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Behavioral Genetics & Development – Lecture 2 Vocabulary

Lecture Scope & Recap

  • Builds directly on Lecture 2 (Tinbergen’s Four Questions, proximate vs. ultimate explanations, hypothesis construction & testing)
  • Today’s focus: developmental and genetic bases of behavior; explicit goal to relate proximate questions to ultimate ones in every example

Learning Objectives (Stated)

  • Identify & define key genetic/developmental terms that influence behavior
  • Contrast and evaluate alternative hypotheses for any behavioral phenomenon
  • Understand methodological pipelines: observation → hypothesis → prediction → experimental/quantitative test → inference
  • Prepare for Canvas quiz covering this material (date: July 31, 2025)

Core Terminology & Concepts

  • Genotype = complete DNA sequence inherited from parents (fixed across lifespan)
  • Environment = all external (abiotic & biotic) and internal (social, hormonal) conditions influencing gene expression
  • G \times E interaction = phenotype results from non-additive interplay of genotype and environment
  • Ontogeny = organism’s developmental trajectory from early stage onward
  • Phenotypic plasticity = capacity of one genotype to express >1 phenotype depending on cues
    • Polyphenism = discrete, discontinuous plasticity (e.g., queen vs. worker)
  • Maternal effect = offspring phenotype change caused by mother’s phenotype/physiology independent of alleles inherited
  • Epigenetics = heritable changes in gene activity without DNA-sequence alteration (DNA methylation, histone modification, non-coding RNA)
  • Supergene = linked block of genes inherited as one unit due to suppressed recombination; often generated by chromosomal inversions and underlies fixed polymorphisms

Molecular Foundation – Central Dogma

  • DNA regions (genes) are transcribed → mRNA (nucleus) → translated by ribosomes → protein
  • Standard schematic: \text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}
  • Poll check-in: students click the cellular location where translation occurs (ribosome in cytoplasm/RER)

Hypothesis Logic in Animal Behavior

  • Hypotheses must be mutually exclusive; poll used to test student recognition of this principle
  • Development of alternative hypotheses and clear predictions critical before data collection
  • Ways we test hypotheses:
    • Controlled lab manipulations (e.g., RNAi knockdown in bees)
    • Field manipulations (e.g., hormone implants in squirrels)
    • Natural experiments/comparative studies (e.g., burrowing in Peromyscus spp.)

Genotype vs. Environment Thought Exercise

  • Slide prompt “Genotype OR Environment – Behavior 1/2” used to emphasize false dichotomy; modern view: behaviors arise through G \times E synergism

Case Study 1 – Honey Bee Worker Ontogeny

  • Eusocial caste system: workers (sterile females), queens (fertile females), drones (haploid males)
  • Age-polyethism in workers (cleaning → nursing → food packing → foraging)
    • Probability curve of task performance vs. age provided (Fig 3.1)
  • Gene-expression profiling (Fig 3.2):
    • Young (Y) vs. Old (O); Nurses (N) vs. Foragers (F)
    • Differentially expressed genes correlate with behavioral state more than chronological age ⇒ behavior-linked transcriptional program

Royal Jelly & Caste Determination

  • All larvae receive some royal jelly; future queens receive it throughout larval life, workers switched to honey/pollen
  • Proteins in royal jelly:
    • Silence Dnmt3 (DNA methyltransferase) → global hypomethylation
    • Initiate epigenetic cascade: altered DNA accessibility, histone tail modifications, small ncRNA action
  • RNAi Experiment (Fig 3.4):
    • Silenced Dnmt3 in larvae + normal worker diet
    • Outcome: queen phenotype frequency ≈ larvae fed full royal jelly ⇒ supports hypothesis that DNA methylation state, not nutrient calories, directs caste
  • Additional hypothesis: diet-specific phytochemicals in honey/pollen may actively suppress queen development; suggests dual regulatory model (promotion + suppression)

Case Study 2 – Genetically Based Burrowing & Migration

  • Peromyscus mice (Fig 3.6):
    • Oldfield mice build long, complex burrows early (precocious)
    • Deer mice build short/simple burrows later (delayed)
    • Trait persists in lab ⇒ strong genetic component
  • Black vs. common redstart:
    • Species-specific night “migratory restlessness” timing (days 0–225 post-hatch chart) indicating innate migratory program

Evo-Devo Approaches

  • Forward genetics (phenotype → gene): e.g., search for loci controlling mouse burrow length
  • Reverse genetics (gene → phenotype): e.g., CRISPR/RNAi knockdown of Dnmt3 or vasopressin receptors in voles
  • Vole example: prairie vole (monogamous) vs. montane vole (polygynous); differing vasopressin receptor distribution identified via reverse genetics

Early-Life Developmental Hypotheses

  1. Developmental Constraint: poor early environment → lifelong fitness cost (no predictive match)
  2. Predictive Adaptive Response (PAR): early cues tune phenotype to expected adult environment

Empirical Tests

  • Amboseli baboons:
    • Females born during drought show reduced fertility if adult drought recurs ⇒ supports constraint, not PAR
    • Discussion prompt: what result would support PAR? (Higher fertility in drought for drought-born)
  • North-American red squirrels (Dantzer 2013):
    • High cone years → population density ↑ → maternal cortisol ↑
    • Elevated maternal cortisol experimentally increases offspring growth rate
    • Outcomes: faster growth ⇒ higher first-winter survival, but reduced adult lifespan ⇒ adaptive trade-off; qualifies as maternal effect possibly favored in fluctuating density environments

Phenotypic Plasticity & Polyphenism

  • Three distribution patterns (Fig 3.28):
    1. High variance continuous trait (plastic or genetic)
    2. Low variance continuous trait (canalized)
    3. Discontinuous multi-modal trait (polyphenism)
  • Types of polyphenism:
    • Density-dependent (locust solitary ↔ gregarious)
    • Switch initiated by social tactile cues; maintained via epigenetic + ongoing social input (Ernst 2015)
    • Predator-induced, food-induced, socially-induced in fishes & amphibians
  • Social-status plasticity in cichlid Astatotilapia burtoni:
    • Removal of dominant male → subordinate male escalates aggression within minutes
    • Surge in egr\text{-}1 gene activity in preoptic brain region; returns to baseline after dominance established (Fig 3.15)

Moving Beyond Plasticity – Behavioral Polymorphism

  • Behavioral polymorphism = fixed alternative strategies; plasticity lost
  • Often underpinned by supergenes (chromosomal inversions blocking recombination)

Ruff (Philomachus pugnax) Example (Fig 3.17)

  • Three male morphs: Independent (territorial, dark plumage), Satellite (non-territorial, light), Faeder (female-mimic)
  • Each satellite & faeder morph controlled by distinct supergenes (~100 linked genes) influencing plumage + reproductive physiology

White-Throated Sparrow Supergene

  • Tan-striped (TS) vs. White-striped (WS) morphs; obligate disassortative mating (TS×WS only)
  • WS possess rearranged chromosome 2 (ZAL2m) locked with ancestral ZAL2 → supergene
  • Key genes within inversion:
    • ESR1 (estrogen receptor-α) — modulates aggression/parental care
    • VIP (vasoactive intestinal peptide) — affects prolactin & social behaviors
  • WS show higher ESR1 & VIP expression in aggression centers → behavioral dimorphism maintained genetically

Genetic Basis of Diet Choice – Garter Snakes

  • Coastal California neonates prefer banana slugs, inland neonates avoid
  • Slug-cube assay on lab-born offspring:
    • Coastal genetic line → tongue-flick response high
    • Inland line → low response
  • Supports heritable chemosensory preference; discussion on allelic variation vs. gene-by-experience effects

Poll / Activity Highlights (Formative Assessment)

  • “Define gene.” → consensus: DNA segment coding for protein/RNA
  • True/False: “Organisms are set and cannot change phenotype throughout life.” → class overwhelmingly answered False (reinforces plasticity concept)
  • Snowball Q-A exercise: students anonymously contribute one question + one biology fact; promotes engagement & identifies misconceptions

Ethical, Philosophical & Practical Considerations

  • Epigenetic inheritance blurs line between genetic determinism and environmental influence; societal debates on trans-generational effects (nutrition, toxins)
  • Understanding maternal effects crucial for wildlife management under climate change (e.g., squirrel density fluctuations)
  • Supergene research informs conservation genetics because suppressed recombination may limit adaptive potential
  • Bee caste manipulation via diet/epigenetics underpins apicultural practices and raises questions on pesticide impact on developmental pathways