Sexually antagonistic (SA) selection arises when alleles that increase fitness in one sex impose a cost on the other. One of its most consequential effects in evolutionary genetics is its potential to reshape the architecture of sex chromosomes. When SA loci reside on autosomes, natural selection can favor chromosomal fusions that bring those loci into physical linkage with the sex-determining region, because such linkage ensures that each allele is reliably transmitted to the sex it benefits. This logic extends to unusual sex-chromosome systems: in organisms with univalent sex chromosomes (YO or WO systems, where one sex carries only a single sex chromosome with no pairing partner), SA-driven fusions between autosomes and the univalent Y or W would be positively selected whenever a beneficial-to-males (or beneficial-to-females) allele exists nearby. Over time, such fusions would convert a YO or WO system into a canonical XY or ZW system, providing a mechanistic explanation for why univalent systems appear evolutionarily transitory rather than stable endpoints. This theoretical argument draws on foundational work by Charlesworth & Charlesworth (1980) and van Doorn & Kirkpatrick (2007), and is articulated in the context of broader questions about why certain sex-chromosome configurations are rarely observed across taxa (Why not Y naught 2022, Finding 1).
The implication is that SA selection functions not merely as a force maintaining sex-limited alleles within existing sex chromosomes, but also as an active driver of sex chromosome turnover — continuously remodeling the genomic regions that differentiate the sexes by pulling new autosomal segments into the sex-determining linkage group.
Sexual antagonism happens when a gene that helps one sex hurts the other. This conflict can reshape how sex chromosomes are organized in a genome. Imagine an autosome — a chromosome that isn’t a sex chromosome — carries an allele that benefits males but harms females. Natural selection can favor chromosomal fusions that physically link this allele to the male sex-determining region, because that linkage ensures males reliably inherit the helpful copy and females reliably avoid it.
This principle matters especially in organisms with univalent sex chromosomes, like YO or WO systems, where one sex carries a single sex chromosome with no pairing partner. When sexually antagonistic alleles exist nearby, fusions between autosomes and the unpaired Y or W chromosome would be favored by selection whenever the fused allele benefits males (or females, in WO systems). Over evolutionary time, repeated fusions like these would gradually convert a YO or WO system into a standard XY or ZW system — the sex-chromosome configurations we see in most animals today. This theory, developed from foundational work by Charlesworth & Charlesworth (1980) and van Doorn & Kirkpatrick (2007), helps explain why univalent systems seem evolutionarily temporary rather than stable long-term arrangements (Why not Y naught 2022, Finding 1).
The bigger picture: sexual antagonism is not just a force that maintains existing sex-specific genes within sex chromosomes — it actively drives sex chromosome turnover by continuously pulling new autosomal segments into the sex-determining region.
Sexually Antagonistic Selection
Current understanding
Sexually antagonistic (SA) selection arises when alleles that increase fitness in one sex impose a cost on the other. One of its most consequential effects in evolutionary genetics is its potential to reshape the architecture of sex chromosomes. When SA loci reside on autosomes, natural selection can favor chromosomal fusions that bring those loci into physical linkage with the sex-determining region, because such linkage ensures that each allele is reliably transmitted to the sex it benefits. This logic extends to unusual sex-chromosome systems: in organisms with univalent sex chromosomes (YO or WO systems, where one sex carries only a single sex chromosome with no pairing partner), SA-driven fusions between autosomes and the univalent Y or W would be positively selected whenever a beneficial-to-males (or beneficial-to-females) allele exists nearby. Over time, such fusions would convert a YO or WO system into a canonical XY or ZW system, providing a mechanistic explanation for why univalent systems appear evolutionarily transitory rather than stable endpoints. This theoretical argument draws on foundational work by Charlesworth & Charlesworth (1980) and van Doorn & Kirkpatrick (2007), and is articulated in the context of broader questions about why certain sex-chromosome configurations are rarely observed across taxa (Why not Y naught 2022, Finding 1).
The implication is that SA selection functions not merely as a force maintaining sex-limited alleles within existing sex chromosomes, but also as an active driver of sex chromosome turnover — continuously remodeling the genomic regions that differentiate the sexes by pulling new autosomal segments into the sex-determining linkage group.
Supporting evidence
- SA-driven autosome–univalent fusions as a turnover mechanism. Theory predicts that if SA loci reside on autosomes, fusions between those autosomes and a univalent Y or W chromosome will be selectively favored, because they create linkage between the SA locus and the sex-determining locus. This mechanism would destabilize YO/WO systems and convert them to XY or ZW configurations. Indirect empirical support comes from phylogenetic patterns in Polyneoptera, where XX/XO systems appear to transition to XY systems, consistent with SA-driven fusion dynamics (Why not Y naught 2022, Finding 1).
Contradictions / open disagreements
- The primary tension here is between the theoretical plausibility of SA-driven fusion and alternative explanations involving genetic drift. The verbal model synthesized in Why not Y naught 2022, Finding 1 does not provide quantitative estimates of the relative contributions of SA selection versus drift, especially in small populations where stochastic forces could dominate. Empirical evidence directly demonstrating SA-driven transitions in YO or WO systems (as opposed to XO systems) is currently lacking, leaving the generality of the mechanism uncertain.
Tealc’s citation-neighborhood suggestions
- Charlesworth & Charlesworth (1980) — foundational SA theory underpinning the fusion-drive model.
- van Doorn & Kirkpatrick (2007) — modeling of SA loci and sex chromosome turnover.
- Sylvester et al. (2020) — empirical phylogenetic analysis of XX/XO-to-XY transitions in Polyneoptera, cited as indirect support.