drosophila as a model organism

7/30/2019 drosophila as a model organism

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Drosophila as a model system

Paul Adler

[email protected]

Gilmer245

982-5475


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Why is Drosophila a valuable

model system? It is an animaltherefore it can be used to

study development, physiology and

behavior. Many genes only have functionsin multicellular organism e.g. cadherins.

Drosophila has been a particularly valuable

model system for development. 90 years of genetics


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Features shared by Drosophila and

other animals and higher plants:

Obligate diploid.

Sexually dimorphic gametes.

Pleiotropy and redundancy.


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Homework

Go to FlyBase and learn about cadherins in

flies.


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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


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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 giveDrosophila its wonderful cytogenetics.


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Polytene Chromosomes

A consequence of lack of cell division inlarval life (2000N).

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

5200 kb).

Great cytologyin favorable regions canrecognize a 15 kb deletion.

Uneven Amplification


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Cytogenetics

Chromosomes divided into 102 numbered

divisions each of which is divided into

lettered subdivisions.

Individual bands are numbered within each

lettered subdivision


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Cytogenetics

X 1-20

2L 21 - 40

2R 41 - 60

3L 61 - 80

3R

81 - 100 4 101 - 102


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X

2

3

4

1 20

21 40 41 60

61 80 81 100

101-102

L

L

R

R


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Polytene Chromosomes

Identifying Chromosome Aberrations

Mapping physical location of mutations and

genes.

Substrate for nucleic acid and antibody

probes


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Chromosome aberrations

Pairing of maternal and paternally derived

homologs a big help

Deficiency (Df) (known as a deletion inother organisms).

Duplications.

Inversions.

Translocations.


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Df

How can you tell if you have a mutation

that is a deletion?

Molecular mapping

Failure to recombine with two point

mutants

Cytologyloop in meiotic or polytene

chromosomes.


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1 2 5 6 7 8 9 10

1 2 5 6 7 8 9 10

3 4

Df/+


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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

Paracentric Inversion

Pericentric Inversion


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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

7

8 9 10


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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


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How to maintain a lethal ?

Retest every generation?

Balanced lethal state l1 +/+ l2X 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


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L1 +

L1 +

L1 +

+ L2

+ L2

+ L2

L1 +

+ L2

L1 +

+ L2

L1 +

+ L2X


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L1 +

+ L2

L1 +

+ L2

L1 L2

+ +


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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 + +


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Balancer Chromosomes

1. Multiply inverted

2. Dominant and recessive markers

3. Recessive lethal (sterile)

FM7a, y sc w v B

CyO, Cy dp pr cn

SM6, al Cy dp cn sp

Tm3, ri p sep bx Sb e

TM6B, Hu Tb e

3 1 d 8 a

l v l 2

2 l v l 2 2

p 3 4 e


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Mutations and Nomenclature

Wild type often not stated.

Semicolon between chromosomes

Descriptive and humorous names.

Dominants are capitalized.

Allele names superscripts


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y w f

y w f; cn bw

y w f; TM3/DcxF

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

Dr/TM3


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Morphs

Loss of function

hypomorphs - leaky, weak

amorphs - phenotypic nulls, tight, strong

null - no gene product


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Gain of Function

Hypermorph - extra activity

Neomorph - new activity

antimorph - dominant negative


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Mutation Nomenclature in Drosophila

Loss of function:

Amorphicnull

m/m = m/Df

Hypomorphicsome activity remains

m/m < m/Df


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Gain of function

Hypermorphic (increased activity) m/m>m/+>m/Df

Neomorphic (new activity)

Antimorphic (dominant negative)

m/+ >Df/+

m/+>m/Dp


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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.


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Resources

Sequence well annotated.

Genome project cDNA clone collection.

Expression patterns in embryos.

Deletion collection.

drosophila as a model organism

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