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Chapter 9. Sex allocation/(ratio) distorters. 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. Nuclear genes. Sex chromosome meiotic drive:

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chapter 9

Chapter 9

Sex allocation/(ratio) distorters

sex ratio distorters
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
nuclear genes
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

b chromosomes
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
cytoplasmic genes
Cytoplasmic genes
  • Only transmitted trough the maternal line > selection for SR distortion
  • Include mitochondria and micro-organisms (Wolbachia, cardinium)
  • Several mechanism found to increase the amount of female offspring produced
  • 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
maternal sex ratio
Maternal Sex Ratio
  • Influences the fertilization rate
  • Found in some parasitoids
  • Should rapidly spread to fixation



male killers
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
parthenogenesis induction
Parthenogenesis induction
  • In haplodiploids: unfertilized eggs develop into females
  • Genome duplication
  • Found in several insect taxa
cytoplasmic incompatibility
Cytoplasmic incompatibility
  • Not strictly SR distorter
  • In haploids male unaffected

>leads to male biased SR

/only males

genomic imprinting
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
spread of sr distorters
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
  • Spread dependent on fertilization rate
  • It can only invade when FR > 0.5
  • LMC causes female biased SR, but small patch size selects against PSR
  • Presence of MSR, although PSR selects against MSR
male killing
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
the consequences of sr distorters
The consequences of SR distorters
  • Compensatory SR adjustment

Only under imperfect transmission

Under high transmission, no selection

>no gene flow between infected and uninfected part population

other effects of sr distorters
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)
  • 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

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