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Axis Specification and Patterning II
Segmentation and Anterior
posterior axis specification in the
Drosophila embryo
Segmentation and anterior-posterior body plan
The hierarchy of genes identified in the genetic screens pioneered by Christiane Nϋsslein-Volhard
1) Genes that establish anterior posterior polarity
2) Genes that divide the embryo into specific number of segments each with a different identity
Zygotic genes
Classical experiments:
Klaus Sanders (1975)
Ligated the egg early in development from the middle.
One half became an anterior end of the embryo and other
the posterior half the middle segments were missing.
Destruction of RNA in the anterior of insect eggs with
Ultraviolet light or RNAse
Embryos lacked a head and thorax and developed into
embryos with two abdomens and two telson (tails)
Normal
Irradiated
http://10e.devbio.com/article.php?ch=9&id=93
Postulate: Morphogen gradients emanating from the two
poles interacting to produce positional information.
What were these morphogens present in gradients in the early embryo?
The genetic screen carried out by Christiane Nϋsslein-Volhard and colleagues.
Maternal gradients: polarity regulation by oocyte cytoplasm
Bicoid as the anterior morphogen was established by the strategy of “find it, lose it, move it”
“find it”
Bicoid protein and bicoid
mRNA were both found to
be present in a gradient
Highest in the anterior
(head-forming region)
“lose it”
“move it”
Anterior determinant:
Localization of the bicoid
mRNA to the anterior of
the oocyte
 3’ UTR of bicoid mRNA is critical
for its localization.
 bicoid mRNA synthesized in
nurse cells interacts with
Exuperantia and Swallow
proteins.
 This complex is transported out
of nurse cells on microtubule
rails riding a Kinesin.
 In the oocyte bicoid mRNA
associates with the dynein
proteins present at the minus
end of microtubules in the
anteriror.
Posterior determinant: nanos mRNA is localized to the posterior pole of the oocyte
 nanos mRNA seems to be “trapped” in the
posterior end by passive diffusion.
 nanos mRNA interacts with products of
oskar, valois, staufen, vasa and tudor.
 The oskar mRNA and Staufen protein are
transported first to the posterior of the
oocyte via Kinesin motors.
 Staufen permits the translation of oskar
mRNA. The Oskar protein then binds
nanos mRNA and traps it in the posterior.
Gradients of specific translational inhibitors: Localized translation of hunchback and caudal transcripts
http://www.tmd.ac.jp/artsci/biol/textlife/develop2.htm
Bicoid
Caudal
http://www.mun.ca/biology/desmid/brian/BIOL3530/DEVO_0
2/devo_02.html
How is it that different concentrations of Bcd at different points along the A/P
axis of the embryo lead to transcription of different target genes?
 The Bcd gradient provides positional information along the axis in a dosedependent manner and efforts have been made to understand how this could be
achieved. As the Bcd protein encodes a DNA-binding transcription factor, it was
initially proposed that the thresholds of Bcd concentration required for the
expression of its target genes depend on the number and on the affinity of Bcd
binding sites found in their regulatory regions.
 However, it is now clear that other elements in target gene promoters and the
integration of positive and negative transcriptional inputs from proteins bound to
these elements are major determinants for the interpretation of positional
information along the anterior-posterior axis.
Segmentation genes
Gap genes
The gap genes are activated or repressed by
maternal effect genes and are expressed in one or
two broad domains along the anterior posterior
axis.
http://www.discoveryandinnovation.com/BIOL202/notes/lecture21.html
The transcription pattern of different gap genes are
initiated by different concentrations of Hunchback
and Bicoid.
Interactions between AP polarity
genes and gap genes
 High levels of Bicoid and Hunchback induce the
expression of giant
 Kruppel transcripts appear where Hunchback
transcripts decline.
 The Caudal protein which is highest in the
posterior activates the giant and knirps genes in
the posterior.
Figure 6.29 Architecture of the gap gene network
Pair rule genes: The first indication of segmentation
Primary pair rule genes: hairy, even-skipped and runt
Expressed in 7 stripes each.
Secondary pair rule genes: fushi tarazu, odd-skipped,
odd-paired, sloppy-paired and paired
Figure 6.30 Messenger RNA expression patterns of two pair-rule genes,
even-skipped (red) and fushi tarazu (black) in the Drosophila blastoderm
Each stripe pattern is the result of distinct
enhancers which are modular in nature.
Genetic studies:
Deletion of each enhancer leads to
disappearance of a particular stripe.
Biochemical studies:
LacZ reporter placed under an enhancer is
expressed in a single stripe
Figure 6.31 Specific promoter regions of the even-skipped (eve) gene control specific
transcription bands in the embryo
Segment polarity genes:
 They reinforce the paprasegment boundaries established by earlier transcription factors.
 They establish cell fates within each parasegment through cell-cell signaling.
 Encode members of the Wingless and Hedgehog signaling pathways.
 engrailed is expressed in those cells with
high levels of Eve, Paired or Ftz and
inhibited in cells with high levels of
Odd–skipped, Runt or Sloppy-paired.
 Thus engrailed marks the anterior part
of each parasegment.
 wingless is transcribed in cells that see
little or no Eve or Ftz but which have
Sloppy-paired.
 Thus wingless is transcribed in a row of
cells just anterior to those making
Engrailed.
The key to maintaining the pattern of engrailed and wingless is in activation of engrailed (en) in the cells that will
express Hedgehog.
The diffusion of the Wingless and
Hedgehog proteins provide the
morphogen gradients by which cells
acquire distinct identities with a
parasegment.
 At Stage 9-10 the Wg protein gradient is symmetric
and present anterior as well as posterior to the cells
expressing Wg.
 At Stage 11 the Wg protein gradient becomes
asymmetric such that Wg protein is only present in a
gradient anterior to the wg expressing cell.
 This asymmetry in Wg distribution and correspondingly
signaling activity is a result of its rapid endocytosis and
degradation in cells posterior from which it is secreted,
in a process that is promoted by Hh signaling.
 Wg signaling, in turn, attenuates Hh signaling anterior
to the En/Hh-expressing cells, thereby allowing its
activity in only the posterior direction.
Cold Spring Harbour Perspectives
Wnt/Wingless Signaling in Drosophila
Sharan Swarup and Esther M. Verheyen
Department of Molecular Biology and Biochemistry, Simon Fraser
University, Burnaby, British Columbia,V5A1S6, Canada
EMBO reports vol. 2 | no. 12 | pp 1083–1088 | 2001
Generating patterns from fields of cells
Examples from Drosophila segmentation
Bénédicte Sanson
University of Cambridge, Department of Genetics, Downing Site, Cambridge CB2 3EH, UK
 Wg in the cells anterior to it inhibits ser
expression while Hh also inhibits ser
expression in cells posterior to the Hh
expressing cells thus restricting Ser to the
middle of the parasegment. Rho expression
is repressed by Wg signaling and is
positively reinforced through a
combination of Hh and Ser signaling.
 Thus during stage 12, each parasegment is
divided into four domains that express
specific genes that are responsible for the
intra-parasegmental patterning of the
embryo.
 Within each segment the binary decision
between specification of naked cuticle or
denticle cell fates is dependent on the
expression of a transcription factor
encoded by the shaven baby (svb) gene.
Wg signaling specifies naked cuticle by
repressing the expression of svb.
Homeotic selector genes
After the segmental
boundaries are set the pairrule and gap genes interact to
regulate the homeotic
selector genes.
Ultrabithorax
Antennapedia misexpresed
in the head and the thorax
Antennapedia
Loss of expression of Ubx . T3 acquires
identitiy of T2.
Initiation and maintenance of homeotic gene expression patterns
 pair-rule and gap genes interact to regulate the homeotic selector genes
 abdA and AbdB are inhibited by Krϋppel and Hunchback which prevent their expression in
the head and thorax region.
 Antennapedia is activated by particular levels of Hunchback thus only in parasegment 4
which becomes T2.
 The expression domains of homeotic genes are dynamic.
 There is also reciprocal interactions between homeotic genes such as bithorax complex
genes repress the expression of Antennapedia.