Dorsal-ventral patterning of the Drosophila melanogaster embryo begins with nuclear translocation of the ubiquitously distributed Rel-family transcription factor Dorsal. Following a complex signaling cascade, an asymmetric nuclear concentration gradient is formed, with high levels of Dorsal protein in ventral nuclei and progressively less Dorsal protein in lateral and dorsal nuclei. This Dorsal gradient differentially regulates zygotic gene expression to establish three embryonic tissue types along the dorsal-ventral axis. Of the 80 nuclei around the circumference of the precellular embryo, 18–20 ventral nuclei will form mesoderm, 14–16 lateral nuclei on each side will become neuroectoderm, while the remaining 32–34 nuclei are fated as dorsal ectoderm.; Formation of the mesoderm, neuroectoderm, and dorsal ectoderm depends upon localized expression patterns of zygotic Dorsal target genes. Studies of the mesoderm and dorsal ectoderm suggest that spatially restricted target gene expression within these tissues foreshadows subsequent subdivisions into particular cell types. The aim of this research was to determine if zygotic expression patterns of neuroectodermal Dorsal target genes similarly “pre-pattern” cell types of the ventral nerve cord prior to gastrulation. Based upon their expression patterns and mutant phenotypes, ventral nervous system defective (vnd), intermediate neuroblasts defective (ind), muscle segment homeobox (msh), and single-minded ( sim) were selected as candidates for neuroectodermal subdivision.; The experiments described here indicate that the Dorsal nuclear concentration gradient patterns the ventral nerve cord prior to gastrulation through differential regulation of transcriptional repressors. In transgenic assays, vnd was shown to establish the ventral neuroblast column by acting as a groucho dependent transcriptional repressor. Addition of vnd binding sites repressed a heterologous enhancer and ectopic vnd expression repressed both ind and msh expression. Furthermore, the Vnd protein contains a Groucho interacting domain similar to the eh1 repression domain of engrailed. The identification of a potential eh1 domain within the hid protein suggests that transcriptional repression may also be important for distinguishing intermediate neuroblasts from lateral neuroblasts. However, localized expression of Dorsal dependent transcriptional repressors is not the only mechanism subdividing the neuroectoderm. Experiments with the Dorsal target gene sim confirmed a role for the Notch signaling pathway in determining mesectodermal fate.
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