Lecture 7 Development of a body plan

Question 1

Segmentation is an … and…. way of building bodies

Answer 1

ancient

conserved

Question 2

Define saturation mutagenesis

Answer 2

‘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

Question 3

What was identified in saturation mutagenesis x5

Answer 3

• 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

Question 4

Why were most mutation not embryonic lethal

Answer 4

Most are lethal later as they are able to survive off maternal contributions e.g. RNA in the first few hours

Question 5

Define a complementation group

Answer 5

• 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

Question 6

What occurs if the mutation is in different genes

Answer 6

Complement

No phenotype

Question 7

What occurs if mutation is in the same genes

Answer 7

No complementation

Phenotype

Question 8

How is bicoid loaded

Answer 8

Maternally loaded into the developing oocyte

Question 9

Name 3 maternal genes and their location

Answer 9

Bicoid - anterior
Torso - middle
Nanos - posterior

Question 10

When there are 0 copies of the bicoid gene what occurs to the segmentation pattern

Answer 10

• 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

Question 11

Explain what happens if bicoid is overexpressed

Answer 11

• 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)

Question 12

What are gap genes

Answer 12

Gap genes read the maternal gene gradients to define broad ‘blocks’ or domains of gene expression

Question 13

How is the expression of pair rule genes controlled

Answer 13

stripe by stripe

Question 14

What are HOX genes controlled by

Answer 14

A combination of gap and pair-rule genes

Question 15

Mutation in antennapedia complex

Answer 15

mutation leads to legs instead of antenna (segment is the same but the combination of selector genes is wrong)

Question 16

Describe long germ band insects

Answer 16

• 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

Question 17

Describe short and intermediate germ band insects

Answer 17

• 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

Question 18

Describe how delta signals via Notch

Answer 18

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

Question 19

Describe feedback loop segmentation clock

Answer 19

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