Phenotypic Plasticity

Presented by: Tom Reed, School of BEES, UCC, ZY4021
Examples: Christian Bale as an illustration of phenotypic plasticity, showing the same genes expressed differently at two time points.

Phenotypic plasticity: Definition - The property of a genotype to produce different phenotypes in response to different environmental conditions (Bradshaw 1965).
Significance: Study of plasticity reveals how nature (genetics) and nurture (environment) interact to form the morphology, physiology, behavior, and life histories of organisms.
- Single-locus genotypes: Examples include AA, Aa, aa.
- Multi-locus genotypes: Examples include AABB, AaBB, AABb, AaBb, aabb.

Historical debate over the roles of nature (genetics) and nurture (environment) in shaping characters.
John Locke (1632-1704): Proposed that humans are born as "blank slates" (tabula rasa) influenced entirely by environmental experiences.
The naturistic perspective has often approached dangerous concepts like "genetic determinism" seen in movements such as eugenics.

Current consensus acknowledges that both nature and nurture contribute to shaping human behavior and traits across all life forms.
Key contemporary questions include:
- What is the relative importance of genes versus environment for different traits in various contexts?
- How do genes and environment interact to produce complex phenotypes?

Example: Genetically identical individuals of Daphnia retrocurva exhibit cyclomorphosis, showing seasonal body shape changes.
- Development of helmet/crest in response to predator presence (inducible defense).
- Changes triggered by chemical cues in water, even in the absence of predators, reducing predation risk but incurring metabolic costs or reduced swimming efficiency.

Introduced by Woltereck in 1909 based on cyclomorphosis in Daphnia.
Three Genotypes: Display varying degrees of plasticity in relation to their environments.
- Genotype 1: Most plastic
- Genotype 2: Less plastic
- Genotype 3: Not plastic
Norm of reaction illustrates how phenotypes vary across different environmental contexts.

Plasticity itself is a character trait, specific to certain traits and environments.
Some genotypes show plasticity in some traits, while others do not exhibit the same flexibility.

Emphasizes different phenotypic expressions based on environmental conditions.
Relevant studies: Via, Sara, and Russell Lande on genotype-environment interactions and evolution of phenotypic plasticity.
Genetic Correlations:
- $r_G = 1$ (perfect correlation)
- $0 < r_G < 1$ (partial correlation)
- $r_G < 0$ (negative correlation)

Adaptive plasticity may evolve under the following conditions:
- Populations experience variable environments.
- Environments provide reliable cues.
- Selection favors different phenotypes for varied environments.
- No single phenotype demonstrates superior fitness across all settings.
- Costs of plasticity are manageable.
Example: Shade-avoidance in plants involves changes in growth responses influenced by changing light ratios (R:FR).

Light conditions can trigger differential growth adaptations:
- Under normal daylight conditions, the R:FR ratio is approximately 1.15.
- R:FR ratios in vegetation canopies typically range from 0.05 to 0.7 (Smith, 1982).

Costs:
- Maintenance Costs: Energetic costs from sensory and regulatory mechanisms.
- Production Costs: Additional expenses from producing structures plastically compared to fixed genetic structures.
- Information Acquisition: Energy and time spent in environmental assessments.
- Developmental Instability: Potential inaccuracies in development in various settings.
Limits:
- Reliability of environmental cues may be insufficient or fluctuating.
- Lag times in response to environmental changes.
- Range of developmental expression in plastic genotypes may be limited compared to specialists.

Langerhans & DeWitt (2002) studied freshwater snails with different sunfish species.
- Induced responses included:
- Reduced growth: Beneficial against molluscivores but detrimental to fecundity.
- Rotund shells: Increased resistance to specific predators but heightened vulnerability to shell-entry predators.

A vital area of research focuses on the genetic and molecular basis of adaptive plastic responses.
Schlichting & Pigliucci define plasticity genes as regulatory loci triggered by environmental stimuli that initiate specific morphogenic changes.
Research approaches include mutagenesis and Quantitative Trait Loci (QTL) mapping through inbred line crosses.

Genes controlling trait expression may behave differently under varying environmental conditions, influencing overall biochemistry significantly.

Regulatory switches activate or deactivate based on environmental factors, leading to specific gene activations depending on context.
- Example: Different gene expressions under Environment A vs. Environment B due to the presence of master gene switches.

Example: Eyespot size in tropical butterflies (Bicyclus anynana) and wing color changes in Junonia coenia, demonstrating seasonal adaptations.
Discussed by Hartfelder & Emlen regarding the endocrine control mechanisms guiding these transformations.

Definition: Adaptive responses passed from one generation to the next based on environmental exposures faced by parents.
Study shows the influence of maternal light environments on offspring life histories, where plants harboring proper cues are significantly more fit (3.4 times greater fitness) in native light environments.

Acute temperature increases (1.5°C and 3.0°C) resulted in decreased aerobic capacity; however, full compensation occurred when both generations were raised in similar elevated temperatures.

James Baldwin's Hypothesis: Plasticity mediates adaptation and persistence in novel environments.
Discussion of future research influencing our understanding of plasticity in evolutionary processes and ecological responses.
- Importance of demonstrating how plasticity preexists environmental changes and contributes to evolutionary success (soft vs. hard plasticity).

Snowshoe hares show plasticity in spring color changes but vulnerability due to mismatched coloring in altered climatic conditions, stressing the dual need for evolutionary adaptation and plastic responses to climatic shifts.