Drosophila as a developmental and genetic model

Question 1

Drosophila melanogaster - history and overview

Answer 1

Thomas Hunt Morgan - established Drosophila as genetic model
Won 1933 Nobel Prize
He was the first to show genes are on chromosomes and are basis of heredity and inheritance
Can fly for several hours
Can fly for more then 12 km
Only weigh 1 mg

Question 2

We are more like flies than you think

Answer 2

1980/90s - many genes that control fly development discovered and very similar to genes control development of vertebrates including humans
Flies and humans developing separately for over 700 million years yet still have very similar genes that control development
Evolution reuses and adapts genes and principles rather than invent new ways
Means by understanding developmental principles in flies can learn about our own development

Question 3

Why do we know so much about fly development?

Answer 3

Christiane Nüsslein-Volhard and Eric Wieschaus
Christiane realised Drosophila genetics could be applied to find genes controlling development
Simple but brilliant idea: if the gene is mutated they should lead to developmental defects that illustrate their function
Concept called reverse genetics

Question 4

Forward and reverse genetics

Answer 4

Forward genetics:
- Function know but gene sequence needs to be determined
- Positional identification/cloning
Reverse genetics:
- Gene sequence known but gene function needs to be determined
- Gene knockout

Question 5

Saturation screening

Answer 5

SS - creating mutations in animals to study them, but don’t know which gene it is damaging
To make mutation chemical EMS used, it acts randomly
Treatment adjusted so that an average gene would be destroyed with chance of ~1/500 (done by Christiane and Eric)
E.g. if there was 1 gene specifically required to form head and search 1 line there is only a 0.2% chance of finding (1 in 500)
If you search:
- 10 lines then 2 %
- 100 lines 19%
- 500 lines 64%
- 1000 lines 87.5%
- 2000 lines 98%
If there 5 genes required for making head you’d fine all
Mutated genes need to be homozygous to show
Important part innovation was developing method for keeping and screening eggs from 100s of lines efficiently

Question 6

Mutant screens led to

Answer 6

1. Basic understanding how genes controlling elaboration of body plan
2. Molecular identification of many new genes and biological signalling pathways
3. Confirmed “Genetics” as extremely powerful way dissect biological processes
   Soon after discovery, became clear very similar genes and principles acting in other animals
   These genes not just important during development but often remain important throughout life:
   
   • Homeostasis
   • Cancer
   • Regeneration
   • Aging
     Therefore principles and genes identified and also methods employed drove lot of progress in understanding human disease

Question 7

Drosophila melanogaster life cycle

Answer 7

In total around 9 days
Live for max of 140 days if kept low temperatures
Embryos initially develop as a syncytium (united cell) - important when A/P axis set up
3 hours post fertilisation embryos consist single layer of cells enclosing yolk, containing a few nuclei
At posterior pole cells separated off - these will form germ line

Question 8

Fly segments

Answer 8

Anterior - ante=before
Posterior - post=after
Dorsal - dorsum=back
Ventral - Venter=belly

Ectoderm - Epidermis, Nervous system
Endoderm - Gut
Mesoderm - Muscle

Once a cell has been specified to one of these fates - M,E or E - all descendants will adopt that fate

Question 9

Further development in flies - pupa

Answer 9

Once lavi hatched it becomes a pupa
Different imaginal diskd form different parts in adult fly
During pupation they grow massively and take over pupa

Question 10

Screening stage in flies

Answer 10

Each end of larva has unique structures, each segment has characteristic pattern of denticles
Denticles have particular orientation - allows recognition of AP polarity in each segment
Dorsal side is different from ventral side
Segmentation:
Head - T1 T2 T3 A1 A2 A3 A4 A5 A6 A7 A8 - End

Question 11

Mutations

Answer 11

Can deduce from gap gene mutant that this gene important for creation of segments T2 and T3
Pair-rule gene mutant showed there was one gene responsible for forming every other segment

Question 12

Summary

Answer 12

Drosophila melanogaster was initially employed to understand the basic principles of Mendelian Genetics
Applying such genetic approaches to understand how the fly egg develops into a larva, revolutionised our understanding of developmental processes in general.
Drosophila develops indirectly, it first forms a larvae that grows by moulting 2 times before undergoing pupation to form and adult fly
The egg initially develops as a syncytium: a single “cell” with a cytoplasm the contains a large number of nuclei
After 1 day a larva is formed that has clearly visible body segments (3 thoracic, 8 abdominal)