MULTIGENIC CONTROL OF THE LOCALIZATION OF THE ZONE OF POLARIZING ACTIVITY IN LIMB MORPHOGENESIS IN THE MOUSE

We report here that in three preaxial polydactylous mutants in the mou se, namely, Ist, Ix and Xpl, ectopic expression of the Shh and Fgf-4 g enes can be detected at the anterior margin of limb buds. These and th ree other mutants, namely, Rim4, Hx, and Xt(l), which rye described in our previous study, all appeared to form a duplicated zone of polariz ing activity (ZPA) at the anterior margin of the limb bud. We studied the spatial and temporal pattern of expression of the Gli3 gene, which is affected in a loss-of-function type of mutation, Xt(l). The expres sion domain of Gli3 appeared to be complementary to the ZPA region and the gene was expressed prior to Shh. The results support the hypothes is that GLI3 functions in the anterior portion of Limb mesoderm to sup press the expression of Shh. In Drosophila, the gene ci, the fly homol ogue of Gli, functions to repress hh, suggesting that the negative reg ulation of the expression of hedgehog by genes belonging to the GLI-kr uppel family has been conserved from flies to mice. Finally, we found that the polydactylous phenotype of the mutants Rim4 Xt, Ist, and lx; could be abrogated by the crossing with an inbred strain derived from wild mouse, MSM, whereas the phenotype of Xpl could not. These results indicate the presence of a modifier gene(s) that can influence the mu tant phenotype and also that the mutations could be classified into tw o categories with regard to the mode of interaction with the modifier gene(s). Thus, this study revealed a multigenic control in the establi shment of the anteroposterior axis in mouse limb development. (C) 1997 Academic Press.