Dorsal cell fate specified by chick Lmx1 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 fate1,2. 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.