Dorsal cell fate specified by chick Lmxl during vertebrate limb development

THE positional cues that govern the fate of cells along the dorsoven-tral axis of the developing vertebrate limb are established in the mesoderm before outgrowth of limb buds. In Drosophila, a LIM/-homeodomain gene, apterous, expressed in the dorsal compartment of the wing disc, specifies dorsal cell fate^1,2. Here we report the isolation of a vertebrate LIM-homeodomain containing gene, Chick Lmxl (C-Lmxl). Transcripts for C-Lmxl are detected in the presumptive dorsal limb mesoderm and are restricted thereafter to the dorsal mesoderm of the developing chick bud. C-Lmxl expression is regulated by the overlying ectoderm where Wnt7a messenger RNA is localized³. Wnt7a, required for normal development of the dorsoventral axis in mouse limbs⁴, can induce ectopic expression of C-Lmxl in ventral mesoderm. Misexpression of C-Lmxl during limb outgrowth causes ventral to dorsal transformations of limb mesoderm. We propose that C-Lmxl specifies dorsal cell fate during chick limb development.