXPRESSION OF CHICK TBX5 AND TBX4

It has been reported that members of the fibroblast growth factor (FGF ) family can induce additional limb formation in the flank of chick em bryos, The phenotype of the ectopic limb depends on the somite level a t which it forms: limbs in the anterior flank resemble wings, whereas those in the posterior flank resemble legs, Ectopic limbs located in t he mid-flank appear chimeric, possessing characteristics of both wings and legs; feather buds are present in the anterior halves with scales and claws in the posterior halves, To study the mechanisms underlying the chimerism of these additional limbs, we cloned chick Tbx5 and Tbx 4 to use as forelimb and hindlimb markers and examined their expressio n patterns in FGF-induced limb buds, We found that Tbx5 and Tbx4 were differentially expressed in the anterior and posterior halves of addit ional limb buds in the mid-flank, respectively, consistent with the ch imeric patterns of the integument. A boundary of Tbx5/Tbx4 exists in a ll ectopic limbs, indicating that the additional limbs are essentially chimeric, although the degree of chimerism is dependent on the positi on, The boundary of Tbx5/Tbx4 expression is not fixed at a specific po sition within the interlimb region, but dependent upon where FGF was a pplied, Since the ectopic expression patterns of Tbx5/Tbx4 in the addi tional limbs are closely correlated with the patterns of their chimeri c phenotypes, it is likely that Tbx5 and Tbx4 expression in the limb b ud is involved in determination of the forelimb and hindlimb identitie s, respectively, in vertebrates.