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Social Insects and the Evolution of Sterility
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• Social Insects and the Evolution of Sterility
• I. Characteristics of social insects
  Hymenoptera Isoptera
• Cooperative brood care
• Overlap of generations
• Reproductive castes and division of labor
  fertile queen and sterile workers

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• A. evolved from solitary ancestor (mass
  provisioning)

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Primitively social
• small, annual colonies
• indistinct morphological castes
• no complex communication systems
• reproductive division of labor based on dominance
  and maintained by aggression
• worker sterility may be facultative

Exoneura
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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• large, perennial colonies

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• distinct castes
• -- reproductive castes

fire ant
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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• distinct castes
• -- sterile worker castes
• polyethism division of labor based on age

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• distinct castes
• -- sterile worker castes
• polymorphism division of labor based on
  morphological castes

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• distinct castes
• -- sterile worker castes
• polymorphism division of labor based on
  morphological castes

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• reproductive division of labor maintained, not by
  dominance and aggression, but by pheromones from
  queen and brood
• workers are obligately sterile (may still be able
  to produce eggs, but cant mate can lay only
  unfertilized eggs ? males)

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• Social Insects and the Evolution of Sterility
• II. Levels of Sociality
• B. Advanced (highly) social eusocial
• complex communication systems

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 1. haplodiploidy
• -- if workers share same father and queen
  mother, r 0.75
• supersisters (for sisters and brother, r
  ¼)
• -- if workers have different father, r ¼
  (half-sisters)
• -- if females reproduce themselves, r
  0.50
• -- ? female workers more closely related to
  supersisters than to own offspring (but males are
  not)

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 1. haplodiploidy
• limitations to haplodiploidy and kinship
• multiple matings by queen
• r 0.25 0.5/me (me no. of matings)
• -- if queen mates with 2 males, average r for
  sisters 0.50
• ? unless workers can discriminate super- from
  half-sisters, will on average raise siblings that
  are related to them by same amount as their own
  offspring

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 1. haplodiploidy
• limitations to haplodiploidy and kinship
• investment in supersisters and brothers if queen
  mates once
• -- females related to supersisters by ¾ to
  brothers by ¼
• -- if workers invest equally in each,
• average r (3/4 ¼) 0.5
• 2
• ? unless workers can bias their brood care
  towards females over males, will on average raise
  siblings that are related to them by same amount
  as their own offspring
• haplodiploidy will favor evolution of sterility
  only if workers can preferentially rear
  reproductive super-sisters over half-sisters or
  brothers.

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 2. philopatry
• -- results in groups of related females
• -- can cooperate to build and defend nest
• -- can lead to dominance hierarchy and
  reproductive division of
• labor

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 3. progressive provisioning
• -- results in overlap of generations

progressive provisioning
Mass provisioning
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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 4. size inequalities
• -- size of adults can vary widely
  determined by food received as
• larva
• -- small individuals have little chance of
  independent reproduction
• -- will select for
• remaining with nest
• and helping can gain
• inclusive fitness
• benefits
• -- could be maternally
• manipulated

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• A. Intrinsic factors
• 5. internal symbionts
• -- Isoptera shed gut symbionts with each
  molt
• -- must reseed gut via proctodeal feeding
• -- requires staying at nest and overlap of
  generations

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• B. Extrinsic factors
• 1. the nest (cost of independent reproduction
  requires defense)

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• Social Insects and the Evolution of Sterility
• III. Factors contributing to the evolution of
  eusociality
• B. Extrinsic factors
• 2. nest predation and nest parasitism
• -- favors cooperation
• -- can lead to dominance and reproductive
  division of labor
• -- can favor remaining at nest and not
  trying to engage in independent reproduction

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• Social Insects and the Evolution of Sterility
• IV. Hypotheses for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• -- workers more related to reproductive
  super-sisters than own offspring
• -- will be selected to forego personal
  reproduction and help raise
• reproductive super-sisters (directly and
  indirectly)
• -- but only if workers can skew investments
  toward super-sisters over half-sisters and
  brothers (only if workers can capitalize on
  relatedness asymmetries)

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• prediction 1 only females should be sterile
  helpers, never males
• TRUE

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• prediction 2 sterile altruists should occur in
  non-hymenopteran insects that are haplodiploid
• Ex Gall-forming thrips

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• Gall-forming thrips
• Thrips are haplodiploid
• Within gall, some individuals function as
  soldiers
• Soldiers are sterile
• All soldiers are female never male

Reproductive female
Soldier
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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• prediction 3 if queen mates once, workers and
  queen should
• favor different investment
  strategies for reproductive females and
• males
• -- queen selected to prefer 11
• investment ratio
• -- workers selected to prefer 31
• investment ratio
• -- workers should win

male
In many ant species, queen mates once and
colonies have 31 investment ratio in female and
male reproductives
female
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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• prediction 4 if queen mates multiply, workers
  should not strongly bias investment ratios toward
  reproductive sisters over brothers
• r 0.25 0.5/me (me no. of matings)
• if Q mates 1X rs 0.75 rb 0.25
• if Q mates 2X rs 0.50 rb 0.25
• if Q mates 3X rs 0.42 rb 0.25
• if Q mates 4X rs 0.375 rb 0.25
• if Q mates 5X rs 0.35 rb 0.25
• if Q mates 12X rs 0.29 rb 0.25

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• Formica sp.
• Queens can be either
• monogamous or polyandrous
• Workers provide all brood
• care
• Colonies with monogamous
• queens workers heavily
• bias investment toward
• reproductive sisters
• Colonies with polyandrous
• queens no bias toward
• female vs. male reproductives

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• prediction 5 if queen mates multiply, workers
  should preferentially
• raise super-sister queens over half-sister
  queens (should show kin
• discrimination)

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• -- Use instrumental insemination to create
  colonies that contained
• two patrilines of workers (distinguished
  morpholoigcally or genetically)
• -- manipulate colonies to raise new queens some
  will be super-sisters (SS)
• and some half-sisters (HS) of the workers
• -- monitor worker interactions with developing
  queens
• feed queen larvae
• incubate sealed cells
• perform vibration signals on cells
• Compare observed and expected number of SS and HS
  interactions

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Workers feeding developing virgin queens (VQs) (N
16 colonies)
(P lt 0.05 overall, but not consistent between
colonies)
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Workers incubating developing virgin queens
(VQs) (N 16 colonies)
(P gt 0.05)
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Workers vibrating queen cells (N 16 colonies)
(P lt 0.05 overall, but not consistent between
colonies)
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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• prediction 6 if queen mates multiply, workers
  should police
• worker laid eggs (haploid) and raise only
  queen-laid haploid eggs
• -- workers related to queens sons
  (brothers) r ¼
• -- if workers lay eggs, vast majority will
  be nephews (r 1/8)
• -- in honey bees and some ants, queen marks
  eggs with a
• pheromone worker-laid eggs lack pheromone
• -- workers eat worker-laid eggs (worker
  policing), but not queen
• laid eggs

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• A. Kin selection hypothesis (haplodiploid
  hypothesis) for Hymenoptera
• Much evidence suggests that haplodiploidy has
  facilitated the evolution of eusocialty in
  hymenoptera.
• But, haplodiploidy is not a prerequisite or a
  guarantee of eusociality
• -- all hymenoptera are haplodiploid, but most
  are solitary
• -- termites have sterile workers, but are
  diploid
• -- some predictions for hypothesis are not
  upheld
• In some species, ancestrally queen mated once and
  haplodiploidy contributed to evolution of sterile
  castes. Subsequently, polyandry may have evolved
  for increased genetic diversity in offspring
• (may explain honey bee and some ants)

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• B. Mutualism and Reciprocity hypotheses
  (emphasis is on environmental constraints, not
  just relatedness)
• Individuals help and forego own reproduction
  because
• -- they have little chance of reproducing alone
• -- they have a chance of inheriting the nest if
  the queen dies

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• B. Mutualism and Reciprocity hypotheses
• Ex Polistes dominulus primitively social wasp
• groups of females join together to help build
  nest may be related or unrelated
• high nest predation rates almost 100 of small
  nests are lost
• all founding females are mated and fertile, but
  nest has one dominant individual (queen) who
• lays 95 of eggs others
• raise her young
• (facultatively sterile)
• helpers form dominance
• hierarchy subordinate females
• will lay own eggs, but queen eats
• them

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• B. Mutualism and Reciprocity hypotheses
• Ex Polistes dominulus primitively social wasp
• in 45 of nests, queen is lost
• in 77 of these nests, top-
• ranking helper becomes new
• queen
• ? can evolve sterile helpers
• even with low or no relatedness
• because of environmental
• constraints

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• B. Mutualism and Reciprocity hypotheses
• -- the role of the nest
• Valuable resource filled with brood and food
• If required for successful reproduction, and if
  cannot be constructed or maintained alone, then
  can favor staying and helping even with low
  levels of relatedness
• All eusocial species characterized by elaborate
  nest architecture
• Predict that eusociality should also evolve in
  other species with elaborate nest structure

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• Ex naked mole rate
• subterranean mammal
• eusocial one breeding female and sterile
  workers
• build elaborate system of tunnels

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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• C. Colony-level selection
• -- sterility evolves from selection acting at
  level of colony colonies with sterile workers
  produce more new colonies than those without
• -- can explain evolution of castes among sterile
  workers
• -- helping relatives (kin selection) could
  facilitate evolution of sterility, but not be the
  primary driving force.
• -- in fact, high levels of nepotism
  (preferential treatment of closer relatives)
  could actually lower colony success and
  reproduction

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• Ex Queen replacement in honey bees
• When replacing an old queen, colonies raise
  multiple VQs (virgin queens)
• When VQs emerge, they battle to the death until
  there is a sole survivor who inherits natal nest

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• Ex Queen replacement in honey bees
• Workers interact with VQs at high rates and may
  determine outcome of battles
• Perform vibration signals on VQs

• -- within same colony, some VQs receive
• hundreds of signal/hr others few or none
• -- workers preferentially direct signals toward
• certain VQs
• -- function of signal unclear, but VQs vibrated
• at higher rates
• survive longer
• produce more piping
• kill more rivals
• become new laying queen

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• Ex Queen replacement in honey bees
• Give workers a choice of interacting with SS and
  HS queens of higher and lower quality
• If Kin selection (haplodiploidy hypothesis) is
  primary driving force, then workers should
  vibrate SS queens regardless of quality
• If Colony-level selection is primary force, then
  workers should vibrate higher quality queens
  regardless of relatedness

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• Instrumentally inseminated queens with equal
  volume of semen from one African and one European
  drone (distinguished by color)
• Colonies contained European-patriline workers
  (yellow) and African-patriline workers (dark)
• Manipulated to raise VQs, some of African and
  some European paternity.

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• African-patriline VQs kill more of their rivals
  and are more likely to become new laying queen of
  their colony. A-patriline queens may be superior
  (at least in fighting ability)
• Workers therefore had opportunity to interact
  with SS and HS VQs of higher (African queens) and
  lower (European queens) quality
• Compared observed and expected number of
  vibration signals performed on VQs

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Kin preferences in vibration signal performance
A-paternity workers
E-paternity workers
(P lt 0.01)
(P lt 0.01)
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VQ race preferences in vibration signal
performance
(P lt 0.01)
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• Social Insects and the Evolution of Sterility
• IV. Hypothesis for the evolution of eusociality
• Summary
• Evolution of sterility is unclear
• Involved mutualism and reciprocal altruism
• Kin selection enhanced the evolution, but did not
  drive it
• Colony level selection probably more important
  than realized