Lecture 12 - Drosophila - setting up the body axis

Question 1

What parts of the body are formed through the development of the A/P axis?

Answer 1

• a head, a tail, a thorax and abdominal region are formed
• the thorax & abdomen are SEGMENTED

Question 2

What 4 regions are formed through the development of D/V axis?

Answer 2

• the ventral most mesoderm
• the ventral ectoderm
• dorsal ectoderm
• aminoserosa (an extra embryonic tissue)

Question 3

What is the initial expression of genes that result in the expression of GAP genes that define different regions in the embryo?

Answer 3

initial maternal gradient (maternal genes)

Question 4

Where does the expression of maternal genes occur?

Answer 4

occurs in syncytium - multiple nucleus in one cytoplasm

Question 5

What does the expression of GAP genes lead to?

Answer 5

GAP gene expression leads to the periodic expression of the Pair-Rule gene.

Question 6

What do the GAP genes do?

Answer 6

divide the embryo

Question 7

What does a mutation in GAP genes lead to?

Answer 7

loss of an area in embryo

Question 8

What does the Pair-Rule gene do?

Answer 8

• continue to refine the body - these specify the so called ‘para-segments’ and foreshadow segmentation of the larva

Question 9

What follows the Pair-Rule gene?

Answer 9

the segmentation genes

Question 10

What does the segmentation genes do?

Answer 10

• elaborate patterning within each para-segment
• patterning of these segments happens when the embryo has cellularised.
• so cell-to-cell signaling is essential to coordinate this patterning process

Question 11

What follows segmentation genes?

Answer 11

homeotic seletion genes

Question 12

What do homeotic selector genes do?

Answer 12

determine segment identity

Question 13

Where does the cascade (initiated by maternal genes) occur?

Answer 13

Occurs in syncytium - multiple nuclei in one cytoplasm (NOT IN THE ZYGOTE ITSELF)

Question 14

Where are the maternal genes located?

Answer 14

mother - genes that need to be present in the genome of the mother to provide the initial information

Question 15

How were initial mother genes identified?

Answer 15

identified by specially adapted genetic screens where homozygous mutant mothers were screened for defects in all their progeny

Question 16

What are the 3 relatively independent classes of maternal genes that set up the A/P axis?

Answer 16

• anterior
• posterior
• terminal system

Question 17

How were the 3 classes of maternal genes defined?

Answer 17

these were defined by mutations that led to the corresponding defects either in anterior, posterior or terminal structures in the larva

Question 18

A mutation in what gene leads to a defect in anterior?

Answer 18

Bicoid - an example of a MORPHOGEN

Question 19

What is a morphogen?

Answer 19

a molecule that emanates (originates) from a specific region and is present in a concentration gradient.
- a morphogen should be able to induce more than one fate

Question 20

What type of gradient does Bicoid form?

Answer 20

a protein gradient across the A/P axis of the syncytial embryo

Question 21

Where is RNA of Bicoid localised in the egg?

Answer 21

at the anterior

Question 22

Where is Bicoid transcribed?

Answer 22

transcribed at the anterior and diffuses towards the posterior

Question 23

What is Bicoid?

Answer 23

GENE - transcription factor - switches on different genes at different THRESHOLD CONCENTRATIONS
- Bicoid can only function like a morphogen, because the egg is a syncytium
- UNUSUAL as a morphogen, as it is a transcription factor

Question 24

What gene acts on the anterior?

Answer 24

Bicoid

Question 25

What gene acts on the posterior?

Answer 25

Nanos

Question 26

What is the role of Nanos?

Answer 26

- preventing Hunchback translation in the posterior of the egg

Question 27

Where does Nanos travel from?

Answer 27

- from posterior to anterior (creating posterior gradient) - this create a lot of the anterior & posterior pattern

Question 28

What does the Torso signal do?

Answer 28

specifies the extreme ends of the egg

Question 29

Where is the Torso receptor present?

Answer 29

everywhere on the outside of the egg

Question 30

What binds to the Torso receptor?

Answer 30

Trunk protein

Question 31

What is required to release the ligand 'Trunk Protein', which is attached to the 'Torso receptors'?

Answer 31

the protease 'Torso-like'

Question 32

Where is the protease 'Torso-like' located?

Answer 32

only present at the poles - leading to torso activation only at the poles

Question 33

Describe the basic principles of cell-cell signalling

Answer 33

- ligand can't cross membrane - attaches to receptor - ligand changes structure (depending on signalling pathway) - activation of transcription factor - leads to action in cell

Question 34

What is the receptor on the outside of the egg that determines D/V polarity?

Answer 34

Toll

Question 35

What is the ligand that attaches to Toll, which is involved in creating D/V polarity?

Answer 35

Spatzle

Question 36

What is the localised enzyme which creates an active ligand on ventral side?

Answer 36

Pipe

Question 37

What is Dorsal?

Answer 37

a protein and transcription factor that moves into the nuclei on the ventral side (nuclear localisation)

Question 38

What side does 'Dorsal' the protein act on?

Answer 38

ventral - high levels of Dorsal on ventral side - low levels of Dorsal on dorsal side

Question 39

What does differing levels of Dorsal on the ventral & dorsal side create?

Answer 39

creates a gradient of the Dorsal protein in nuclei

Question 40

How is Dorsal activated?

Answer 40

via nuclear localisation

Question 41

Why is toll activated only on the ventral side?

Answer 41

Toll is activated on ventral side cos PIPE is only found on that side (pipe only on ventral side of egg)

Question 42

What is a summary of the creation of D/V polarity?

Answer 42

PIPE (enzyme) - only ventral side Spatzle (ligand) - everywhere Toll (receptor) - everywhere Dorsal (protein) - nuclei on ventral side Pipe activates spatzle which activates toll which activates Dorsal

Question 43

How does the polarity in the oocyte needed to form A/P axis come about?

Answer 43

polarity of the ovaries is somehow transmitted to the egg - this occurs through nuclear positioning. the side where the nucleus is closer to, is the side where the dorsal side develops

Question 44

What is the one cell-cell singal that is required to set up BOTH anterio-posterior and dorso-ventral axes?

Answer 44

Gurken

Question 45

Where does Gurken come from?

Answer 45

nucleus