5.2 Drosophilla Flashcards
Bicoid gene mutants: What happens when you -remove bicoid -add bicoid to antihero end -add bitcoin to middle -add bitcoin to
• If the maternal gene bicoid (bcd-) is deleted, embryo
lacks head or thorax structures, and telson forms at
anterior end of embryo.
• If bicoid mRNA was injected into the anterior end of bcd
mutant eggs, then normal development resumed.
• If bicoid mRNA was injected into the center of the bicoid
deficient egg, head structures formed there, forming a
mirror-image body pattern at site of injection.
• Simplest interpretation is that: the bicoid gene is
necessary to to establish a gradient of bicoid protein
which is highest in anterior end of embryo
•added to posterior, the egg forms 2 heads, one at each end.
How does the gradient of bicoid protein lead to anterior determination?
• Bicoid is a transcription factor, it binds to the regulatory
region of a gap gene called hunchback (hb), and
activates its transcription.
• Hunchback protein is also produced in the anterior 1/3 of
the embryo.
• Mutants of hunchback also produce a phenotype with no
head and thorax, this is because hunchback protein
represses the action of abdominal-specific genes.
anterior-posterior axis formation in Drosophila, the genes
• Bicoid protein promotes transcription of hunchback.
• Nanos protein inhibits hunchback mRNA translation.
•Bicoid prevents the translation of caudal
•opposing caudal and Hb gradients
•Nanos mRNA is localized (tethered) at the posterior
end of the egg during oogenesis: note that nanosmutants
have no abdominal segments
• it is translated after fertilization to give a gradient of
nanos protein with highest concentration at the
posterior end
• nanos suppresses the translation of a maternal hb
mRNA, which is found at a low level throughout the
embryo, thus keeping hunchback protein at a low level at
the posterior end of the embryo
• nanos protein binds to hb mRNA at 3’-untranslated
region via pumilio protein (encoded by gene pumilio)
Mutations in nanos
- Nanos- mutants have no abdominal segments.
* Nanos suppresses the translation of a hb mRNA
Regulation of hunchback translation by nanos protein
ANTERIOR
- pumilio binds to nanos responce element on the Hb mRNA.
- Adenylation and translation into a Hb protein.
- This promotes anterior structures.
POSTERIOR
- Increases Nanos interacts with pumilio
- causes deadenylation, so no translation and no Hb
- allows for abdominal formation
Summary of hb regulation:
both at transcription and translation levels
ANTERIOR
- Bicoid mRNA makes Bicoid protein
- protein activates Hb gene which transcribes into Hb mRNA
- pumilio binds to nanos responce element on the Hb mRNA.
- Adenylation and translation into a Hb protein.
- Transcription of Head and Thorax genes
POSTERIOR
- Nanos mRNA males Nanos protein
- Increases Nanos interacts with pumilio
- causes deadenylation, so no translation and no Hb
- allows for abdominal formation
Formation of the terminal regions of a Drosophila
embryo by torso signalling
- Huckbein (hkb) and tailless (tll) along with bicoid protein form the acron.
- Huckbein (hkb) and tailless (tll) are activated by torso signal at area of low Capicua expression form telson
- Hkb+Tll act of own = forms telson
- Hkb+Tll+bicoid = forms acorn
Hirearchy of genes in drosophila
• Maternal effect genes (bicoid, nanos) control the anteriorposterior,
dorsal-ventral polarity of the embryo.
• Segmentation genes (gap genes, pair rule genes and segment polarity genes) control the number and polarity of segments.
• Homeotic genes control the identity of each segment
Segmentation Genes
Cell fate commitment in Drosophila from specification to determined is mediated by segmentation genes.
• Segmentation genes divide the embryo into
repeating segments.
• Determined by mutants that disrupted the body plan
Mutations in segmentation pattern of Drosophila
Gap genes mutations cause “gaps” in the segmentation.
(A) Blocks of segments are missing.
Pair-rule gene defines pattern and also determine the number of segments. mutations in pair-rule genes results in half the normal number of segments.
(B) Parts of alternating segments are missing.
Segment polarity gene
(C) Regions of each segments are missing
Major genes affecting segmentation pattern in
Drosophila
Gap Genes:
- hunchback (hb)
- tailess (yll)
Pair Rule Genes Primary:
- hairy (h)
- runt (run)
Pair Rule Genes Secondary:
- fushi tarzan (ftu
- odd paired
Segment Polarity Genes:
- engraled (en)
- wingless (eg)
Parasegments and the different segments
A parasegment includes the posterior region of one
segment and the anterior region of the immediately
posterior segment. there’s 14
Segments: Ma, Mx, Lb, T1, T2, T3, A1, A2, A3, A4, A5, A6, A7, A8
Model for transcription of Segment Polarity Genes
- One row of cells each produce wingless (wg) and engrailed (en). En is expressed if either Ftz or eve proteins are present.
- The cells that express en also express hedgehog.
- Wg is expressed when there is neither Ftz or eve proteins present.
Hedgehog signaling: Drosophila early development
- in absence of Hh binding to patched, Ci is weathered to mtubules by Cos2 and fused proteins.
- allows PKA and Slimb portions cleave Ci into a transcriptional repressor that blocks transcription of particular genes
- when Hh binds to patched, conformational changes releasing inhibition of smoothened protein. •Smoothened then releases Ci from m.tubules (P Cos2 and Fused)
- Inactivates cleavage proteins PKA and Slimb
- Ci origin enters nucleus binds CBP proteins and acts as a transcriptional actuator of a particular gene.
Paracrine regulation of wingless and hedgehog
transcription
- Reciprocal interactions between neighboring cells:
- Cells secreting hedgehog activate the transcription of wg.
- wg binds to frizzled, activates Dsh inhibts GSK which activates B-cat which activated Hh
- Cells secreting wg activate the transcription of en and hedgehog.
