Several target genes are expressed in response to different nuclear concentrations of the Dorsal protein, dividing the embryo into regions of different fate. twist is one of the earliest target genes of Dorsal.
Twist
is activated in response to high levels of Dorsal in a gradient at the ventral
side of the embryo, where it is required for the specification of mesodermal
fate. Dorsal and Twist protein function synergistically to regulate the
expression of snail, another mesodermal dorsal target gene. The expression
pattern of snail shows a steep fall-off at the edge. This sharp limit
demarcates the boundary between mesoderm and neural ectoderm (mesectoderm).single minded (sim) is expressed in a single row of cells at either side of the presumptive mesoderm, bordering snail expression. This row of cells, termed mesectoderm, will converge at the ventral midline when mesodermal cells invaginate during gastrulation. The mesectodermal cells will contribute to the formation of the VNC and will generate specialized neurons and glial cells, the midline cells. sim is positively regulated by dorsal and twist and repressed by snail.
A further threshold of gene activation through dorsal is represented by rhomboid (rho), which is expressed in the ventral part of the neurogenic ectoderm. The neurogenic ectoderm is the region that will give rise to the nervous system and the ventral epidermis (see nervous system development). Similar to the regulation of sim, rho is activated by dorsal and twist and its expression excluded from the presumptive mesoderm by repression through snail. Different sensitivity to Dorsal and Twist as well as regulation by additional factors accounts for the differences in dorsal extension of Sim and Rho.
The neurogenic region is demarcated at the dorsal side by the presence of short gastrulation (sog) and absence of zerknüllt (zen) and decapentaplegic (dpp). zen and dpp are expressed only in the dorsal ectoderm. The same low levels of Dorsal that activate sog repress zen and dpp.
Dpp gradient
dpp is required for the establishment of dorsal cell fate in the embryo. In the absence of dpp, the complete dorsal region is transformed into neurogenic ectoderm. In partial loss of function alleles, progressively more dorsal fates are lost, indicating that different levels of Dpp activity can elicit different responses (morphogen). At least two different cell fates are determined by dpp: amnioserosa and dorsal epidermis.
dpp is expressed at uniform levels throughout the dorsal region of the embryo, the presumptive dorsal ectoderm and amnioserosa. How can a uniform expression of dpp elicit different responses? This apparent contradiction can be resolved by a simple explanation: the short gastrulation gene product modifies Dpp post-transcriptionally to produce an activity gradient.
short gastrulation, which is expressed in the neurogenic region abutting dpp expression, encodes an inhibitor of Dpp function. It acts non-autonomously, that is, its gene product diffuses into the region of dpp expression. There it binds Dpp and renders it inactive.
Media list
Dorsal gradient (0.2 MB)..........additional
information
Dpp gradient (0.1 MB)Genes discussed
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Gene
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Gene product
- Domains
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Function
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Links
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decapentaplegic (dpp)
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ligand - TGF-ß homolog
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Dpp establishes the dorsal / ventral pattern and defines
compartment boundaries
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dorsal (dl)
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transcription factor - NFkappaB/rel homolog
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establishment of dorsal/ventral polarity
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rhomboid (rho)
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serine protease - transmembrane protein
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required for processing of the EGF-receptor ligand
Spitz
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| short gastrulation (sog) | extracellular matrix protein, chordin homologue |
Dpp antagonist, involved in the determination of dorsal/ventral
polarity - regulates amnioserosa and dorsal ectodermal fate |
Interactive Fly |
| single-minded (sim) | transcription factor - bHLH - PAS homology |
required for the developmental specification of the
CNS midline |
Interactive Fly |
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snail (sna)
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transcription factor - zinc finger
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transcriptional repressor, required for mesoderm development,
restricts neural fate in the invaginating mesoderm
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twist (twi)
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transcription factor - bHLH
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required for mesoderm development, high Twi levels
block formation of visceral mesoderm and heart and induce somatic
myogenesis
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| zerknüllt (zen) | transcription factor - homeodomain - Antp class - HoxA3, HoxB3 and HoxD3 homolog | involved in DV patterning - specifies the amnioserosa
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Interactive Fly |