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Drosophila melanogaster

Eye genetics and mosaics

General characteristics

Eye structure and development

Eye mutants

Mosaic analysis

Drosophila characteristics

Life cycle 10 days

Hardy and prolific

Four chromosome pairs

Polyteny in larval tissues

Special chromsomes

P-elements

Genome = 1.7x108 bp

Drosophila eye structure

Each eye composed of 600-700 simple units (ommatidia)

Ommatidia form a highly regular array

Drosophila eye structure

Core of 8 photoreceptors (R1-R8)

Surrounded by 4 cone cells (lens) then by pigment cells

Each photoreceptor cell has a unique identity

Drosophila eye development

Adult Drosophila epidermis derived from imaginal discs

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Drosophila eye development

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AntennaEye

Morphogenetic furrow

The eye forms from the eye-antennal disc

Differentiation is from posterior to anterior

A furrow marks the boundary betweendifferentiating and undifferentiated ommatidia

Drosophila eye development

Anterior to furrow cells divide synchronouslyIn furrow division stops

Cells go through two further synchronous cell divisions

Photoreceptors differentiate in a fixed order ending with R7

The eye is a good model for:-

Organogenesis

Cell-cell interactions

Cell division

Cell death

Mutations affect many different aspects of eye development and function

sine oculis Bar

Mutations affecting the R7 photoreceptorcan be isolated by behavioural screensfor UV insensitivity

Wild type bride of sevenlesssevenless

UV insensitive mutants include two in which the R7 cellcompletely fails to develop

X-ray induced mitotic recombination

In Drosophila mitotic recombination can be induced by X-rays

Heterozygous cells give rise to recombinant daughters

One daughter homozygous wild type one homozygous mutant

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X-ray induced mitotic recombination

Single homozygous mutant daughter cell gives rise toa clone of genetically distinct cells in the adult eye

Clones can include incomplete ommatidia

X-ray induced mitotic recombination

In flies heterozygous mutant for botha marker gene and a more distal biologically interesting geneinduced mitotic recombinationproduces marked daughter cells, which are homozygous mutant at both loci

This gives rise to genetically mosiac individuals with marked clones of mutant cells in a background of normal cells

Applications of mosaic analysis

Inducing recombination after embryogenesis allows theeye phenotype of lethal mutations to be analysed

Analysing the phenotype of mutant clones shows whether the activity of the affected gene is required cell-autonmously

Applications of mosaic analysis

Mosaic adults generated by hedgehog clone induction during 1st larval instar are viable

Hedgehog ommatidia arein the centre of large cloneshave an abnormal structure

Applications of mosaic analysis

A signal from R1/6/8 instructs the neighbouring cell to be the R7

Genes involved in producing the signal need to be active inR1/6/8 (non-cell autonomous)

Genes involved in transducing the signal need to be active inR7 (cell autonomous)

Applications of mosaic analysisChaoptic is a recessive mutation on chromosome 3 just proximal to boss, which affects rhabdomere morphology

X-irradiation used to inducemosaicism in flies, which areheterozygous for boss and chaoptic

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Applications of mosaic analysis

In mosaicommatidiaif R8 chaopticR7 isabsent

Applications of mosaic analysis

R7 cells present in ommatidiawith R1-R6 chaoptic

R7 cells present in ommatidiawith R7 chaoptic

Applications of mosaic analysis

Boss is required non-cell autonmously in R8 but not R1/R6

Sevenless is required cell autonomously in R7

A signal from R8 instructs the neighbouring cell to be R7

Boss codes for a membrane bound ligand

Sevenless codes for a transmembrane receptor

Summary

Drosophila eye development is a good modelfor analysis of cell-signalling and many other aspects of biology

Mosaic analysis allows the adult phenotype of embryonic lethal mutations and the cell-autonomyof gene action to be determined

References

http//www.sdbonline.org/fly/aimorph/eye.htm

Eye development genetics

Thomas BJ and Wassarman DA (1999)A fly's eye view of biologyTrends in Genetics vol 15 pp184-190