Phenotypic Plasticity

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Phenotypic Plasticity

• Genotypes produce different phenotypes in
  response to different environmental conditions

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Most intuitive way of visualizing phenotypic
plasticity is through a norm of reaction
G1
Phenotype
G2
G3
Environment
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Historical Overview

• Study of phenotypic plasticity is the modern
  incarnation of the ancient philosophical debate
  about the roles of nature versus nurture
• John Locke (1632-1704) suggested that humans are
  born as blank slates on which the environment
  writes their character early representation of
  the nurturistic position
• Thomas Hobbes (1588-1679) was a pioneer of the
  application of mechanistic principles to explain
  human motivation a forerunner of modern genetic
  determinism, the naturistic school
• Resolution of the debate has been achieved by the
  study of phenotypic plasticity

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Historical Overview

• James Mark Baldwins (1896, American Naturalist)
  new factor in evolution individuals differ
  not only in their phenotypic attributes (as
  Darwin knew well) but in the way those attributes
  are altered by changing environmental
  circumstances (in their reaction norms)
• Those organisms more adaptable (i.e., plastic)
  to new circumstances are bound to leave more
  progeny
• Baldwin effect is a clever Darwinian
  interpretation of situations that might otherwise
  appear Lamarckian
• Baldwins arguments however were intuitive
  without empirical evidence

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Scientific Study of Genotype X Environment
Interactions

• Began with the introduction of the concept of the
  reaction norm (called phenotypic curves) by
  Richard Woltereck (1909)
• Initial experiment (1909) under low to high food
  availability

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A
A
B
Head Size
C
Low Intermediate High
Algae
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Woltereck 1912

• Studied the phenomenon in Daphnia, known today as
  cyclomorphosis
• When exposed to the presence of a predator they
  respond by altering the shape of their body to
  produce a helmet or neck teeth effective in
  reducing predation pressure

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Historical Overview

• Experimental support came predominantly from Ivan
  Schmalhausen (1949) and Conrad Waddington (1952)
• Schmalhausen argued that evolution proceeds by
  altering the developmental systems of organisms
  changing the norm of reaction to cope with and
  anticipate environmental stimuli e.g., case of
  Arrowhead, Sagittaria sagittifolia

Heterophylly
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Historical Overview

• Schmalhausens notion of change from the old norm
  to the new one (stabilizing selection) through
  an evironmentally induced response, mirrors
  Waddingtons notion of genetic assimilation
• Genetic assimilation is defined as a phenotypic
  character that is initially produced in response
  to some environmental influence, then stabilized
  due to natural selection, and finally occurs in
  the absence of the previously necessary external
  influence

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Genetic Assimilationin Drosophila
Selected
60
Crossveinless
30
Unselected
0 22
Generations
Waddington 1952
Heat shock during larval development 40 vs. 25C
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Historical Perspective

• Reaction norms and phenotypic plasticity did not
  play a prominent role during the neo-Darwinian
  synthesis of the 1930s and 1940s
• It was not until Anthony D. Bradshaw published an
  influential review in 1965 that research on
  phenotypic plasticity was brought into the main
  stage of evolutionary theory

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Historical Perspective

• Bradshaw was the first to clearly state two
  fundamental concepts of plasticity
• First, plasticity is a character in its own
  right, genetically controlled, and it can
  therefore evolve somewhat independently of other
  aspects of the phenotype
• Second, plasticity is not a property of an entire
  genotype it needs to be studied in reference to
  specific environments and traits a given
  genotype can be plastic for one trait in response
  to one set of environmental conditions but not to
  another set, or it can be plastic for some traits
  but not others in response to the same set of
  conditions

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Modern Concepts in Plasticity Research

• One of the most controversial and difficult areas
  of study in plasticity concerns the possibility
  that plasticity may be an adaptive character
  directly targeted by natural selection

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Some Examples

• Plant compensatory responses (i.e.,
  over-compensation)
• Nemoria arizonaria twig and catkin caterpillar
  morphs
• Spadefoot Toads Tiger Salamanders larvae
  carnivorous omnivorous morphs
• Aphids - winged and non-winged morphs
• Daphnia helmet neck tooth forms

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Testing the Adaptive Plasticity Hypothesis

• Dudley, S.A. and J. Schmitt. 1996. Testing the
  adaptive plasticity hypothesis density-dependent
  selection on manipulated stem length in Impatiens
  capensis. American Naturalist 147445-465

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Shade Avoidance in Plants

• Type of plasticity in which individual plants can
  perceive the presence of other plants
  (competitors) by means of detecting changes in
  the spectral quality of light
• Created elongated and shortened (suppressed)
  plants by altering the red to far red ratio of
  light and placed them each at high and low plant
  densities

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Cumulative Fitness
Elongated

Suppressed
High Low
Density
RFR elongated 1.11 - Nigrosin RFR suppressed
6.9 Copper Sulfate RFR natural 1.24 full sun
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A Second Concept in Plasticity Research

• There are costs associated with plasticity
• DeWitt et al. 1998 proposes the following costs
• Maintenance energetic costs of sensory and
  regulatory mechanisms
• Production excess cost of producing structures
  plastically (when compared to the same structures
  produced through fixed genetic responses)
• Developmental Instability plasticity may imply
  reduced canalization of development within each
  environment, or developmental imprecision
• Genetic deleterious effects of plasticity genes
  through linkage, pleiotropy, epistasis with other
  genes

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Problem Is That They Are All Difficult To Test
Empirically No One To Date Has Successfully Done
So

• Students of phenotypic evolution will surely find
  this area of research particularly challenging
  and potentially rewarding

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A Third Area of Plasticity Research Molecular
Basis of Adaptive Plastic Responses

• Plasticity Genes
• Regulatory loci that directly respond to a
  specific environmental stimulus by triggering a
  specific series of morphogenic changes

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How do we find plasticity genes?

• Candidate genes - based on previous knowledge of
  the function of genes
• QTL mapping
• Microarrays
• RNA Seq

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Wu, R. 1998. The detection of plasticity genes in
heterogeneous environments. Evolution 52967-977.

• Used QTLs to assess molecular genetic mechanisms
  associated with phenotypically plastic
  differences in height, basal area, stem allometry
  and volume indices in Populus
• QTLs active in only one environment likely
  constitute regulatory plasticity whereas QTLs
  active across environments may be good candidates
  for allelic sensitivity
• Showed that most of the genes were of a
  regulatory nature

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Phytochromes Plasticity Genes?

• Shade avoidance made possible by phytochromes
• Five have been described A,B,C,D,E
• Shade avoidance primarily due to phytochrome B
  but A and possibly C play a role in complex
  morphological response

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Pigliucci, M. and J. Schmitt. 1999. Genes
affecting phenotypic plasticity in Arabidopsis
pleiotropic effects and reproductive fitness of
photomorphic mutants. Journal of Evolutionary
Biology 12551-562
Blue Receptor Mutant Wild Type Phy B Mutant Phy
A-E Mutant
No. of Fruits
Low RFR
High Light
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Two Conclusions

• First, the shade avoidance response is
  characterized by molecular redundancy, given that
  only the elimination of all five phytochromes
  completely flattens the reaction norm yielding a
  non-plastic genotype
• Second, under high light, the blue receptor
  actually acts in opposition to the phytochromes
  in respect to the wild type, so that eliminating
  its functionality actually prolongs the
  vegetative phase in that environment