Lecture 7- Development of the Drosophila body plan

Question 1

Briefly outline the segmentation of Drosophila

Answer 1

1. Segmentation occurs very quickly from the unpatterned egg and further grooves develop between segments.
2. Segmentation is easily observed in drosophila larvae where they have 8 clear abdominal sections
3. Additional segments in the head and thorax
4. Each thoracic segment has a pair of legs attached to it

Question 2

Why is complementation testing completed?

Answer 2

To determine if two mutations associated with a specific phenotype represent two different forms of the same genes (alleles) or are variations of two different genes

Question 3

What occurs if a mutation occurs in the same genes in complementation testing?

Answer 3

Phenotype was observed due to failure to complement

Question 4

What occurs if a mutation occurs in different genes in complementation testing?

Answer 4

No phenotype was observed as complementation occurred

Question 5

What were the 3 major classes of phenotype discovered in the drosophila complementation testing?

Answer 5

1. Knirps
2. Paired
3. Goosebury

Question 6

Describe the Knirps phenotype and the type of gene that causes it

Answer 6

• Normal head and tail but only one abdominal stripe
• Large middle chunk is missing
• Type of gap gene

Question 7

Describe the Paired phenotype and the type of gene that causes it

Answer 7

• Every second segment pair is missing

* Type of pair-rule gene

Question 8

Describe the Gooseberry phenotype and the type of gene that causes it

Answer 8

• Naked cuticle which goes between the denticle belts is missing
• Belts are fused together
• Polarity between the segments has been destroyed
• Type of segment polarity gene

Question 9

Outline the order of genes involved in segmentation

Answer 9

1. Maternal genes
2. Gap genes
3. Pair rule genes
4. Segment polarity genes (however these do not directly feed into the process)
5. Selector genes

Question 10

Give examples of 3 maternal genes, there location and what happens if mutated

Answer 10

1. Bicoid: located anteriorly so anterior structures are missing when mutated
2. Nanos: located posteriorly so tip remains but most posterior structures are missing when mutated
3. Torso: located in the middle, when mutated anterior and posterior are missing by torso remains

Question 11

Describe the expression of Bicoid

Answer 11

• Bicoid is a DNA binding transcriptional activator which is maternally loaded into the developing oocyte
• Bicoid mRNA is deposited into the anterior part of the embryo by the nurse cells
• When the egg is laid, bicoid mRNA is translated into protein which diffuses throughout the syncytial blastoderm

Question 12

Describe the phenotype when an anterior cytosol from a wild type egg is transplanted to the anterior end of a bicoid mutant egg

Answer 12

The anterior structures are partially rescued

Question 13

Describe the phenotype when an anterior cytosol from a wild type egg is transplanted into the middle of a bicoid mutant egg

Answer 13

Ectopic head structures are formed and a mirror image of the thoracic segment forms either side of the central head

Question 14

What happens to the drosophila segmentation pattern when there are 0 gene copies of bicoid?

Answer 14

No bicoid protein so the patterning has been pushed to the anterior side

Question 15

What happens to the drosophila segmentation pattern when there is 1 gene copy/wiltype of bicoid?

Answer 15

Clear gradient of bicoid protein so segmentation patterning is normal

Question 16

What happens to the drosophila segmentation pattern when there are 4 gene copies/overexpression of bicoid?

Answer 16

More bicoid protein anteriorly so the bicoid gradient extends further. The segmentation patterning is pushed more posteriorly and the high levels of bicoid anteriorly inhibits the differentiation/formation of head structures

Question 17

How are low and high affinity binding sites activated by different concentrations of bicoid?

Answer 17

High affinity binding sites: activated at lower thresholds of biocoid

Low affinity binding sites: activated/require high concentrations of bicoid

Question 18

Describe the expression of gap genes

Answer 18

• Expression of gap genes is dependant on the maternal genes

* Gap genes read maternal gene gradients to define broad blocks/domains of gene expression

Question 19

Describe the expression of pair rule genes

Answer 19

• Expression of pair rules genes is controlled stripe by stripe
• Each stripe is controlled by a complex system (e.g bicoid/giant/hunchback/kruppel) which work together to produce a singe stripe of gene expression
• Expression is dependant on the interaction of positively and negatively acting acting transcriptional regulators (e.g gap genes)

Question 20

Describe the expression of segment polarity genes

Answer 20

• Repeated pattern where each half of each segment is deleted and a mirror-image is duplicated and reversed to replace the missing half segment

Question 21

Describe the expression of Wnt and Hh as an example of segment polarity genes

Answer 21

1. Both Wnt and Hh cause loss of naked cuticle and replaced with hairy cuticle
2. Hh and Wnt feedback to each other to maintain each others expression and refine segment borders
3. In the posterior end, Hh is on and is secreted and diffuses to the new cells anterior to it
4. Hh causes the expression of Wnt. Wnt is secreted and diffuses backwards to the more posterior cell to maintain Hh
5. Hh maintains Wnt expression which surporsess denticle development
6. Wnt diffuses anteriorly, where there is Wnt there is naked cuticle, as the distance away increases, hairs start to develop and get bigger

Question 22

Which genes pass information and enable the expression of hox/selector genes?

Answer 22

Gap genes and pair rule genes

Question 23

Describe the expression of hox/selector genes

Answer 23

• Provide the ‘who am I’ information to each segment
• Expression of hox genes along the A/P body axis occurs in the same order as genes that are within the genome
• Encode for DNA binding TFs

Question 24

Is drosophila a long or short germ band insect and why?

Answer 24

• Long germ band as all 14 segments are defined at once

* Provides an explanation for why drosophila embryogenesis occurs so quickly

Question 25

Describe the development of short/intermediate germ band insects

Answer 25

• Start with head and thoracic segments
• Add abdominal segments sequentially- posterior disc (proctodaeum) appears to bud off segments as it gets smaller
• Moderate complexity and not too slow
• Segments not added at once, instead are added over time
• As evolution advances, less reliance of segmentation clock

Question 26

Describe the segment addition in Strigamia maritima

Answer 26

• Notch activation causes down regulation of notch ligand
• Adjacent striped of Delta and Her1 set up feedback loop necessary for oscillation
• Time lag in response causes oscillation between strong and weak signalling levels
• Propagation of signal between cells causes wave of activation
• This pattern is due to a segmentation clock…

Question 27

What 2 molecule rings surround the proctodaem

Answer 27

Delta (ligand) and notch (receptor)

Question 28

Describe the Notch/Her1 segmentation clock

Answer 28

1. Notch is activated and causes the expression of pathway target genes (e.g Her)
2. Her1 protein is made which switches off its own transcription and the transcription of delta
3. Therefore the level of delta/ligand presented to this cell goes down
4. The level of activation of neighbouring cell go down
5. This mean there is less Her made so no inhibition of delta
6. Which means the delta goes up, which then re-stimulates the notch again
7. And this pathway recycles like a clock

Question 29

Where do the majority of candidate pacemaker genes lie?

Answer 29

In the Notch pathway

Question 30

Explain the evolution of segmentation

Answer 30

• Notch segmentation clock is evolutionarily conceived in vertebrates and short germ band insects
• This strongly suggests that segmentation must have evolved early on in evolution