L20: Pattern Formation of Drosophila Flashcards

1
Q

what are the two differences in early embryo patterning mechanisms?

A
  1. egg fertilization - egg polarity (fly egg is highly polar where as a mouse/human egg is nonpolar)
  2. Early embryo - most animals go through cleavage divsions but the fly goes through syncytium
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2
Q

why do we care about Drosophila development?

A

-short generation time - 9 days to sexual maturity
-sophisticated genetic and molecular biology tools
-studies in drosophila have revealed how genetic control mechanisms govern development
-many basic mechanisms, including the functions of specific molecules (Hox genes), are conserved in humans

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3
Q

fate map of fly development

A

questions:
* How does this segmented pattern forms?
* How does each segment know its identity?
* How is the pattern maintained throughout development?

approach: genetic screens (looking for mutants with defects)

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4
Q

drosophila early embryo is a syncytium

A
  1. unfertilized fly egg is already polarized
  2. fertilized egg
  3. many nuclei divide rapidly in a syncytium
  4. nuclei migrate to periphery, where cell boundaries will eventually form
  5. 13 divisions prior to cellularization, forms multiple somatic cells
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5
Q

bristle patterns reveal…

A

distinct anterior-posterior (A-P) difference among and within segments

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6
Q

egg-polarity gene (bicoid)

A

missing anterior or posterior segments

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7
Q

what are the three segmentation genes?

A
  1. gap gene - missing several contiguous segments
  2. pair-rule gene - missing alternating segments
  3. segment-polarity gene - polarity defect in every segment
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8
Q

hierarchy of gene regulatory interaction patterns

A

first, egg polarity genes organize the A-P axis of early embryo (prior to fertilization)

this affects expression of three groups of genes which control segmentation along A-P axis: gapm pair-rule, seegment-polarity genes

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9
Q

hox genes…

A

give identity to each segment

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10
Q

egg polarity genes…

A

encode maternal molecules deposited in the egg

bicoid mutants lack anterior
segments

  • Maternal bicoid mRNA is localized
    to the anterior of the egg prior to
    fertilization
  • Bicoid protein is made after
    fertilization and diffuses in the
    syncytium, forming a gradient
    from anterior to posterior
  • Bicoid is a transcription regulator
  • Bicoid regulates the expression of
    gap and pair-rule genes
  • Bicoid acts as a morphogen!
    Activates expression of genes in a
    concentration-dependent manne
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11
Q

egg-polarity gene products form opposing gradients

A
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12
Q

egg polarity gene product gradients induce epression of zygotic genes and turn on —– genes

A

GAP genes

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13
Q

gap genes are…

A
  • Expressed in broad domains (multiple segments)
  • Encode transcription regulators
  • Their expression is regulated by
    egg-polarity genes
  • They regulate the expression of
    other gap genes and pair-rule genes
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14
Q

pair-rule genes…

A
  • Mutants lack every other segment
    (either odd or even segments
    depending on the gene)
  • Expressed in 7 segments (either
    even or odd) ~ or 7 ‘stripes’
  • Encode transcription factors
  • Their expression is regulated by
    gap genes and egg-polarity genes
  • They regulate the expression of
    other pair-rule genes and
    segment-polarity genes
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15
Q

how to determine which factors regulate the expression of a gene?

A

example: Gene A

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17
Q

pair rule genes….

A

*Mutants lack every other segment
(either odd or even segments
depending on the gene)

  • Expressed in 7 segments (either
    even or odd)
  • Encode transcription factors
  • Their expression is regulated by
    gap genes and egg-polarity genes
  • They regulate the expression of
    other pair-rule genes and
    segment-polarity gene
18
Q

how do segment polarity genes differ from the other three types of genes?

A

they maintain their expression through development

encode transcription factors and signaling pathway components

other three gene types remain inmemory of cells but do not remain expressed, only transiently expressed

19
Q

a signaling loop maintains expression of segment polarity genes

A

Engrailed, Wingless and Hedgehog, are segment polarity genes expressed at posterior end of each
segment in neighboring cells

  • Engrailed = transcription factor; Wingless and Hedgehog are secreted proteins and paracrine
    signaling molecules
  • MUTUALLY REINFORCING SIGNALS between Wingless expressing cells and Hedgehog expressing
    cells maintains the narrow stripes of expression (cell memory) throughout development
20
Q

when are each type of gene expressed in terms of the fly cell cycle of development

21
Q

how do the segments know their identity (i.e. what they should become?)

23
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expression pattern and function of Hox proteins
Hox proteins are.... -transcription regulators that bind to DNAA through their homeodomains -act as master regulators by controlling the expression of multiple genes
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hox gene is maintained in adults, occurs via...
Hox gene expression must be maintained throughout development This occurs via epigenetic inheritance of histone modifications & recruitment of chromatin remodeling complexes The Hox genes are present and play a role in anterior-posterior patterning in all bilaterally symmetric animals, including vertebrates first molecular evidence that development of all animals was based on common principles and molecular mechanisms
28
28
Hox genes regulate limb development in vertebrates
mutations of different Hox genes will result in malformations of specific regions of the limb (S, Z, A regions)
28
what are paralogs
similar gene sequences that have similar expression patterns