Download Chapter 10 (Conflict II)

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
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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
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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
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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
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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
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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
/only males • In haploids male
unaffected
>leads to male
biased SR
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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
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
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Male killing
• Spread dependent on transmission rate
– High transmission: fixation, population extinction
– Low transmission: intermediate frequency
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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