Evo-Devo: Evolutionary Development

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Evo-Devo Evolutionary Development

• DNA
• Regulatory genes code for signal proteins and
  transcription factor proteins
• SP target particular groups of cells for gene
  expression
• Regulatory sequences binding sites of
  transcription factors (enhancers, promoters).
• Structural genes Activated by binding of TF RS
  
• Responsible for producing phenotypic characters.

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Dll expression
En/Inv expression
Eyespots on a butterfly wing
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Homeotic Genes and Animal Body Plans

• Multicellular animals develop in four dimensions.
• 3 spatial time
• Each cell has to have
• 1. information where it is relative to other
  cells
• 2. where it is in the developmental sequence.
• Information provided by Homeotic genes (Hox genes)

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Cells along these major body axes assume
positional information during development
(After Strickberger.)
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Hox genes in Drosophila (body segmentation) Positi
onal information
Colinearity 1. Expressed first 2. Anterior to
posterior 3. Greater quantity of transcription
factors

Gene location in hox cluster
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Colinear Hox gene expression

• Hox genes provide information on location.
• 1. The transcription factor from the first Hox
  gene
• required to express downstream Hox genes.
• 2. The effectiveness in initiating sequential
  gene expression declines with distance.

Paralogous and evolutionarily conserved
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• Each Hox gene contains a highly conserved 180 bp
  sequence the homeobox.
• Codes for a DNA binding segment (aa sequence) in
  the transcription factor.
• The transcription factors activate structural
  genes.
• Structural genes produce structures appropriate
  for that location.
• Mutations in Hox genes result in inappropriate
  structures for that location.

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Hox gene products activate genes responsible for
making a particular structure.
Mutations in Hox genes bx, pbx, and abx
Mutation of Hox gene antp
Wings normally appear on T2 Hox mutations change
identity of T3 cells to T2 cells. An extra pair
of wings is produced. Ancestors of dipteran
flies had 4 wings.
Identity of a head segment changed to that of a
thoracic segment
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Hox gene diversification ? diversification of
animals

• Hox paraloges (homologs) in everything from
  SpongeBob Squarepants to humans to fungi and
  plants (MADS-box genes).
• Therefore, Homeobox genes predate the origin of
  animals.

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• Representative arthropods What is the basis of
  their diversity?
• 1 million sp. described maybe 50 million still
  to be named.
• Exoskeleton segmented body (H T A) and
  segmented legs
• Paired appendages on body segments open
  circulatory system

Crustaceans
Hexapods
Myriapods
An onychophoran (velvet worm) Closest living
relative of arthropods 1 pr. unjointed legs on
each of the similar body segments
Chilicerates
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Evolutionary diversification of arthropods partly
based on sites of Hox gene expression
Hox cluster of 9 loci for all arthropods
abdA always expressed on ventral side of segment
Ubx and abdA not expressed in posterior segments
Evolutionary change in where a Hox gene is
expressed
Mutation legless abdominal segments
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Homeotic genes and Flower formation C. 300,000
sps. of Angiosperms
Four concentric whorls of modified leaves Normal
order sepals, petals, stamens, carpels
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MADS-box mutants (Hox genes)