Studying Segmentation Mutants in Balanced Stocks

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Studying Segmentation Mutants in Balanced Stocks
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Drosophila Development

• Each egg is surrounded by a chorion.
• The anterior end has two filaments to allow
  oxygen to enter the cell.
• Sperm enter through the micropyle at the anterior
  end.

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Early Drosophila Development

• It takes 1 day for the embryo to develop into a
  larva.
• The larva hatches, feeds, and sheds its skin
  twice.
• After 5 days, the larva becomes immobile and
  forms a pupa.
• During the pupal stage, cells in the imaginal
  discs differentiate into adult structures.

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Maternal Gene Activity in Development

• Materials transported into the egg during
  oogenesis play a major role in embryonic
  development.

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Maternal-Effect Genes

• Maternal-effect genes contribute to the formation
  of healthy eggs effects of mutations in these
  genes may not affect the phenotype of the female
  making the eggs but may be seen in the next
  generation.
• A maternal-effect mutation causes a mutant
  phenotype in the offspring of a female with a
  mutant genotype.

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The dorsal GeneOffspring of dl/dl Females are
Dorsalized and Inviable
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Segmentation Genes

• Segmentation genes are required for segmentation
  along the anterior-posterior axis.
• They are classified into three groups based on
  embryonic mutant phenotypes.
• Gap genes
• Pair-rule genes
• Segment-polarity genes

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Gap Genes

• Gap genes define segmental regions in the embryo.
• Mutations in the gap genes cause a set of
  contiguous body segments to be missing.
• Four gap genes have been well characterized
  Krüppel, giant, hunchback, and knirps.
• Gap gene expression is controlled by bicoid and
  nanos.
• The gap genes encode transcription factors.

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Pair-Rule Genes

• Pair-rule genes define a pattern of segments
  within the embryo.
• Pair-rule genes are regulated by the gap genes
  and are expressed in seven alternating bands,
  dividing the embryo into 14 parasegments along
  the anterior-posterior axis.
• In pair-rule mutants, every other parasegment is
  missing.
• The pair-rule genes encode transcription factors.

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Expression of fushi tarazu (ftz) in a Drosophila
Blastoderm Embryo
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Segment-Polarity Genes

• Segment-polarity genes define the anterior and
  posterior compartments of individual segments.
• Mutations in segment-polarity genes cause part of
  each segment to be replaced by a mirror-image
  copy of an adjoining half-segment.
• Segment-polarity genes refine the segmental
  pattern established by the pair-rule genes.
• These genes encode transcription factors and
  signaling molecules.

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Segmentation Gene Mutants
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Chapter 21The Genetic Control of Animal
Development
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Sex Determination in Drosophila and C. elegans

• The sex determination signal in both animals is
  the ratio of X chromosomes to autosomes. If the
  ratio is 1.0 or greater, the animal is a female
  if the ratio is 0.5 or less, the animal is a
  male.CLASSIC Definition
• But wrong
• In Drosophila, the key genes in sex determination
  encode proteins that regulate RNA processing.

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Sex Determination in Drosophila

• Components of the sex-determination pathway
  include
• A system to ascertain the XA ratio ,
• A system to covert this ratio into a
  developmental signal, and
• A system to respond to this signal by producing
  either male or female structures.

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Ascertaining the XA Ratio

• The system that ascertains the XA ratio involves
  interactions between maternally synthesized
  proteins in the egg cytoplasm and embryonically
  synthesized proteins encoded by several X-linked
  genes.
• The X-linked gene products are called numerator
  elements and are twice as abundant in XX embryos
  as in XY embryos.
• The autosomal gene products are called
  denominator elements and antagonize the products
  of the numerator elements.

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The Sex-lethal (Sxl) Gene

• Sxl is the mater regular of the sex
  determination pathway in Drosophila.
• The XA ratio is converted into a molecular
  signal that controls the expression of the
  X-linked Sxl gene.

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Function of SXL

• SXL regulates splicing of its own transcript to
  maintain SXL protein expression in XX embryos.
• SXL also regulates splicing of the transformer
  (tra) gene.

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Differentiating in Response to the Signal

• TRA, along with TRA2, regulate splicing of
  doublesex (dsx) and fruitless (fru).
• In XX embryos, where TRA is present, dsx
  transcripts are processed to encode a DSX protein
  that represses the genes for male development.
• In XY embryos, where TRA is absent, dsx
  transcripts are processed to encode a DSX protein
  that represses the genes for female development.

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Fruitless (fru)

• Males homozygous for the fru mutation court other
  males.
• The fru gene encodes a zinc-finger transcription
  factor that regulates the genes for male sexual
  behavior.

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Loss-of-Function Mutations in Sex-Determination
Genes in Drosophila

• Mutations in Sxl prevent SXL protein from being
  made in males homozygous mutants would develop
  into males but die as embryos.
• Mutations in transformer and transformer2 cause
  both XX and XY animals to develop into males.
• Mutations in dsx cause both XX and XY embryos to
  develop into intersexes.

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Key Points

• In Drosophila the pathway that controls sexual
  differentiation involves some genes that
  ascertain the XA ratio, some that convert this
  ratio into a developmental signal, and others
  that respond to the signal by producing either
  male or female structures.
• The Sex-lethal (Sxl) gene plays a key role in
  Drosophila sexual development by regulating the
  splicing of its own transcript and that of
  another gene (tra).

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