Sex Bias

males and females differ in

• physical traits

• behavior

—> occurs most higher in eukaryotes

male: extravagant / exaggerated secondary sexual characteristics (e.g. peacock’s tail)

—> these traits often deletarious (gefährden) to individuals survival

Theory of sexual selection, 2 components:

• Male-male competition - males use trait to compete with each other for food, territory, access to females, ….

  —> traits evolve fast

• Female choice - females prefer to mate with males with ‘attractive’ phenotypes

Yes, with exception of genes on Y chromosome

most differences —> differential expression of genes that are in both

genes that are expressed at higher level in one sex than in the other

fold-change cutoff / statistical test

• M/F > 2 = male-biased gene (MBG)

• M/F < 0.5 = female-biased gene (FBG)

• 0.5 < M/F < 2 = unbiased gene (UBG)

often M/F presented on log scale:

• log2(M/F) > 1 = MBG

• log2(M/F) < -1 = FBG

• -1 < log2(M/F) < 1 = UBG

2-fold cutoff

• 2.000 MBG (avg M/F = 6)

• 2.000 FBG (avg M/F = 2.5)

• 9.000 UBG

—> 30% are sex-biased

statistical

• 2.800 MBG

• 4.000 FBG

—> 85% are sex-biased

—> MBG tend to have stronger expression bias than FBG

under-presentation of MBG on X-chromosome —> demasculinization

enrichment of FBG on X-chromosome —> femininization

X contains 16% of genes of genome

• 11% of MBG on X

• 21% of FBG on X

• 16% of UBG on X

• Dosage

• X-inactivation (or X suppression)

• Sexual conflict

males: 1 copy of X, females: 2 copies of X

expectation: X-linked genes have 2X expression in females

Dosage Compensation in Drosophila: in somatic tissues, male X transcription is doubled to equalize expression

—> does not occur in male germline

—> explain under-representation of MBG on X (when have reproductive organs)

X-chromosome becomes transcriptionally inactive in male germline

—> expression of MBG suppressed during spermatogenesis if X-linked

—> lead to under-representation of MBG on X, if many MBG expressed in testes/sperm

X spends 2X time in females —> female outweigh male interests

—> lead to reduced expression of X-linkd MBG

Ka/Ks

possible differences in mutation rates

MBG —> faster rate of protein evolution in MBG

FBG slightly faster than UBG

strongest difference in reproductive organs

• less selective constraint

  —> accumulate many neutral amino acid changes that have no effect on fitness —> faster evolution

• more positive (or sexual) selection

  accumulate more adaptive amino acid replacements —> rapid evolutionary changes

selection more efficient in regions of high recombination (each mutation selected independently)

positive selection —> positive correlation between divergence and recombiantion rate

in Dros

FBG and UBG slightly negative correlation —> most amino acid changes are deleterious and removed by purifying selection

compares ratio of divergence at nonsynonymous and synonymous sites (Dn/Ds)

to polymorphism at nonsynonymous and synonymous sites (Pn/Ps)

for Drosophila MBG (Dn/Ds)(Pn/Ps) ≈ 2

• for FBG ≈ 1.2

• for UBG ≈ 0.9

MBG excess of nonsynonymous divergence

X-linked MBG stronger signal of positive selection than autosomal MBG

X-linked MBG (Dn/Ds)(Pn/Ps) ≈ 3.4

—> highest divergence

• Fast-X effect

  rate of adaptive evolution expected faster on X chromosome

• Large-X effect

  large excess of incompatibile loci on X chromosome relative to autosomes