Cells from the cap of the animal hemisphere of the early Xenopus embryo are determined to form ectodermal lineages. When these cells are explanted and cultured in the presence of various growth factors a change in fate to cells of mesodermal lineage can be observed. Proteins of the fibroblast growth factor (FGF) family belong to this class of fate altering compounds. The ability of FGFs to change animal cap cell fate is in part due to an alteration in the program of genes expressed in these explanted cells. Several genes that are known to be pattern regulating in other systems have been shown to be induced by FGFs in the animal cap assay. We have utilized a PCR-based sib-selection and cloning protocol to identify a large number of cDNAs of the HOX, POU and FKH families that are present in animal caps very early during bFGF-induced mesodermal differentiation. A total of 11 different HOX, 7 POU and 4 FKH cDNAs were identified in an induced animal cap cDNA library. In several cases, pairs of highly related sequence variants were identified that presumably represent expression from the duplicated alleles of the ancestrally tetraploid Xenopus genome. In this report we characterize the temporal and spatial expression of three novel Xenopus genes during early development as well as during bFGF-induced mesodermal differentiation.
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