Sexual Antagonism
Current understanding
Sexual antagonism arises when alleles beneficial to one sex impose fitness costs on the other. One prominent genomic signature of sexual antagonism is the predicted enrichment of sex chromosome–autosome (SA) fusions relative to autosome–autosome fusions. If autosomally located alleles have sex-specific fitness effects, fusion with a sex chromosome can lock those alleles into a sex-limited transmission context, reducing the cost paid by the disadvantaged sex and thereby spreading under selection.
Inversions on sex chromosomes represent a parallel mechanism for resolving intralocus sexual conflict — but their dynamics depend critically on the dominance of the sexually antagonistic alleles. When the male-beneficial allele is recessive (h < 0.3), an X chromosome inversion capturing the female-beneficial allele cannot fix; instead it is maintained as a stable polymorphism (Blackmon & Brandvain 2017, Finding 1). This dominance threshold helps explain why sexually antagonistic inversions accumulate asymmetrically on Y chromosomes relative to X chromosomes: Y-linked inversions face no such constraint because the Y is always hemizygous. The maintenance of polymorphism under intermediate dominance regimes means that sexual antagonism may leave a signature not only of rapid fixation events, but also of long-lived balanced variation on X chromosomes.
Empirical work on Habronattus jumping spiders provides a rigorously tested case at the chromosomal level. Using a probabilistic null model anchored to the reconstructed ancestral karyotype (XXO, 2n=26), researchers found that 8 of the 10 observed chromosomal fusions in the clade were SA-fusions. The probability of observing ≥8 SA-fusions under the null is p < 10⁻⁵, constituting strong statistical rejection of fusion-type indifference (The probability of fusions 2020, Finding 1). This transforms a previously qualitative observation about Habronattus karyotypes into a formally tested, quantitative conclusion and is consistent with sexual antagonism acting as a selective filter on chromosomal fusions.
Taken together, these lines of evidence — inversion dynamics at the allelic level and fusion enrichment at the karyotypic level — paint a coherent picture: sexual antagonism shapes both the fine-scale and large-scale organization of sex chromosomes, but the specific outcomes depend heavily on dominance relationships and the chromosomal context in which conflict-resolving mutations arise.
Supporting evidence
- Blackmon & Brandvain 2017, Finding 1: When the male-beneficial allele is recessive (h < 0.3), an X chromosome inversion capturing the female-beneficial allele cannot fix and is instead maintained as a stable polymorphism — establishing a dominance-dependent threshold governing whether sexual antagonism is resolved or perpetuated on the X chromosome.
- The probability of fusions 2020, Finding 1: In Habronattus, 8 out of 10 total chromosomal fusions are sex chromosome–autosome fusions, and the probability of observing this many or more under a null model of fusion-type indifference is p < 10⁻⁵, providing strong quantitative evidence for non-random fusion accumulation consistent with sexual antagonism.
Contradictions / open disagreements
No direct contradiction exists between these two findings, but they operate at different levels and their relationship is worth flagging. The Habronattus fusion result (The probability of fusions 2020, Finding 1) treats chromosomal fusions as the unit of analysis and does not specify the dominance architecture of the underlying alleles. The inversion model (Blackmon & Brandvain 2017, Finding 1) shows that the outcome of sexually antagonistic variants on the X depends strongly on that dominance architecture (h < 0.3 threshold). If the fusions in Habronattus involved recessive male-beneficial alleles on the autosomes being fused, the theoretical model would predict polymorphism rather than fixation of those variants — yet fixation is implicitly assumed in the fusion-count framework. Reconciling these frameworks requires empirical data on the dominance coefficients of the specific loci involved, which are not yet available.
Additionally, the Habronattus p-value is sensitive to the assumed ancestral karyotype, and the h < 0.3 threshold from the inversion model derives from a symmetric, fully recombination-suppressing scenario; alternative parameter regimes could shift both results.
Tealc’s citation-neighborhood suggestions
- Theoretical models of intralocus sexual conflict resolution via sex-linkage (e.g., Rice 1984; van Doorn & Kirkpatrick 2007) are the conceptual backbone for interpreting both the Habronattus fusion result and the inversion-dominance threshold.
- Empirical estimates of dominance coefficients for sexually antagonistic loci in natural populations would help adjudicate between fixation and polymorphism scenarios on X chromosomes.
- Broader comparative studies of SA-fusion enrichment across Araneae or other taxa with frequent karyotype changes could contextualize whether Habronattus is exceptional or representative.