L7 - Drosophila Body Plan

Question 1

What is the Drosophila adult segmentation pattern?

Answer 1

Pattern of pigmentation on the abdomen and thorax

Each thoracic segment has a pair of legs associated with it

Question 2

What is the Drosophila embryo segmentation pattern?

Answer 2

Denticle belts in embryo

• Small hairs that emerge from the otherwise naked cuticle
• Help larva crawl too food

Question 3

What is the history of segmentation?

Answer 3

Segmentation is an ancient and conserved way of building bodies
Go from an unpatterned egg to a patterned egg in 24 hours

Question 4

What were the results of the 1980 mutagenesis screen carried out by Nusslein-Volhard and Wischaus?

Answer 4

Total lines established – 27,000
Lethal mutations – 18,000
Mutations causing embryonic lethality - 4,000
Mutations causing embryonic phenotypes – 600
Complementation groups – 139

Question 5

Complementation analysis -If AxA = mutant phenotype

If BxB = mutant phenotype

Answer 5

AxB if in same gene = mutant phenotype

Fail to complement

Question 6

Complementation analysis -If BxC = no phenotype

If AxC = no phenotype

Answer 6

Complement - if different genes then
B mutant is over a wildtype on the opposite chromosome
C mutant is over a wildtype on the opposite chromosome

Question 7

What were the results of the complementation analysis?

Answer 7

A and B are mutations of same gene

C is different from A and B

Question 8

What is saturation?

Answer 8

Hitting the genome often enough to ensure we are not finding any new mutations

Question 9

What were the 3 different genes discovered in the mutagenesis experiments?

Answer 9

Didn’t find consistent defects – range of phenotypes
Gap genes
Pair rule gene
Segment polarity genes

Question 10

What are gap genes?

Answer 10

E.g. knirps

Question 11

What are pair rule genes?

Answer 11

E.g. paired

Missing the paired segments

Question 12

What are segment polarity genes?

Answer 12

E.g. gooseberry

Lose the naked cuticle in between the hairs

Question 13

What are maternal genes?

Answer 13

Bicoid – anterior

Nanos – posterior

Question 14

What is the order of the hierarchy of genes that progressively define the egg?

Answer 14

Maternal genes define gap genes
Gap genes define pair rule genes
Segment polarity genes control the fine details

Question 15

What is Bicoid?

Answer 15

A DNA binding transcriptional activator

Question 16

What are the two bicoid binding sites?

Answer 16

High affinity binding sites – activated at lower concentrations of bicoid
Low affinity binding sites – activated at high concentrations of bicoid

Question 17

How does Bicoid enter the embryo?

Answer 17

Maternally loaded into developing oocyte

Question 18

What are the characteristics of the bicoid mutant?

Answer 18

Lose head structures

Lose thoracic structures

Question 19

What was the first experiment used to identify bicoid mutant?

Answer 19

Transplantation of cytoplasm from anterior end into bicoid mutant
- Partial rescue

Question 20

What was the second experiment used to identify bicoid mutant?

Answer 20

Took donor cytoplasm and transplanted into middle of bicoid mutant

• Head like structures in the middle
• Mirror image thoracic segments

Question 21

What is the expression pattern of pair rule genes in egg with 1 gene copy of bicoid?

Answer 21

7 stripes

Equal gaps at posterior and anterior end

Question 22

What is the expression pattern of pair rule genes in egg with 0 gene copies of bicoid?

Answer 22

5 stripes

Fairly equal spacing

Question 23

What is the expression pattern of pair rule genes in egg with 4 gene copies of bicoid?

Answer 23

7 stripes

Large gap at anterior end

Question 24

How do gap genes show a readout of bicoid gradient? - e.g. Kruppel

Answer 24

Bicoid above threshold – Kruppel not on
Bicoid below threshold – Kruppel not on
Bicoid at standard – Kruppel on
You get a stripe effect of gap genes

Question 25

Pair rule genes - transcriptional repressors example

Answer 25

Anterior and posterior ends E.g. Kruppel and giant Switch off gene expression of target gene

Question 26

Pair rule genes - transcriptional activators example

Answer 26

In the middle E.g. bicoid and hunchback Switch on gene expression of target gene

Question 27

Overview of pair rule gene control

Answer 27

Expression of pair rule genes is controlled stripe by stripe Dependent on the interaction of positively and negatively acting transcriptional regulators Many of which are gap genes

Question 28

What are segment polarity genes?

Answer 28

Parasegments and segments are half a segment out of place | Parasegments are where the gene expression barriers are

Question 29

Where are Hh and Wg expressed?

Answer 29

Hh maintains Wg expression | Wg is expressed in a cell which directly neighbours a cell expressing Hh

Question 30

What is the role of Wg?

Answer 30

Forms a morphogen gradient which is asymmetric - Further anterior than posterior Wg suppresses denticle development

Question 31

What are selector genes?

Answer 31

Hox genes

Question 32

Where are selector genes expressed?

Answer 32

Expression of hox genes along A/P body axis occurs in the same order as the genes are within the genome

Question 33

What controls the expression of selector genes?

Answer 33

A combination of gap and pair-rule genes

Question 34

What does the homeobox contain?

Answer 34

DNA binding transcription factors

Question 35

What is an example of a long germ band insect?

Answer 35

Drosophila - All 14 segments are defined at once - Quick - embryogenesis complete in just 24 hours - Complicated - maternal, gap and pair rule genes interact for every segment

Question 36

How do short and intermediate germ band insects develop?

Answer 36

Start with head and thoracic segments - Via an ancestral version of the system Drosophila uses Abdominal segments added sequentially - Posterior disc (proctodeum) bud off segments as it gets smaller Moderate complexity Not too slow

Question 37

Segment addition in Strigamia maritima

Answer 37

Adjacent stripes of Delta and Hes4 set up feedback loop necessary for oscillation

Question 38

Segmentation clock - feedback loops

Answer 38

Notch activation causes down regulation of Notch ligand Time lag in response causes oscillation between strong and weak signalling levels Propagation of signal between cells causes wave of activation

Question 39

How do Strigamia get their segmentation?

Answer 39

Use segmentation clock

Question 40

How do Tribloium get their segmentation?

Answer 40

Use segmentation clock and a few gap genes

Question 41

How do Drosophila get their segmentation?

Answer 41

Segmentation all occurs at once using many gap genes

Question 42

Segmentation in vertebrates

Answer 42

The majority of known candidate pacemaker genes lie in the Notch pathway