Chapter 9 Sex allocationratio distorters Sex ratio distorters

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: 1. 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 2. 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 • 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 • 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

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

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

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

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

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 • 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

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

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 1. 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 • 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)

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

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