LT11 Phenotypic Plasticity

Phenotypic Plasticity

…and Norm of Reaction

 Phenotypic plasticity: Exhibited when a
genotype produces different phenotypes
in different environments
 Reaction norms: range of phenotypes the
results from a given genotype (coined by
Ivan Schmalhause, 1949)/depict the
phenotypes produced by each genotype in
2 or more environments
- If genotypes vary in the degree to
which their phenotype is altered by environment, then norm of reaction can
evolve
 Norm of reaction of the level of transcription of a gene involved in cell wall
maintenance in budding yeast (Landry et al., 2006)
- 6 2n strains studied across of range of environments representing different
levels of stress (mainly nitrogen starvation)

Half-sib breeding design

 Data analysis comes from trait values of
members of pairs of families
 Pairs have one parent (father) in common
 Progeny of paired families have known average degree of relatedness
- Offspring of each female = full-sibling families (same mother and father)
- These are nested within half-sibling families (all are progeny of same father)
- Grow progeny of each cross (family) in each environment
- Variance among half-sib families estimates VA
 Relatives resemble each other: they share genes and environments
- Can quantify genetic resemblance between relatives by estimating genetic
covariance between specified relatives
 Compare half-siblings
- Covariance among half-sibs = (1/4)VA
- If measure cov[half-sibs] and multiply by 4 then obtain an estimate of V A
- Use half-sib design to estimate VA of traits and compare




Reaction Norms

, In the 4 hypothetical examples: family (genotype) members are reared in each of 2
environments
- Line joining up mean value of each genotype in each environment depicts reaction
norm of that genotype
- Slope of line for each family indicates degree of plasticity for that family

A. Populations lack plasticity
 Lines joining mean family performance are a series of
parallel lines
 Phenotypic mean of each family is the same in each
environment
 There is variance between families – spread of genetic
values within environments
- This variance is similar across 2 environments
 Overall phenotypic mean for each environment is similar
(mean of E1 = mean of E2)

B. Populations show high degree of plasticity
 Lines joining mean family performance have steep slopes
 All families respond to 2 environments in similar way,
decreasing by similar amounts - their slopes are parallel
 No genetic variation in plasticity – phenotypic family means
are very different between environments
 In genetic variance – variance between families, in each
environment, Magnitude of this variance is similar across
the 2 environments
 Overall phenotypic mean of E2 is much lower than that of
E1

C. Genotypes often respond differently to different environments
 There are large differences between families in plasticity
– different slopes of reaction norms
 Lines cross (no longer parallel), so phenotypic rank of
families different in 2 environments
 Families respond differently to the environments =
genotype-by-environment interaction (GxE)
 Still have genetic variance – variance between families –
within each environment
 Overall phenotypic mean of E1 similar to that of E2

D. Another pattern of GxE interaction
 Still large differences between families in plasticity

,  Lines rarely cross and phenotypic rank of families is
different in 2 environments
 Still have genetic variance – variance between families –
within each environment
 Low variance in E1 and high variance E2
 ~equal number of families increased and decreased their
phenotype – hence overall phenotypic means of E1 and E2
are similar

To summarise…

 Via 2 –way ANOVA: 2 main factors are Genotype (sire/family) and Environment
- Individuals at each level of factor are represented at each level of the other
factor
- Members of each half-sib family in each environment
 Outcomes
- Significant sire/family effect (Genotype, G)? Evidence for overall additive
genetic variance for the trait
- Significant environment € effect? Evidence of overall plasticity
- Significant G-by-E interaction? Evidence for additive variance for plasticity




Examples of G by E interactions (common)

 If there is G by E, plastic responses can
evolve
 Can study the evolution of plasticity using
artificial selection
 Body size and rearing temperature in 1 strain
of Drosophila
- If grown at low temperature, flies have
small body size, if grown at high temperature, flies are larger
- Scheiner and Lyman – reared families and then reared siblings at either 19 oC or
25oC
- Difference in size of warm and cold-reared siblings varied from family to family
- Artificial selection – after 22 generations
High/increased plasticity – only flies with greatest size difference between
temperatures allowed to breed
Low/reduced plasticity – only flies with smallest size difference between
temperatures allowed to breed
- See a direct response to selection in high and low lines – amount of phenotypic
plasticity had evolved in response to selection