Lecture 8 - Invertebrate Early Development II: The Drosophila body plan or Segmentation Flashcards
(53 cards)
How does patterning occur?
patterning happens in a series of steps
What is segmentation?
An ancient & conserved way of building bodies
- segmentation is like building blocks of bodies and a segment is a repeating structure. A common group of repeating structures can then develop into legs etc.
Describe patterning of insects
Insects - like vertebrates are bilaterally symmetrical (two distinct & largely independent axes)
- A/P & D/V
Broad building blocks are symmetrical - e.g. arms & legs on each side
How is patterning already set up in the egg?
Fertilized egg (cleavage) - syncytial blastoderm (gastrulation) - embryo (hatching - 1st,2nd,3rd instar (Larva), Pupa (metamorphosis) - adult fly
How were almost all genes known to be involved in setting up the body axis identified?
in the Heidleberg screen
What did Heidelberg screen identify?
Both maternal & zygotic screens required for body patterning.
- researchers undertook a saturation mutagenesis to identify genes involved in the development & patterning of the larval cuticle
30,000 lines established & tested - 139 genes
What can you tell if an egg was dying?
A gene that is essential for development must’ve been hit
What are the 3 types of mutations that can be seen on a gene?
Mutations A - identical to another mutation A
Mutation B - same gene, different place
Mutation C - different gene
Each mutation alone will be lethal - we only have 2 copies of each gene.
2 lethal mutations on the same gene leads to death, but 2 lethal mutations in different genes doesn’t lead to death
What does a Mutation A & Mutation A give?
Phenotype (disease) - fail to complement
What does a Mutation A & Mutation B give?
Phenotype (disease) - fail to complement
What does a Mutation B & Mutation C give?
No Phenotype (no disease) - complement
What does a Mutation C & Mutation A give?
No Phenotype (no disease) - complement
Describe maternal vs zygotic genes
- Mother puts RNA proteins into egg, before sperm entry. Genes that are put in are called MATERNAL GENES, while genes required in the embryo for lifetime are ZYGOTIC GENES.
Healthy mother no mutations - always deliver normal maternal contribution to embryo and any processes that require this will always happen as normal.
Mutations in mother and zygotes have both been screened
Describe maternal contribution vs zygotic genes
Many phenotypes seen can be put into different classes of mutants:
Gap genes (e.g. knirps)
Pair rule genes (e.g. paired)
Segment polarity genes (e.g. gooseberry)
Where is affected in a bicoid mutant?
Anterior - get 2 ends (no head)
Where is affected in a nanos mutant?
Posterior (lacks whole middle of embryo - much shorter embryo)
Where is affected in a torso mutant?
Terminal (lack head and back end - only torso)
How do maternal genes lay down the co-ordinates?
Maternal genes put in by the mother, and polarized in the egg, dictate what the ends of the embryo will be - alongside head & tail.
What is Bicoid?
Bicoid is a maternal gene which determines anterior.
Bicoid is a DNA binding transcriptional activator - maternally loaded into developing oocyte (anterior)
What is a classic experiment that shows that Bicoid acts as a morphogen?
- pricking of front of egg and some cytoplasm leaked out, same phenotype as Bicoid mutant
- concluded that whatever was making anterior of egg was in the cytoplasm.
- if you take a Bicoid mutant egg & cytoplasm of normal egg - what happens? Found that cytoplasm was not just required for anterior, but also sufficient to rescue the loss of head & anterior structures found in Bicoid mutants - PARTIAL RESCUE
Steps in different experiment:
- normal development
- removal of cytoplasm in the front lead to loss of anterior structures
- partial rescue
- ectopic head structures and mirror imagine thoracic segments (if placed in the middle)
How can Bicoid morphogen activity be linked to segmentation?
An embryo with no Bicoid and stains for abdominal segments, move up in the embryo and lose where anterior part of egg should be.
1 gene copy of Bicoid leads to some rescue of anterior & abdominal sections pushed back.
4 gene copy of Bicoid leads to expansion of anterior region and pushing back of posterior regions.
THIS REINFORCED THE IDEA THAT BICOID IS ACTING IN A GRADIENT
How does Bicoid function as a DNA binding transcription factor?
Functions via differential binding affinity in its target genes - some of its targets have weak affinity for Bicoid
Weak (low) affinity binding sites - requires lots of Bicoid (high concentration) to be activated - only activated in most anterior part where there are high levels of Bicoid
Strong (high) affinity binding sites - requires small amounts of Bicoid (low concentration) - it will be marked up and genes will get activated.
In the more middle regions of the embryo - small amounts of Bicoid - these genes are activated
What are the threshold concentrations of Bicoid needed at different affinity binding sites?
High affinity binding sites - activated at lower threshold concentrations of Bicoid
Low affinity binding sites - need high concentration of Bicoid to be activated
What are Gap genes activated by?
(largely) activated by the maternal genes , but also activate each other.
