Drosophila as a model

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Drosophila as a model

• Paul Adler
• Gilmer 245
• Pna_at_virginia.edu

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figure 9.6
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figure 9.7
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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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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
  its wonderful cytogenetics.

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Drosophila Genetic Tools
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Figure 14-16

• P-element structure

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figure 5.16
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figure 5.17
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Gal4
Galactose
Gal80
UAS
No galactose Gal4 does not activate
transcription
In presence of Galactose, Gal80 does not bind
Gal4, GAL4 activates transcription
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Genetic Mosaics

• Provides a cell marker that cannot be diluted
  out. Very valuable for tracing cell lineage.
• Can use to study gene function.
• Gets around some aspects of pleiotropy.
• Allows additional functional tests of genes and
  pathways.

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Eric Wieschaus and Christiane Nüsslein-Volhard use
d genetics to identify proteins that set up the
embryonic body plan
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maternal effect genes cytoplasmic polarity
Basic Body Axes
Hunchback protein gradient
Specify Broad Body Regions
Kruppel knirps hunchback giant tailless
Gap genes
orthodenticle hairy even-skipped runt fushi tarazu
Split Body into Segments
Pair rule genes
Determine Segment Orientation
Define Segment Identity
Homeotic genes
Segment polarity genes
engrailed
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wild-type
bic mutant
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Bicoid is localized to the anterior pole as an
mRNA
anchoring to microtubule network requires
exuperantia, swallow, staufen
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Bicoid is a transcriptional regulator
AND a translational regulator
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Nanos inhibits the translation of
maternally-provided hunchback
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• caudal maternal mRNA uniform
• translated only in posterior
• gradient of protein
• translation in anterior inhibited by bicoid

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protein gradients after fertilization
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A gradient of the bicoid transcription factor
turns on different genes at different
"thresholds"
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Expression pattern of proteins ecoded by gap genes
Hunchback
These bicoid target genes are known as the gap
genes
Kruppel
Knirps
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Gap gene mutants are missing different regions
of the body
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Wildtype
knirps
Krüppel
hunchback
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Figure 9.17Model of Drosophila
Anterior-Posterior Pattern Formation
Cytoplasmic polarity
Hunchback protein gradient
Gap genes
Pair-rule genes
Segment polarity genes
Homeotic genes
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fig. 9.13
D
Dorsal protein
wild-type
dorsalized
ventralized
V
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fig. 9.11
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Production of the body plan is a stepwise process
of cell commitment. Major subdivisions are
specified and then further refined. Cells need
to communicate and coordinate their commitments.
Use intercellular signaling systems, that
ultimately influence transcriptional regulatory
hierarchy.
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Bicoid protein gradient
Hunchback protein/Kruppel protein
Pair rule gene Fushi tarazu
Segment polarity gene Engrailed