Drosophila as a model system

• Paul Adler

• pna@virginia.edu

• Gilmer245

• 982-5475

Why is Drosophila a valuable model system?

• It is an animal – therefore it can be used to study development, physiology and behavior. Many genes only have functions in multicellular organism e.g. cadherins. Drosophila has been a particularly valuable model system for development.

• 90 years of genetics

Features shared by Drosophila and other animals and higher plants:

Obligate diploid.

Sexually dimorphic gametes.

Pleiotropy and redundancy.

Goals for my lectures

• Understand Drosophila well so that you can understand a paper or seminar.

• Hopefully you will be comfortable enough so that you are likely to keep up with the fly literature on problems and approaches that are relevant for your research.

• Drosophila has very sophisticated classical genetics and cytogenetics. These topics are often ignored these days, but they remain important in biomedical research.

• Because of their sophistication and power they are essential for fly genetics.

Homework

• Go to FlyBase and learn about cadherins in flies.

The Drosophila Genome

• 3 sets of autosomes – 2 and 3 - large metacentric chromosome– 4 - very small telocentric chromosome

• X/Y sex Chromosomes– X is a large telocentric chromosome

Unusual Features of Drosophila

• No crossing over in male meiosis

• larval cells (e.g. salivary gland cells) do not grow by mitotic cell division– they increase in size and become polyploid– the many chromosome strands line up to form

the giant polytene chromosomes that give Drosophila it’s wonderful cytogenetics.

Polytene Chromosomes

• A consequence of lack of cell division in larval life (2000N).

• DNA strands line up in register• Giant chromosomes, banding pattern (bands

5 – 200 kb).• Great cytology – in favorable regions can

recognize a 15 kb deletion.• Uneven Amplification

Cytogenetics

• Chromosomes divided into 102 numbered divisions each of which is divided into lettered subdivisions.

• Individual bands are numbered within each lettered subdivision

Cytogenetics

• X 1-20

• 2L 21 - 40

• 2R 41 - 60

• 3L 61 - 80

• 3R 81 - 100

• 4 101 - 102

X

2

3

4

1 20

21 40 41 60

61 80 81 100

101-102

L

L

R

R

Polytene Chromosomes

• Identifying Chromosome Aberrations

• Mapping physical location of mutations and genes.

• Substrate for nucleic acid and antibody probes

Chromosome aberrations

• Pairing of maternal and paternally derived homologs a big help

• Deficiency (Df) (known as a deletion in other organisms).

• Duplications.

• Inversions.

• Translocations.

Df

• How can you tell if you have a mutation that is a deletion?

• Molecular mapping

• Failure to recombine with two point mutants

• Cytology – loop in meiotic or polytene chromosomes.

1 2 5 6 7 8 9 10

1 2 5 6 7 8 9 103 4

Df/+

1 5 4 3 2 6 7 8 9 10 11

1 8 7 6 5 4 3 2 9 10 11

1 2 3 4 5 6 7 8 9 10 11

Pa ra c e ntric Inve rsio n

Pe ric e ntric Inve rsio n

1 2 3 4 5 6 7 8 9 10

1 2 7 6 5 4 3 8 9 10

1 2 3

4

56

78 9 10

Sex determination

• Males X/Y, 2A

• Females X/X, 2A

• Y chromosome is not male determining– X/0, 2A is a sterile males– X/X/Y, 2A is a fertile female– ratio of X to autosomes determines sex– Y chromosome is needed for male fertility

How to maintain a lethal ?

• Retest every generation?

• Balanced lethal state l1 +/+ l2 X l1 +/+ l2

• If no crossing over you would get l1 +/+ l2, l1 +/ l1 + (die), + l2/+ l2 (die)

• Problem is that crossing over generates + + chromosomes and these ruin the scheme

L1 +

L1 +L1 +

+ L2

+ L2

+ L2

L1 +

+ L2

L1 +

+ L2

L1 +

+ L2X

L1 +

+ L2

L1 +

+ L2

L1 L2

+ +

How to maintain a lethal?

• Balancer chromosomes to the rescue.

• + l2 /CyO X + l2 /CyO this cross yields + l2 /CyO, + l2/+ l2 (die), CyO/CyO (die)

• CyO prevents crossing over so no + +

Ba la nc e r C hro m o so m e s

1. M ultip ly inve rte d2. Do m ina nt a nd re c e ssive m a rke rs3. Re c e ssive le tha l (ste rile )

FM 7a , y sc w v B

C yO , C y d p p r c n

SM 6, a l C y d p c n sp

Tm 3, ri p se p b x Sb e

TM 6B, Hu Tb e

3 1 d 8 a

l v l 2

2 l v l 2 2

p 3 4 e

Mutations and Nomenclature

• Wild type often not stated.

• Semicolon between chromosomes

• Descriptive and humorous names.

• Dominants are capitalized.

• Allele names superscripts

y w f

y w f; cn bw

y w f; TM3/DcxF

y w f; In(3L)fzK21/TM6C

Dr/TM3

Morphs

• Loss of function

• hypomorphs - leaky, weak

• amorphs - phenotypic nulls, tight, strong

• null - no gene product

Gain of Function

• Hypermorph - extra activity

• Neomorph - new activity

• antimorph - dominant negative

Mutation Nomenclature in Drosophila

Loss of function:

Amorphic – null

m/m = m/Df

Hypomorphic – some activity remains

m/m < m/Df

• Gain of function

• Hypermorphic (increased activity)

• m/m>m/+>m/Df

• Neomorphic (new activity)

• Antimorphic (dominant negative)

• m/+ >Df/+

• m/+>m/Dp

Fly Resources

1. Flybase (http://flybase.bio.indiana.edu/)

2. Genome Project (http://www.fruitfly.org)

3. Allied databases (e.g. Interactive Fly – there are links for all of these on Flybase)

4. Stock Center.

Resources

• Sequence well annotated.

• Genome project cDNA clone collection.

• Expression patterns in embryos.

• Deletion collection.