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
20
1
L
2
R
L
3
4
61
60
41
40
21
R
80
81
101-102
100
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
4
6
7
8
9
10
3
Df/+
1
1
2
5
3
4
4
3
5
2
6
6
7
7
8
8
9
10
9
10
11
11
Parac entric Inversion
1
8
7
6
5
4
3
2
9
Peric entric Inversion
10
11
1
2
3
4
5
6
7
8
9
10
1
2
7
6
5
4
3
8
9
10
5
6
4
1
2
3
7
8
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
+
+
L2
L1
+
+
L2
L1
+
+
L2
X
L1
L1
L1
+
+
L2
+
+
+
L2
+
L2
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 + +
Balanc er Chromosom es
1. Multiply inverted
2. Dom inant and rec essive m arkers
3. Rec essive lethal (sterile)
3 1 d
FM7a, y
8
a
sc w v B
lv l
2
CyO, Cy dp pr cn
2
lv l
2
SM6, al Cy dp cn sp
p
3 4 e
Tm3, ri p sep bx
TM6B, Hu Tb e
2
Sb e
Mutations and Nomenclature
•
•
•
•
•
Wild type often not stated.
Semicolon between chromosomes
Descriptive and humorous names.
Dominants are capitalized.
Allele names superscripts
ywf
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.