Lecture 7 Development of a body plan
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Segmentation is an … and…. way of building bodies
ancient conserved
Define saturation mutagenesis
‘induction + recovery of large no. of mutations in 1 area of the genome or in 1 biological function to identify all the genes in that area or affecting that function’. You statistically hit the genes many times so you can be statistically confident that there are no more genes – as these would have also been hit
What was identified in saturation mutagenesis x5
• 26,978 lines (with mutations in them) • 18,136 lethal mutations • 4,332 mutations causing embryonic lethality • 580 mutations caused embryonic phenotypes • 139 complementation groups (genes) were identified
Why were most mutation not embryonic lethal
Most are lethal later as they are able to survive off maternal contributions e.g. RNA in the first few hours
Define a complementation group
• A complementation group is a collection of mutant alleles that fail to complement and restore the WT when tested in all pair-wise combinations. The mutation will be in the same gene. Crossing 2 species will form a heterozygous mutant that will show the phenotype
What occurs if the mutation is in different genes
Complement No phenotype
What occurs if mutation is in the same genes
No complementation Phenotype
How is bicoid loaded
Maternally loaded into the developing oocyte
Name 3 maternal genes and their location
Bicoid - anterior Torso - middle Nanos - posterior
When there are 0 copies of the bicoid gene what occurs to the segmentation pattern
• 5 stripes instead of 7 which have been pushed forwards - This corresponds with the loss of A head structures as the 1st 2 stripes have gone
Explain what happens if bicoid is overexpressed
• Overexpressing bicoid • 7 stripes still form but pushed more posterior i.e. there is a large gap between A tip and 1st pair rule gene • Because bicoid is now too high at the extreme anterior to aloe for dev of head structures (jack Q wrong here)
What are gap genes
Gap genes read the maternal gene gradients to define broad ‘blocks’ or domains of gene expression
How is the expression of pair rule genes controlled
stripe by stripe
What are HOX genes controlled by
A combination of gap and pair-rule genes
Mutation in antennapedia complex
mutation leads to legs instead of antenna (segment is the same but the combination of selector genes is wrong)
Describe long germ band insects
• Drosophila is a ‘long germ band’ insect i.e. all 14 segments are defined at once • Quick- embryogenesis complete in just 24 hours • Complicated- maternal, gap, pair rule genes interact for every segment
Describe short and intermediate germ band insects
• Start with head and thoracic segments - probably via an ancestral version of the system drosophila now uses • Add abdominal segments sequentially - posterior disc (proctodeum) appears to bud off segments as it gets smaller • Moderate complexity and not too slow • Likely to represent the original ‘ancestral’ segmentation mechanism
Describe how delta signals via Notch
Delta ligand activates the notch receptor Short range signal Immediately adjacent See rings of delta and notch activation and a series of zones of notch activation
Describe feedback loop segmentation clock
Notch activation causes down regulation of notch ligand. This is because Notch activation switches on the expression of Her/Hes (inhibitor) which binds to the promotor of the delta gene to switch it off and so downregulate delta ligand. This means that this cell stops making signal for neighbour so stops seeing activation and so stops making the repressor and so delta comes back on again and Notch is turned back on. This cycle repeats