The response of early neural genes to FGF signaling or inhibition of BMP indicate the absence of a conserved neural induction module

To address the question of necessity and sufficiency of BMP inhibition and FGF signaling, we compared the temporal expression of the five earliest genes expressed in the neuroectoderm and determined their requirements for induction at the onset of neural plate formation in Xenopus. Our results demonstrate that the onset and peak of expression of the genes vary and that they have different regulatory requirements and are therefore unlikely to share a conserved neural induction regulatory module. Even though all require inhibition of BMP for expression, some also require FGF signaling; expression of the early-onset pan-neural genes sox2 and foxd5α requires FGF signaling while other early genes, sox3, geminin and zicr1 are induced by BMP inhibition alone.We demonstrate that BMP inhibition and FGF signaling induce neural genes independently of each other. Together our data indicate that although the spatiotemporal expression patterns of early neural genes are similar, the mechanisms involved in their expression are distinct and there are different signaling requirements for the expression of each gene.Development of the vertebrate central nervous system (CNS) is initiated during gastrulation when dorsal ectodermal cells are converted to the neural fate. There are two prevailing models for the induction of the CNS. The first, the neural default model, arose from experiments demonstrating that in the absence of bone morphogenetic protein (BMP) signaling, amphibian ectodermal explants form neural tissue instead of epidermis [1,2]. Formation of the nervous system by default is highly conserved. In the protostome Drosophila melanogaster, neural tissue forms as a result of inhibition of the BMP homolog Decapentaplegic (Dpp) by the Chordin ortholog Sog [3]. Furthermore, the Xenopus BMP antagonist Noggin is sufficient to inhibit Dpp and induce neuroectoderm in fruit flies [4], and overexpression of Sog induces a secondary axis in Xenopus embryos [5]. The second model, the instr