Worse than nothing at all: the inequality of fusions joining autosomes to the PAR and non-PAR portions of sex chromosomes

Summary

Ingested 2026-04-21. 3 findings extracted and verified.

Findings worth citing

Finding 1 — Fusions joining an autosome to the non-PAR of a sex chromosome are favored under sexual antagonism, whereas fusions to the PAR are disfavored under sexual antagonism.

Our results show a clear pattern where fusions to the non-PAR are favored in the presence of sexual antagonism, whereas fusions to the PAR are disfavored in the presence of sexual antagonism. — p. 1

Why this is citable: This is the paper’s central theoretical claim and is the natural citation for any future work invoking neo-sex chromosome fusions as a mechanism to resolve sexual antagonism — it sharpens the Charlesworth & Charlesworth (1980) prediction by conditioning it on fusion location.

Counter / limitation: The result depends on the strong modeling assumption of obligate recombination in the PAR every male meiosis; in clades with achiasmatic or asynaptic male meiosis (some beetles, marsupials, dipterans) the prediction does not apply, and the simulation is a single two-locus biallelic model rather than an empirical test.

Topics: sex_chromosome_evolution, karyotype_evolution_overview

Finding 2 — A fusion to the PAR under sexually antagonistic selection is worse than no fusion because obligate PAR recombination reconstitutes maladaptive genotypes each generation, increasing rather than reducing recombination load.

Based on the results of our model, we suggest that a fusion to the PAR of a sex chromosome under sexually antagonistic selection will be more detrimental than a state in which no fusion occurred at all. — p. 10–11

Why this is citable: This is the namesake ‘worse than nothing at all’ conclusion and provides a specific mechanistic argument other authors can cite when interpreting why observed sex-autosome fusion breakpoints tend to fall outside the PAR.

Counter / limitation: The claim is derived entirely from a forward-time simulation with symmetrical sexual antagonism and assumes PAR recombination rates do not rapidly evolve after fusion; empirical fitness measurements of PAR-fused neo-sex chromosomes are lacking.

Topics: sex_chromosome_evolution, fragile_y_hypothesis

Finding 3 — Y-autosome non-PAR fusions reach marginally higher frequencies than X-autosome fusions except when the female-beneficial allele is recessive, because X-autosome fusions spend only one-third of their time in males where the selective benefit is realized.

It then follows that fusions to the Y should be more common than X fusions. This pattern is driven by the fact that X autosome fusions are only present in males one-third of the time. — p. 9

Why this is citable: Provides a specific mechanistic explanation for the predicted X-vs-Y asymmetry in non-PAR fusion fixation: X-autosome fusions are present in males only one-third of the time, reducing the opportunity for selection to act on the fusion. This can be cited when comparing empirical counts of X-autosome vs Y-autosome fusions, with the caveat that the authors themselves note the difference is small and convergent at higher selection coefficients.

Counter / limitation: The simulations show the X–Y difference is small (e.g., 13% vs 8% above MDE in one additive scenario) and the two frequencies converge at fixation when s is between 0.3 and 0.7, making the asymmetry likely undetectable in empirical fusion counts against noise from Y degeneration, Y-chromosome size differences, or small sample sizes of documented fusions.