FIG. 1.

NE genes are ectopically induced in the ventral epidermis by different sets of Organizer transcription factors. (a) The percentage of embryos in which an ectopic ventral patch of gene expression (Foxd4, Sox11, Gmnn, Zic2) was observed after mRNA encoding an Organizer transcription factor, indicated by colored bars (key is across the top), was injected into a ventral epidermis progenitor (V1.1; Moody, 1987). The number of embryos analyzed is shown over each bar. (b) Examples of ectopic ventral induction of the four NE genes in response to injection of Organizer transcription factor mRNAs. Induced clones are outlined in black. Foxd4 and Sox11 are ectopically induced by Sia, but not by Gsc, Otx2, Foxa4 or Xnot2, whereas Gmnn and Zic2 are ectopically induced by each of these genes. Images are of the ventral sides of gastrula stage embryos, animal pole to the left. The cells expressing the Organizer transcription factor (noted to the left of each embryo) are identified by pink nuclei (nbgal staining). The NE genes, labeled at the top of each column, were detected by ISH (purple reaction product). Higher magnification insets in the top row show examples of nβgal-positive cells that are also NE gene-positive (red arrows); for these cells the induction is likely cell autonomous. Blue arrows indicate cells that are nβgal-negative and NE gene-positive; for these cells the induction is non-cell autonomous. For Sox11, the blue bar indicates a broad region of ectopic induction in which there are no nβgal-positive cells, demonstrating that most of its induction is indirect. Black arrows indicate the endogenous expression domain of the NE gene on the dorsal side of the embryo.