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L9 - Developmental Axis III Be able to describe the basic process of…
L9 - Developmental Axis III
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.
Sequential expression of different sets of genes post fertilization
Maternal effect localised mRNA & proteins translated after fertilization give positional
information to activate zygotic genes.
Downstream genes activated in temporal sequence:
gap genes
pair rule genes
segmentation polarity genes
homeotic identity genes
Maternal Effect Genes
Gap Genes
Pair Rule Genes
Sedment Polarity Genes
Homeotic Identity Genes
Pair Rule Genes
Gap gene expression results in a gradient of morphogens which directly induces the expression of pair-rule genes
Pair Rule Genes encode transcription factors and are activated in a series of 7 stripes in every second parasegment of Thorax and Abdomen
Primary Pair-Rule Genes
3 primary genes
Even-Skipped (eve)
Even-skipped defines odd-numbered parasegments
Runt
Hairy
Secondary Pair Rule Genes
5 secondary genes
Fushi tarazu
Fushi tarazu expressed in even-numbered parasegments
Protein Expression of Stripes
Influenced by two chief interactions
Enhancers Sequences
Transcription Factors
ENHANCER
Regulating which proteins are expressed in each para-segment is determined by that genes location relative to specific enhancer sequences
E.g. A gene located adjacent or near to the enhancer which regulates the
1st parasegment
, will then be prominent in the
1st parasegment.
If it is also located next to the enhancer regulating the
5th parasegment
, then it will be at the 1st and
5th parasegmen
t.
These enhancer sequences are located both
upstream and downstream
of the
Transcription start site
Enhancer for Stripe 1 translocated next to
lac z
gene
lac z
encoded proteins prominent at stripe 1
Enhancer for Stripe 5 translocated next to
lac z
gene
lac z
encoded proteins prominent at stripe 1
Transcription Factors
When consiudering
eve
expression, the following is true;
Binding of Bicoid and Hunchback proteins stimulates transcription of
eve.
Binding of Giant and Krüppel represses
eve
transcription.
• Trapped in a valley between high levels of the giant and Krüppel proteins,
expression of eve in the third stripe is encouraged
Segment polarity genes
Define the
boundaries and polarities
of the 14 segments
Expression induced by Pair Rule Genes and continually expressed throughout the life of
Drosophila
=>
In contrast to Gap and pair-rule genes, they are not all transcription factors, have more diverse functions
The combined expression of the pair-rule genes regulate transcription of the segment polarity genes
Are expressed in patterns of 14 stripes
Expression begins after cellularization
Wingless (wg)
&
Engrained (en)
Encode diffusible signalling proteins.
These signalling proteins are the principal constituents of the
Hedgehog and Wnt
signalling pathway
Hedgehog and Wnt Signalling Pathway
promotes cell
differentiation through signalling to
the nucleus.
en & wg
mutations affect wing
development in adult
drosophila
Also expressed in many other times/places.
Establishing Segment Polarity
Segment polarity is required for formation of segment boundaries
Within this model,
en
and
wg
help define the
posterior and anterior
compartment of each segment, respectively.
Wg
is released from a cell in the anterior compartment of the segment
Wg
protein It binds to a cell in the posterior compartment of the segment and induces expression of
En
En
protein serves as a transcription factor that binds to the promoter sequence regulating expression of
Hedgehog (Hh)
Hh
is secreted from the cell once its traslation is complete. It binds to recptors on neighboruing cells, and promotes
Wg
transcription and subsequent secretion of
Wg
Homeotic Selector Genes
The sumation of interactions between the Segment Polarity Genes regulate the
homeotic selector genes
Homeotic selector genes encode transcription factors and control
segment identity
The Homeobox Domain
All homeotic selector genes encode for transcription factors with a specific domain commonly referred to as a
"homeobox
"
Homeodomain/homeobox domain
improve the TF's ability of to binding to DNA, and will serve either to activate or repress transcription
Discovery
Mutations of certain genes were observed to delete/convert entire segments into a different type of segment
E.g. Misexpression of
antp
in anterior segments results in legs forming where the antennae should be
Homeosis - development of one body part with the phenotype of another
Developmental Parallels
Homologs of drosophila selector genes have been discovered in virtually all other animals
ATHROPODS
- Homeotic selector genes are arranged in two clusters
(Hom-C Complex)
VERTEBRATES
- Homeotic Selector genes are arraged in 4 clusters
(Hox A/B/C/D)
Arranged in the same order and expressed in the same direction
Homeotic Selector Genes are said to be orthologous developmental genes
Segmentation of the Drosophila Embryo
Segment
- repeated subunits that will form specific structures (Thorax, Abdomen)
Compartments
- segments which have been subdivided into an
Anterior
and
Posterior
portion. A-compartment and a P-compartment = Segment
Parasegment
- Posterior compartment of the preceeding segment combined with the anterior compartment of the following segment.
