Sex allocation/(ratio) distorters

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Chapter 9

• Sex allocation/(ratio) distorters

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Sex ratio distorters

• The ESS SR may differ between the point of view
  of different genes within an individual conflict
  over SR
• SR distorting elements
• Nuclear genes
• Cytoplasmatic elements

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Nuclear genes

• Sex chromosome meiotic drive
• Y chromosome drive leads to male bias
• Y chromosome only transmitted by males so a gene
  on Y that will lead to more male offspring will
  spread
• X chromosome drive leads to female bias
• X drive at the cost of Y
• Spread slower
• Commonly found in Diptera
• More common than Y drive

Aedes aegypti
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B chromosomes

• Supernumerary chromosome, not required for
  fitness
• Generally no effect on SR but
• PSR in Nasonia vitripennis, only male offspring
  produced
• Ultimate selfish element, ensures own
  transmission at cost of the rest of the genome

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Cytoplasmic genes

• Only transmitted trough the maternal line gt
  selection for SR distortion
• Include mitochondria and micro-organisms
  (Wolbachia, cardinium)
• Several mechanism found to increase the amount of
  female offspring produced

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Feminizers

• Override the nuclear sex determination
• Found in woodlice, mites, parasitoids and shrimp
• Frequency often lower than expected, might be
  caused by risk of producing intersexes

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Maternal Sex Ratio

• Influences the fertilization rate
• Found in some parasitoids
• Should rapidly spread to fixation

?
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Male killers

• Two types early and late
• Early resources allocated to sons can be used by
  daughters with related bacteria
• Late males used as vectors for horizontal
  transfer

9
Parthenogenesis induction

• In haplodiploids unfertilized eggs develop into
  females
• Genome duplication
• Found in several insect taxa

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Cytoplasmic incompatibility

• Not strictly SR distorter
• In haploids male unaffected
• gtleads to male biased SR

/only males
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Genomic imprinting

• Differential expression alleles dependent on
  parental origin
• Alleles from different backgrounds can disagree
  over SR
• Imprinting as a battle ground for conflict over
  SR

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Spread of SR distorters

• Often not fixed in populations
• Possible explanation
• Balancing selection
• Reduced fertility/survival infected individuals
• Sexual selection, avoiding infected individuals
• Suppressors
• Sex chromosome linked
• Autosomal Fisherian selection

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PSR

• Spread dependent on fertilization rate
• It can only invade when FR gt 0.5

• LMC causes female biased SR, but small patch size
  selects against PSR
• Presence of MSR, although PSR selects against MSR

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Male killing

• Spread dependent on transmission rate
• High transmission fixation, population
  extinction
• Low transmission intermediate frequency
• Resource reallocation among offspring
• Survival cost
• Mating preference
• Selection for nuclear suppression because of
• Increase in fecundity
• Fisherian advantage of rare sex

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The consequences of SR distorters

• Compensatory SR adjustment
• Only under imperfect transmission
• Under high transmission, no selection
• gtno gene flow between infected and uninfected
  part population

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Other effects of SR distorters

• Sex role reversal, due to biased SR
• The evolution of new sex determination systems
  e.g haplodiploidy
• Adjustment of breeding system e.g. larger
  clutches, multiple mating, reallocation of
  resources among offspring
• Selective sweep, hitchhiking effect, reduced
  recombination (X drive)

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Conclusion

• Main topics for future research
• What controls variation across taxa
• The interplay between different distorters
• Consequences for host biology
• Lots of theory, but need for empirical data

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Final thoughts

• Why so often in haplodiploids?
• Mechanisms how does the drive work, details of
  mechanisms might influence effects