Evolutionary Developmental Biology

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Evolutionary Developmental Biology

• Major questions in Evo-Devo
• HOX genes
• How positional information is
• achieved
• Gene pathways
• How development can evolve
• Developmental constraints

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Major Questions in Evo-Devo

• What is the genetic basis of developmental
  instructions?
• What role has developmental evolution played in
  the history of life?
• How does development bias the production of
  variation or constrain evolutionary trajectories?

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Hox Genes

• Hox (homeobox) genes control the morphology of
  segments in animals.
• Discovered in the 70s and 80s
• Homeotic selector genes - control patterning of
  body structures

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Mutations in Hox genes
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The Homeobox

• A highly conserved amino acid sequence, shared
  among Hox genes, that binds DNA
• Thus, Hox genes regulate the transcription of
  other genes typically, genes that are part of
  gene cascades involved in development.
• Transcription factor a protein that, by
  interacting with a regulatory DNA sequence,
  affects the transcription of the associated gene

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Evolutionary History of Hox Genes
known
partial
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Expression

• Occurs in an anterior-posterior pattern
• Segment-specific expression in mouse hindbrain

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Anterior-posterior patterns of HOX expression
highly conserved.
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Conserved Functions

• Pax6/eyeless

UBX/Hoxc-8
Mouse lacking Pax6
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How is position-specific expression achieved?
Segmentation genes establish early embryonic
spatial patterns of transcription factors.
These transcription factors drive the spatial
pattern of HOX gene expression.
HOX genes then trigger cascades of
developmental gene pathways (via transcription
factors for genes that influence the expression
of yet other genes).
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Developmental Pathways for Drosophila wing
1. Transcription factor Engrailed expressed
2. Expression of Hedgehog activated,
3. which signals cells at boundary to express Dpp,
4. which signals cells to express target genes
encoding transcription factors,
5. which control expression of genes responsible
for wing growth and pattern.
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Ancestral Pathway
New Regulatory Genes
Conserved Regulatory Genes
Changed Pattern of Regulatory Gene Expression
Conserved Pattern Regulatory Gene Expression
New Domains of Expression
Modification of Expression within Conserved
Domain
New Promoter Affected by Transcription Factor
Change in Sequence of Target Gene
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New Regulatory Genes
known
partial
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Temporal Co-option
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Spatial Co-option
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Add image 21.8
EXAPTATION

• Distalless is required to organize limb
  development, but has been co-opted to set up
  color patterns.

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Developmental Constraints

• A bias on the production of various phenotypes
  caused by the structure, character, composition,
  or dynamics of the developmental system -
  Maynard Smith

• This leads to
• Lack of variation
• Correlations among characters

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Constrained Branching
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Developmental Constraints May Explain

• Absence of features in certain lineages
• Directional trends
• Parallel evolution of traits
• Reduction of morphological variation
• Morphological stasis
• Similarities in embryological stages

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Questions for thought

• Using the Cambrian Explosion and the subsequent
  540 million years of evolution as a framework,
  explain how development may have biased the
  production of variation or constrained
  evolutionary trajectories.
• How can our understanding of the genetic basis of
  development help explain the rapid origin of body
  plans in the Cambrian?