Chapter 4 - Behavioral Genetics

Fundamental Equations and Definition of Behavioral Genetics

• Phenotype ($P$): The observed traits of an individual.

• Phenotypes are determined by the combination of the individual's Genotype and the Environment.

• Genotype ($G$): All the alleles possessed by an individual.

• Behavioral Genetics: The study of how genes and the environment lead to individual differences in behavior.

• Behavioral output ($P$) is a result of three primary factors:

  • Genotype ($G$).

  • Environment ($E$).

  • Gene-environment interactions ($G \times E$).

• Total Phenotypic Variance formula: $V_P = V_G + V_E$.

Understanding Heritability

• Heritability ($h^2$): Defined as the proportion of phenotypic variation in a trait specifically due to genetic influences.

• Broad-sense heritability ($H^2$): Includes all genetic effects on the phenotype.

• Genetic effects of alleles are categorized into three factors:

  1. Additive effects ($A$): The average effect of individual alleles on the phenotype.

  2. Dominance effects ($D$): Interactions between alleles located at the same locus.

  3. Epistasis ($I$): Interactions between genes located at different loci (inter-genic interactions).

• Total Genetic Variation formula: $V_G = V_A + V_D + V_I$.

• Dominance and epistatic effects are considered interactions and represent non-additive genetic components.

• Additive effects act independently of other genes and the environment.

• Narrow-sense heritability: Phenotypic variance due solely to additive genetic values.

Assessing Narrow-Sense Heritability

• Higher narrow-sense heritability indicates a greater probability that parents and offspring will resemble one another in the trait.

• This is assessed by determining the similarity of behavior between parents and offspring using Parent/Offspring Regressions.

• Interpretation of regression slopes:

  • A greater slope indicates that offspring significantly resemble their parents.

  • A lower slope indicates that phenotypic variance is not primarily due to additive genetic variance.

• Case Study: Dog behavior heritability (Ruefenacht et al. 2002):

  • Question: Are key behaviors for service and hunting dogs heritable?

  • Methods: Examined 25 years of behavioral field test results for German shepherds of known pedigree.

  • Results: Heritability scores ($h^2$) ranged from $0.09$ to $0.24$ for specific traits, such as reaction to gunshots.

  • Conclusion: Behavior consists of both a genetic component and an environmental component.

The Genetic Basis of Behavior and Instincts

• Instinctual Behaviors:

  • Performed the same way every time.

  • Fully expressed the first time they are performed.

  • Present even in individuals raised in complete isolation (e.g., blinking).

• Reflexes: Involuntary movements occurring in response to a stimulus.

• Fixed Action Patterns (FAP):

  • Behaviors displaying almost no variation.

  • Once initiated, the sequence cannot be stopped until completed.

  • Key researchers: Lorenz and Tinbergen (1957).

Research on Mutant Genotypes: Wild-type vs. Yellow Fruit Flies

• Observation: Mutant yellow forms of fruit flies ($Drosophila\,melanogaster$) are rare in nature but common in labs.

• Research Question (Bastock 1956): Why are yellow forms not common in nature?

• Hypothesis: Gene mutation results in behavioral changes that lead to low reproductive success in wild populations.

• Methods: Interbred wild-type (the typical form occurring in nature) and mutant yellow flies to create inbred lines for mating trials.

• Results:

  • Wild-type males achieved higher mating success than mutant yellow males.

  • Yellow flies exhibited significantly less courtship behavior.

• Conclusion: Differences in courtship behavior cause reduced mating success in yellow individuals; thus, genotype variation is directly associated with variation in behavioral phenotype.

Major vs. Minor Genes and Genomic Mechanisms

• Relationship between genes and behavior:

  • Major Gene: An individual gene responsible for a large fraction of phenotypic variation.

  • Minor Gene: An individual gene that contributes small amounts of variation to the phenotype.

• Key Terms:

  • Epistasis: Interactions between genes at different loci; major genes can affect the expression of several other genes.

  • Pleiotropy: A single gene affects more than one phenotypic trait (e.g., the "yellow" gene affecting both body color and courtship).

• Approaches to understanding major genes:

  • Identifying specific gene alleles that directly influence behavior.

  • Disabling specific genes (knockout studies) to observe behavioral changes.

Case Study: Fire Ant Social Organization

• Social organization types in fire ants ($Solenopsis\,invicta$):

  • Monogyne: Single reproductive queen.

  • Polygyne: Multiple reproductive queens; leads to higher ecological impact.

• Research Question (Ross 1997): Do genes affect variation in social organization?

• Methods: Used starch gel electrophoresis to compare allele frequencies across monogyne and polygyne colonies.

• Results: Genotypes and allele frequencies at the $Gp-9$ locus differed.

  • Monogyne colonies: Displayed $Gp-9BB$.

  • Polygyne colonies: Displayed $Gp-9Bb$.

• Genetic Mechanism: $Gp-9$ codes for a pheromone-binding protein. Workers regulate queen numbers via chemical recognition.

• Concept of the Supergene: A set of linked genes that affect many traits together.

• Conclusion: Alleles at the $Gp-9$ locus appear to regulate fire ant social organization.

Knockout Studies: Anxiety and the AVPR1A Receptor

• Knockout Technique: A procedure that eliminates the expression of a specific gene to examine its effect on behavior.

• Research Question (Bielsky et al. 2004): What is the function of the $AVPR1A$ receptor in mice?

• Context: Arginine vasopressin (AVP) is a peptide hormone affecting behavior under stress.

• Methods: Created a knockout of the $V1aR$ gene and conducted stress tests.

• Key Measure: Thigmotaxis (preference for physical contact/hugging walls).

• Results: Knockout mice spent significantly more time in open, light areas compared to wild-type mice.

• Conclusion: The $V1aR$ gene plays a critical role in behavior regulated by stressful situations.

QTL Mapping and Candidate Genes

• Quantitative Trait Loci (QTL): Stretches of DNA that either contain or are linked to genes influencing a phenotypic trait.

• QTL Mapping: A statistical technique combining genetic information with trait information to identify genomic regions containing relevant genes.

• Candidate Genes: Major genes suspected of contributing to a large portion of phenotypic variation in a specific trait.

• Case Study: Pea aphid feeding behavior (Caillaud & Via 2000):

  • Background: Different races of aphids ($Acyrthosiphon\,pisum$) specialize on alfalfa vs. clover.

  • Methods: Created $F1$ and $F2$ generations from crosses; used AFLP markers to assess genotype.

  • Results: Found four to six QTLs associated with plant acceptance behavior (search time, feeding time, digestion).

  • Percent variation: Individual QTLs accounted for $7\%$ to over $50\%$ of behavioral variation.

  • Conclusion: A few major genes may be involved in plant selection behavior.

Environmental Influence and Gene Expression

• Genes do not directly produce behavior; they code for molecules that alter brain function.

• Gene Expression: The process by which gene products are formed, which is dependent on sensory/environmental inputs.

• Case Study: Zebrafish Aggression (Scientific Process 4.1):

  • Research Question: How does environmental variation (hypoxic/low oxygen vs. normoxic/high oxygen) affect aggression?

  • Hypothesis: Both developmental environment (DE) and behavioral test environment (BE) affect aggression.

  • Predictions:

    • (a) Only DE affects behavior: Aggression highest in normoxic DE regardless of BE.

    • (b) DE and BE act independently: Aggression higher in normoxic BE and highest for fish from normoxic DE.

    • (c) DE and BE interact: Aggression is highest when BE and DE match.

  • Methods: Raised siblings in either normoxic or hypoxic DE; tested aggression (mirror biting) in both environments.

  • Results: Fish displayed higher aggression when in test chambers similar to their developmental environment.

  • Conclusion: There is a significant interaction between developmental and behavioral environments.

Social Environment and Birdsong Development

• Bird Vocalizations:

  • Bird calls: Short vocalizations; usually innate in all birds.

  • Bird song (Passeri): Complex vocalizations.

• Learning Styles:

  • Closed-ended learners: Must hear a conspecific song tutor shortly after hatching (during the "sensitive period") to learn properly.

  • Open-ended learners: Can acquire new song elements throughout their entire life.

• Song System Biology:

  • Posterior nucleus: Sound production.

  • Anterior nucleus: Song learning.

  • Gene Expression: Exposure to song influences genes like $ZENK$ and $FoxP2$.

• Knockdown Research (Haesler et al. 2007):

  • Methods: Used a knockdown technique (inserting short RNA sections via virus) to reduce $FoxP2$ expression in zebra finches ($Taeniopygia\,guttata$).

  • Analysis: Used spectrograms (sonograms) to characterize acoustic structures.

  • Results: Knockdown birds had lower $FoxP2$ expression and omitted specific syllables in their songs.

  • Conclusion: $FoxP2$ is required for normal song development.

Gene-Environment Interactions (GEI)

• Reaction Norm: The range of behaviors expressed by a single genotype across different environments; also known as Phenotypic Plasticity.

• Gene-environment interaction (GEI): Occurs when the environment impacts one genotype more significantly than others.

• Expanded Phenotypic Variance Formula: $V_P = V_G + V_E + V_{GEI}$.

• Case Study: Rover and Sitter fruit flies (Kent et al. 2009):

  • Observation: Larval fruit flies differ in foraging trail length (Rovers = long trails; Sitters = short trails).

  • Methods: Exposed adult phenotypes to different food availability (fed vs. food-deprived).

  • Results:

    • Fed Rovers had high food-leaving scores; food-deprived Rovers had low scores.

    • Sitters showed no difference in behavior based on food availability.

  • Conclusion: Gene-environment interactions affect both behavioral and metabolic traits differently across genotypes.

Genomic Approaches to Behavior

• Genomic correlations help link gene expression patterns with specific behavioral phenotypes.

• Case Study: Scouting behavior in Honeybees ($Apis\,melifera$):

  • Observation: Only $5\%$–$25\%$ of individuals act as food scouts.

  • Methods: Identified scouts through feeding experiments; conducted whole-genome RNA analysis.

  • Results: $16\%$ of mRNA transcripts differed between scouts and non-scouts.

  • Gene Types: Many were related to neurochemical signaling associated with vertebrate novelty-seeking behavior.

• Case Study: Mating tactics in Black-faced blennies ($Tripterygion\,delaisi$):

  • Tactics: Territorial males (bright color, defend territory) vs. Sneaker males (dull color, look/act like females).

  • Results: Territorial males expressed more mRNA than sneakers and females.

  • Findings: Differences in gene expression were greater between the two male phenotypes than between males and females.

  • Conclusion: Phenotypic plasticity in males is more strongly tied to gene expression than to biological sex.

Animal Personalities and Behavioral Flexibility

• Animal Personalities: Consistent individual differences in behavior within a population over time or across different contexts.

• Categories: Bold vs. Shy, Highly Exploratory vs. Non-Exploratory, Active vs. Inactive, Aggressive vs. Docile.

• Case Study: Heritability of great tit exploratory behavior:

  • Question: Is exploratory behavior heritable?

  • Methods: Recorded hops/flights in cages with trees; conducted a selection experiment for "fast" and "slow" lines over four generations.

  • Result: Strong changes in behavior were observed based on selection.

  • Conclusion: Exploratory behavior in great tits is a heritable trait.

Personalities and Predation Risk in Salamanders

• Context: Streamside salamander larvae must balance feeding (to avoid pool drying) and avoiding sunfish predators.

• Research Question: How do larvae respond to predation risk?

• Methods: Exposed larvae to low risk (tap water) and high risk (predator-scented water).

• Results:

  • Larvae spent more time in the open in control treatments.

  • There was a positive correlation in time spent outside refuge across both treatments (those bold in controls stayed bold under risk).

• Conclusion: Salamanders exhibit distinct personalities (behavioral syndromes) with a genetic component.

Models for Personalities: Fitness Tradeoffs

• Research Question (Wolf et al. 2007): Why do personalities exist?

• Assumption: Personalities reflect adaptive behavior tied to life-history tradeoffs.

• Model 1 (No competition/predation):

  • Bold (high exploration) individuals invest in future reproduction.

  • Shy (low exploration) individuals invest in current reproduction.

  • Result: Both can have equal lifetime fitness.

• Model 2 (With competition/predation):

  • Bold individuals invest more in current reproduction.

  • Shy individuals invest primarily in future reproduction.

  • Result: Both can achieve equal fitness; personalities are maintained in the population via tradeoffs.

Case Study: Jumping Spider Personalities ($Marpissa\,muscosa$)

• Research Question: Does rearing environment affect personality?

• Methods: Divided offspring into three environments:

  1. Deprived (reared alone).

  2. Socially enriched (raised with siblings).

  3. Physically enriched (raised with natural/artificial objects).

• Results:

  • Behaviors remained consistent over time.

  • Socially enriched individuals showed greater latency to explore.

  • Physically enriched individuals visited more of the test arena.

• Conclusion: Rearing environment influences the development and expression of personality in jumping spiders.