L8 - Axis Development

  1. Be able to describe the basic process of building an embryo.
  2. Define the body axes and sequential gene expression in development.
  3. Define maternal effect genes & their influence on gene expression inembryos.
  4. 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.
  5. Understand morphogen gradients are important in development, & that many are transcription factors.
  6. Understand the experiments (& outcomes) that highlight the role of bicoid in development.
  7. Be able to define position of dorsal protein (active & inactive) in D-V axis generation.
  8. 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
    D-V axis

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
    Spt-TL
    Spt-TL 2

Activation of Dorsal

  1. Formation of Spätzle-Toll complex on embryo cell surface triggers cytosolic signal transduction
  2. Eventuates in the phosphorylation Dorsal inhibitor protein Cactus (Cactus stabilisds the inactive form of dorsal)
  3. Liberated Dorsal migrates to nucleus, and activates genes for ventral fates.
  4. These are zygotic genes responsible for ventral patterning
  5. 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]

D-V subdivision D-V subdivision 2
D-V subdivision 3 D-V subdivision 4

D–V axes in Drosophila & Vertebrates are related but inverted

Comparison between Vertebrates and Arthropods

the homology in protein strucutre (1,2,3,4) (gene sequencing) between vertegrate and arthropod embryological inducers

Similiarities

Nerve cord running A->P

Anterior Head

GI Tract running A->P

Differences

Arthropod nerve cord is located vetrally

Vertebrate cord is located dorsally

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

image image

image
image
image
image

image

image

image

image

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

Segmentation Genes

Control the development of segments (repeated subunits)


Sequential expression of 5 classes of genes
establishes body plan along A–P axis

2. Pair-rule Genes

Divide embryo into stripes, defining segments image

3. Segment Polarity Genes

Divide segments into Anterior and Posterior halves
image

1. Gap Genes

Divide embryo into broad regions
image

5. Homeotic Selector Genes

Specify the identity of each segment
image

0. Maternal Effect Genes

Estabilish initial basis for A-P axis and influence Zygotic Gene Expression
image

So named because mutations resulted in large “gaps” (contiguous sections missing) in the body plan

image
image

Gap Genes

Gap genes are the first zygotic genes to
be expressed along A–P axis.

Maternal Effect Genes estabilish gradient of bicoid mRNA

Bicoid gradient influences expression first zygotic gap gene, hunchback-zygotic

Hunchback protein activates other gap genes sequentially

Pattern of gene expression is influenced by the interactions between these genes

Giant image

Kruppel

image

Knirps

image

Tailles

image

image


Level of hunchback expression is directly proportional to the level of bicoid TF . High [BICOID] increases the interaction with upstream regulators. Increased interaction amounts to increased expression.

image

image image


  1. Knirps and Hunchback inhibit one another's expression
  2. High levels of Hunchback inhibit Kruppel expression, yet intermediate levels of hunchback activate Kruppel expression
  3. Giant and Kruppel inhibit one another's expression
  1. image
  1. image
  1. image