genetic model organisms

Genetic Model Organisms

worm mouse

fish

yeast

fruit flyweed

Drosophila melanogasterGenetics and Developmental BiologyPhysiology and Behavior

Many disease-causing genes in humans have corresponding homologues in the fly genome

CancerNeurodegenerative diseaseDrug addictionsDiabetesObesity

Life cycle of Drosophila

• embryogenesis • three larval stages • a pupal stage • the adult stage

Groups of cells called imaginal discs are set aside at specific sites in the larval body. From these the various body parts develop during pupation (adult muscle, the nervous system, etc).

• A relatively short life cycle (10 days)• Culturing flies is cost-effective (vs mice)• A variety of genetic and molecular tools

available (functional testing in vivo)• Complete genome sequence (13,500

genes)

Advantages of the fly system

• A relatively short life cycle (10 days)• Culturing flies is cost-effective (vs mice)• A variety of genetic and molecular tools

available (functional testing in vivo)• Complete genome sequence (13,500

genes)

Advantages of the fly system

(A)Bridges (left) and Sturtevant in 1920 (B) Morgan in 1917

Thomas H. Morgan and the Caltech fly group

Nüsslein-Volhard and Wieschaus (along with Edward Lewis) were awarded the 1995 Nobel Prize for Physiology/Medicine.

(1980)

Christiane Nüsslein-Volhard, Eric Wieschaus and the Baden fly group

Saturation Mutagenesis Screenandthe Segmentation Hierarchy

Biological question

Screening assay

Speed

Specificity

Lead to biological insight?

Time table of embryogenesis

Stage Time Developmental events

1- 4 0:00 - 2:10 h Cleavage

5 2:10 - 2:50 h Blastoderm

6 - 7 2:50 - 3:10 h Gastrulation

8 - 11 3:10 - 7:20 h

Germ band elongation

12 - 13

7:20 - 10:20 h

Germ band retraction

14 - 15

10:20 - 13:00 h

Head involution and dorsal closure

16 - 17

13:00 - 22:00 h Differentiation

Drosophila embryogenesis 3 h

10 h

22 h

Cellular blastoderm

Segmented embryo

1st instar larva

pole cells (germ line)

Anterior Posterior

Mutant bicoid embryo

a b

Mutations insegmentation genes

Maternal genes

Gap genes

Pair rule genes

Segment polarity genes

Segmentation Hierarchy

Molecular Cloning of Genes

What is the gene product?

Where and when is it active?

Was the genetic prediction correct?

Molecular Mechanisms

Segmentation fate map of Drosophila embryo

Drosophila early development

ftz (fushi tarazu) gene expression pattern

Ernst Hafen and Walter Gehring (1983)

in situ hybridization (RNA detection) need probe!

even-skipped (blue), ftz (red)

antibody staining (protein detection) need antibody!

hunchback (blue) and Krueppel (green)

Gap Genes

Fluorescent staining

Molecular patterningof the embryo

Bicoid (blue) Even skipped (red) Krüppel (yellow)

Segmentation Hierarchy

Biological question

Screening assay

Speed

Specificity

Lead to biological insight?

Germ-line Transformation and in vivo Genetic Manipulations

Is the striped expression of ftz really necessary?

What happens if you express ftz everywhere?

How would you test this?

For example...

Heat shock promoter-ftz P-element plasmid

Adapted from Wang and Lin, 2004

transformation

ftz

1

Transposase (“helper”) plasmid acts on P-element endsfor integration into genome

2

Syncitial blastoderm Germ cells

Transposable P-element mediated transformation

Allan Spradling and Gerald Rubin (1982)

Drosophila embryogenesis 3 h

10 h

22 h

Cellular blastoderm

Segmented embryo

1st instar larva

pole cells (germ line)

Anterior Posterior

The UAS-Gal4 System:

How you can make flies with eyes on their legs

xpromoter Gal4 UAS gene

Gal4

Regulating Gene Activity with Pinpoint Precision

progeny flies will expressgene in the place of choice

wild-type Misexpression ofeyeless using theUAS-Gal4 system

Specialized chromosomes

Discovered by Balbiany in 1881 in salivary gland of drosophila larva (3rd instars)

Why salivary gland of drosophila larva will

have Polytene Chromosomes????????????????

Polytene chromosomes

Drosophila Polytene chromosome

Drosophila Karyotype

1933, Painter showed the banding pattern in Drosophila

1935, Bridges

Banding pattern is fixed in a particular chromosome in a particular species

Polytene chromosome is found in 3rd instar larva of Diptera

Also found in

Malpighian tubules

Mid Gut epithelium

Rectum

Of Diptera order of insects

And in ovary of some plants

wildtype obese (leptin)