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L8 - Axis Development Be able to describe the basic process of building…
L8 - Axis Development
- Be able to describe the basic process of building an embryo.
- Define the body axes and sequential gene expression in development.
- Define maternal effect genes & their influence on gene expression inembryos.
- Detail the process of pattern formation in wildtype Drosophila,
particularly the A-P axis genes: bicoid, caudal, hunchback, nanos & D-V axis genes: toll, cactus, dorsal, spätzle.
- Understand morphogen gradients are important in development, & that many are transcription factors.
- Understand the experiments (& outcomes) that highlight the role of bicoid in development.
- Be able to define position of dorsal protein (active & inactive) in D-V axis generation.
- Understand the homeotic selector gene & homeodomain function.
Dorso-Ventral axis patterning and cell-to-cell signalling
- Maternally-derived proteins provide signals necessary for dorso-ventral specification
Dorsal, Spatzle, Toll
- Although a simplification:
Maternal Dorsal gene (dl) encodes the D-V morphogen, Dorsal
Dorsal is a transcription factor*
- Dorsal
- Dorsal mRNA & Dorsal protein are evenly distributed throughout the cytoplasm of early embryo
Dorsal TF, however, can only actively influence gene expression when located in the nucleus
- Low concentrations of Dorsal in the nucleus of Dorsal pole
- High concentrations of Dorsal in the nucleus in the Ventral pole
Dorsal's migration from cytoplasm to the nucleus is faclitated by Spatzle ligand and Toll receptor
- SPZ gradient estabilished during oogenesis, localised in ventral pole
- TOLL is uniformly distributed across plasma membrane of early embryo
Activation of Dorsal
- Formation of Spätzle-Toll complex on embryo cell surface triggers cytosolic signal transduction
- Eventuates in the phosphorylation Dorsal inhibitor protein Cactus (Cactus stabilisds the inactive form of dorsal)
- Liberated Dorsal migrates to nucleus, and activates genes for ventral fates.
- These are zygotic genes responsible for ventral patterning
- At the dorsal pole, absence of Spätzle-Toll complex results in silencing of vetral effect genes
Evidence for Dorsal's role in Ventral Development
- Drosphila mutants with absent expression of TOLL, are unable to establish D-V axis
- Drosphila mutats with depleted cactus inhibitor universally express Ventral effect genes
Subdivision of Dorso-Ventral axis into unique regions
Nucleic concetrations of Dorsal influence the expression of certain zygotic genes pertinent in the differentiation of the Dorso-Ventral and lateral segments of the embryo
- Zerknullt = Low [Dorsal]
- Decapentaplegic and tolloid = Low-Medium [Dorsal]
- Rhomboid, Sag = Med-High [Dorsal]
- Twist, Snail = High [Dorsal]
Comparison between Vertebrates and Arthropods
the homology in protein strucutre (1,2,3,4) (gene sequencing) between vertegrate and arthropod embryological inducers
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Differences
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D-V axis is inverted
BMP-4 is homologous to Dpp, and whilst Dpp induces dorsal devlopment in arthropods, in vertebrates, BMP-4 induces ventral development
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Segmentation of the Drosophila Embryo
Once A-P D-V gradients are estabilshed, unique Zygotic genes are activated by specific transcription factors and become localised in specific segments
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