Lecture 16 - Drosophila

Question 1

What does Bicoid develop?

Answer 1

Anterior segments (head & thorax)

Question 2

What does Nanos develop?

Answer 2

Posterior segments

Question 3

Describe how the early Drosophila embryo is a Syncytium?

Answer 3

Syncytial blastoderm (fertilized egg) with 8mins per division. There are many nuclei in the cell. The nuclei then migrate to periphery, and cell boundaries start to form. This forms a Cellular Blastoderm - made up of approximately 600 cells.

Question 4

Where are Bicoid mRNA & Nanos mRNA found?

Answer 4

Bicoid mRNA - tethered from anterior pole of unfertilized oocyte

Nanos mRNA - found in posterior pole of unfertilized oocyte

Translation into protein occurs after fertilization.

Question 5

What is Drosophila development dependent on?

Answer 5

Early development is dependent on 4 functional classes of genes:

Axis formation:
- egg-polarity genes

Segment identity:
- Gap genes
- Pair-rule genes
- Segment polarity genes

Question 6

What is the role of segment identity genes?

Answer 6

gradually define sections of the embryo so that eventually developmental decisions can occur

Question 7

Describe how Drosophila segmentation occurs?

Answer 7

HIERACHICAL REGULATION

Gap genes:
- Broad regions
- Head/thorax/abdomen

Pair-rule genes:
- alternating segments

Segment polarity genes:
- define the adult segment plan

Question 8

Describe segmentation mutations

Answer 8

Mutations in Gap genes:
- result in large deletions of the body plan

Mutations in Pair-rule genes:
- results in deletions of ALTERNATIVE segments

Mutations in Segment Polarity:
- effects FEATURES on each segment

Question 9

What is an example of a GAP gene?

Answer 9

Kruppel (expressed in central region) that defines the abdominal segments - mutations leads to deletion of entire segments

Question 10

What is the difference between segment polarity genes and Gap & Pair rule genes?

Answer 10

Mutations don’t lead to loss of whole sections, however do see a change in the pattern.

Question 11

What are examples of Gap genes?

Answer 11

• kruppel (Kr)
• knirps (kni)
• hunchback (hb)
• giant (gt)
• tailess (tll)

Question 12

What are examples of Pair-rule genes?

Answer 12

• hairy (h)
• even-skipped (eve)
• runt (run)
• fushi tarazu (ftz)
• odd-paired (opa)
• odd-skipped (odd)
• sloppy-paired (slp)
• paired (prd)

Question 13

What are examples of Segment polarity genes?

Answer 13

• engrailed (en)
• wingless (wg)
• cubitus interruptusD (ciD)
• hedgehog (hh)
• patched (ptc)

Question 14

How is the BICOID gradient decoded by promoter elements?

Answer 14

Transcriptional activation:
- orthodenticle mRNA (low affinity binding sites)
- hunchback mRNA (high & low affinity binding sites)

Transcriptional repression:
- Caudal protein

Hunchback is a maternal effect gene, however it is also a target of Bicoid, making it a maternal effects gene & GAP gene. Bicoid can regulate its transcription.

Question 15

Describe the genes involved in the dynamic regulatory network defines GAP gene expression

Answer 15

• Bicoid (hunchback)
• Giant
• Kruppel
• Knirps
• Tailless

Question 16

How do Pair-rule genes initiate segmentation?

Answer 16

• even-skipped (eve)
• fushi tarazu (ftz)

Nuclei have now moved towards the periphery of the syncytium, and cellularization has started to occur. Primary pair-rule genes are the genes that are acted upon & then will further initiate patterning processes.

Question 17

How do enhancers determine EVE segment expression?

Answer 17

Distinct regulatory modules determine EVE expression
- EVE regulatory region is large ~20kB
- Regulatory modules determine expression in specific stripe
- Modules contain binding sites for Maternal Effect & GAP proteins.

Question 18

What is found on Stripe 2 module of EVE?

Answer 18

Binding sites:
- Bicoid
- Hunchback
- Giant
- Kruppel

Some TFs will bind & inhibit the regulation of genes

Positive hunchback + bicoid = Eve stripe 2

Question 19

How do primary pair-rule genes regulate secondary genes?

Answer 19

Primary - e.g. even-skipped (eve) expression determined by Maternal Effect & GAP genes

Secondary - e.g. fushi tarzi (ftz) require primary pair-rule genes to define expression

Even-skipped (eve) inhibits fushi tarzu (ftz)

Question 20

What are the segment polarity genes?

Answer 20

Engrailed, Wingless & Hedgehog

Primary region - thick coarse hairs - denticles
Secondary region - smooth region - no hairs
Tertiary region - small, thick hairs
Quaternary region - fine hairs

Question 21

How were segment polarity genes identified?

Answer 21

Morphological differences are clear. It was through this that mutants were identified in the segment polarity genes - engrailed, wingless & hedgehog - mutations lead to loss in distinction in different morphologies

Question 22

How are segment polarity genes controlled?

Answer 22

Controlled by Pair-rule genes

ENGRAILED - posterior expression (activated by EVE & Ftz) ACTIVATES HEDGEHOG

WINGLESS - repressed by EVE & Ftz - Activated by Hedgehog

Genes higher up in the hierarchy - e.g. pair rule & Gap genes dictate where engrailed & wingless are found (specific regions)

Question 23

What are expression patterns dictated by?

Answer 23

Dictated by:
- genes higher up in the hierarchy
- interaction between genes refines the expression domain of genes.

ENGRAILED is always present where there is Ftz & Eve expression. Pair rules are involved in regulation of expression of engrailed

Small domain where eve & ftz not expressed where wingless can be expressed

Question 24

How are Wnt & Hedgehog reinforced?

Answer 24

Wingless & Hedgehog are secreted. Perceived by receptors in neighbouring cells.

Activate signalling cascade leading to:

Hedgehog –> Wingless
Wingless –> Hedgehog/Engrailed

POSITIVE REINFORCEMENT - stable expression boundaries

Engrailed is a transcription factor. Hedgehog is a secreted molecule.

WINGLESS & HEDGEHOG ESTABLISH A TIGHT BAND OF EXPRESSION, AS THEY BOTH INDUCE THE TRANSCRIPTION OF EACH OTHER THROUGH POSITIVE REINFORCEMENT.