Inversions
Current understanding
Chromosomal inversions play a central role in sex chromosome evolution by suppressing recombination and allowing the accumulation of sexually antagonistic alleles. However, the fate of an inversion—whether it fixes in a population or persists as a polymorphism—depends critically on the dominance relationships of the alleles it captures.
Theory predicts a strong asymmetry between inversions on the X and Y chromosomes. A Y-linked inversion capturing a male-beneficial allele can spread readily because it is always expressed in males. The situation for X-linked inversions is more complex. When a female-beneficial allele is captured by an X-chromosome inversion, its fate is governed by the dominance of the opposing male-beneficial allele. Specifically, when the male-beneficial allele is sufficiently recessive (dominance coefficient h < 0.3), the X-linked inversion cannot fix; instead, it is maintained as a stable polymorphism in the population. This result provides a mechanistic explanation for why inversions might accumulate asymmetrically on Y chromosomes relative to X chromosomes, and for why sexually antagonistic variation can be stably maintained on sex chromosomes under particular dominance regimes. See Blackmon & Brandvain 2017, Finding 1.
Supporting evidence
- Dominance threshold for X inversion fate: Modeling of inversion dynamics on sex chromosomes shows that the outcome for an X-linked inversion hinges on the recessivity of the male-beneficial allele. Below a threshold of h < 0.3, fixation is prevented and a stable polymorphism results. Blackmon & Brandvain 2017, Finding 1
Contradictions / open disagreements
The h < 0.3 dominance threshold is derived from a symmetric fitness model that assumes complete recombination suppression within the inversion. Under asymmetric selection coefficients or partial recombination suppression, the boundary between fixation and stable polymorphism could shift substantially or disappear altogether. Broader parameter exploration is needed before this threshold can be treated as a robust empirical prediction.
Tealc’s citation-neighborhood suggestions
- Studies empirically measuring dominance coefficients for sexually antagonistic alleles on sex chromosomes would help test whether h < 0.3 is a biologically realistic regime.
- Empirical surveys of inversion frequency on X vs. Y chromosomes across taxa (e.g., Drosophila, mammals, beetles) would provide a test of the predicted asymmetry.