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ESTABLISHMENT OF THE BODY AXES – DROSOPHILA
Anterior-Posterior Body Axis
The relationship between the embryo and the adult patterns in drosophila are still not fully
understood- metamorphosis is a stage not shared with mammals, and the adult Drosophila emerges
from the development of the imaginal discs on the embryo
...
The
temporally and spatially controlled activation of these molecules and hence the choreography of
gastrulation movements, is determined by region-specific expression of transcription factors which
turn on a set of downstream targets whose products mediate the successive steps of gastrulation
...

The relationship between the egg and larval pattern
-Egg: The AP axis becomes divided into different regions that later give rise to the head, thorax and
abdomen
-Embryo: After the initial division into broad body regions, segmentation begins (future segments
can be visualised through staining eg
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The ventral ectoderm gives rise to both ventral epidermis and neural
tissue, the dorsal ectoderm to epidermis
-Larva: Egg then develops into a segmented larva
...
One
must prepare the cuticle such that it allows easy tracking of specific structures in the body plan of
the embryo
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Generation of mutants:
 Mutate the males and cross with virgin females
 Take mutant male offspring and mate with virgin female to generate heterozygous males
and females (virgin)
 Cross the heterozygous males and heterozygous virgin females to generate offspring that
are homozygous for the mutation
 Analyse the phenotypes of these homozygous embryos and the embryos laid by the
homozygous mutant females

Results of the screen for maternal determinants of the AP axis:
 Anterior: Bicoid, exuperantia, swallow, staufen
 Posterior: Nanos, oskar, vasa, valois, tudor, pumilio, staufen
Early studies on maternal determinants of the AP axis:
These early studies involved embryological techniques, such as the transfer of cytoplasm:
 Puncturing the anterior to let the cytoplasm escape results in a posteriorised embryo, losing
thoracic and head structures
...
But are they sufficient?
 Removing anterior cytoplasm and transferring posterior cytoplasm in its place generates an
embryo with two posterior ends rather than anterior and posterior, thus proving that the
factors present are also sufficient for specification of this axis
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Are any of the products of the genes from
the screen asymmetrically localised?
Labelling of the bicoid mRNA and nanos mRNA demonstrates anterior and posterior localisation
respectively, a more concentrated localisation than is present with the proteins, though the proteins
still localised along AP axis
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At the
10th cell cycle (512 nuclei, 2 hours) the nuclei migrate to the periphery of the cell, creating the
syncytial blastoderm
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The cellular blastoderm consists of approximately 6000 cells and is formed
within 4 hrs of fertilisation
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It is a morphogen,
since it specifies cell fate in a concentration dependent manner
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A system of repressor gradients spatially organises the boundaries of bicoid dependent target
genes (CHEN H et al
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Runt, Capicua and Kruppel)
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Nanos is necessary and sufficient to induce posterior structures
Nanos is an evolutionarily conserved translational regulator
...

Hunchback and Caudal mRNA is expressed uniformly throughout the embryo, but increasing Nanos
protein from the posterior inhibits the action of Hunchback, such that only caudal is selectively
active in the posterior
...

What is the gene network from the AP morphogens to the segmentation pattern at gastrulation?
 Bicoid and Hunchback regulate the production of anterior structures
 Nanos and Caudal regulate the formation of the posterior part of the embryos
 These proteins (and others) activate or repress the expression of zygotic gap genes which
define broad territories of the embryo (at the syncytial blastoderm)
 Differing concentrations of the gap gene proteins cause the transcription of the pair-rule
gene in seven stripes, which divide the embryo into 14 para-segments (at the cellular
blastoderm)
 The pair-rule genes activate the segment polarity genes (gastrulation), and the products of
the gap, pair-rule and segment polarity genes interact to regulate the homeotic selector
genes which define the developmental fate of each segment
Some of the mutants from the maternal screen affected the localisation of bicoid and nanos mRNAs
in the freshly laid egg
...

Staufen is required for oskar mRNA localisation: Staufen mutants (mutation in the RNA binding
domain) displays the oskar mRNA still in the nurse cells rather than at the posterior of the oocyte
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What is needed for an RNA to be localised?
1
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Polarised cytoskeleton (actin and microtubules)
Drosophila Oogenesis
Bicoid and oskar mRNAs are localised at stage 9 of oogenesis
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This epithelium needs to pattern and adopt the
precise fates for the final “signalling” towards the oocyte:
 Oocyte secreted gurken (TGFa)
 Follicle cells need Notch pathway, Hippo pathway, EGF receptor (Gurken receptor)
 Unknown form of signalling from follicle cells back to oocyte
The follicle cells at the posterior adopt the posterior fate
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Thus results in the migration of the
oocyte nucleus from the posterior to the anterior-dorsal corner, and reorganisation of the
microtubule cytoskeleton
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elegans (in which Par proteins were discovered)
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elegans oocytes
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Summary:
1
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The MT network reorganises with the minus and plus ends at the anterior and posterior
respectively
3
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Oskar is translated and the proteins recruits nanos
4
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The proteins activate or repress the zygotic gap genes to define broad territories of the
embryo
6
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The pair rule genes activate the segment polarity genes (during gastrulation), and the
products of the gap, pair rule and segment polarity genes interact to regulate the homeotic
selector genes which define the developmental fate of each segment

Dorsal-Ventral Body Axis
In the early embryo, following specification of the anterior-posterior axis, the DV axis is divided into
four regions: mesoderm, ventral ectoderm, dorsal ectoderm and amnioserosa
...
The ventral ectoderm gives rise to both ventral epidermis
and neural tissue, the dorsal ectoderm to epidermis
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Spatzle, dorsal, toll, snake and easter
 Ventralised the whole embryo in ventral epidermis and mesoderm: Eg
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The important step is the localisation of Dorsal protein to the
nuclei of the ventral cells (in the cellular blastoderm) where it acts to
specify the different DV regions of the embryo
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It was shown that Toll is responsible for the localisation of Dorsal to
the ventral cells:





Wild type toll: Dorsal protein seen only in ventral cells
Recessive toll mutant: Dorsal not seen in any cells
Dominant toll mutant: Dorsal seen in all cells

Toll is actually an interleukin-like receptor which is expressed everywhere, but only activated at the
ventral side
...

 The nuclear dorsal expression is confined to a region corresponding to the pipe domain of
the follicular epithelium

What is the relation of Pipe, the Ser proteases and Toll?
 Ectopic pipe also activates toll
 Injection of a Toll mutant perivitelline fluid into a pipe
mutant perivitelline space induces DV polarity
 What is in this fluid?
 How to identify it?
-A Pipe-dependent ventral cue induces a protease cascade in the
perivitelline fluid that results in the production of active Spätzle
...
), leading to the activation of Spätzle
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-The Pelle protein kinase phosphorylates the Cactus protein,
leading to its degradation which then allows Dorsal protein to enter
the nucleus
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This reduction of Grk signalling leads to an expansion of the pipe domain,
resulting in toll signalling over a broader region, resulting in two regions receiving very high levels of
nuclear dorsal and therefore two sets of cells begin to invaginate (due to high expression of twist)
during gastrulation
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Summary of Development of Body Axes:
 Stage 8: Nuclear migration to the posterior dorsal corner
 Stage 9: MT rearrangements and localisation of bicoid anteriorly and oskar posteriorly (AP
axis)
 Stage 10a/10b/11: Gurken and Pipe signalling to begin defining the DV axis through
expression of ventral Dorsal
 Fertilised egg: follicle cells change into the perivitelline space and see bicoid mRNA at the
anterior and nanos mRNA at the posterior

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