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Chapter 9

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

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  1. Chapter 9 Sex allocation/(ratio) distorters

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. Maternal Sex Ratio • Influences the fertilization rate • Found in some parasitoids • Should rapidly spread to fixation ? ?

  8. 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

  10. Cytoplasmic incompatibility • Not strictly SR distorter • In haploids male unaffected >leads to male biased SR /only males

  11. 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

  12. 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

  13. PSR • 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

  14. 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

  15. 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

  16. 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)

  17. 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

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

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