Phenotypic Plasticity

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Most intuitive way of visualizing phenotypic plasticity is through a norm of reaction. . . . . . G1. G2. G3. Environment. Phenotype. Historical Overview. Study of phenotypic plasticity is the modern incarnation of the ancient philosophical debate about the roles of nature versus nurtureJohn Loc

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Phenotypic Plasticity

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1. Phenotypic Plasticity Genotypes produce different phenotypes in response to different environmental conditions

2. Most intuitive way of visualizing phenotypic plasticity is through a norm of reaction

3. 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

4. Historical Overview James Mark Baldwin’s (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 Baldwin’s arguments however were intuitive without empirical evidence

5. 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

7. 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

8. 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

9. Historical Overview Schmalhausen’s notion of change from the old norm to the new one (“stabilizing selection”) through an evironmentally induced response, mirrors Waddington’s 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

10. Genetic Assimilation in Drosophila

11. 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

12. 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

13. 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

14. 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

15. 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 147:445-465

16. 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

18. 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

19. 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

20. 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

21. How do we find plasticity genes? Candidate genes - based on previous knowledge of the function of genes QTL mapping Microarrays RNA Seq

22. Wu, R. 1998. The detection of plasticity genes in heterogeneous environments. Evolution 52:967-977. Used QTL’s to assess molecular genetic mechanisms associated with phenotypically plastic differences in height, basal area, stem allometry and volume indices in Populus QTL’s active in only one environment likely constitute regulatory plasticity whereas QTL’s active across environments may be good candidates for allelic sensitivity Showed that most of the genes were of a regulatory nature

23. 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

24. Pigliucci, M. and J. Schmitt. 1999. Genes affecting phenotypic plasticity in Arabidopsis: pleiotropic effects and reproductive fitness of photomorphic mutants. Journal of Evolutionary Biology 12:551-562

25. 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

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