The probability of fusions joining sex chromosomes and autosomes: Sex Autosome Fusions

Summary

Ingested 2026-04-21. 2 findings extracted and verified.

Findings worth citing

Finding 1 — Drosophila exhibit far fewer sex chromosome–autosome fusions than expected under the null model, with an observed SA-fusion proportion of 0.155 versus an expected 0.43 with non-overlapping credible intervals.

Across our 1000 stochastic mappings, we find that the average number of SA-fusions observed is 4.49 and that this equates to a proportion of 0.155 (credible interval 0.12–0.22). Using our formula as described above we also calculated the expected proportion of SA-fusions. The mean expected SA-fusion proportion was 0.43 (credible interval 0.42–0.44). — p. 3

Why this is citable: Directly contradicts the general expectation that sexual antagonism drives an excess of SA-fusions, providing a key counterexample citable in debates about the ubiquity of sexually antagonistic selection.

Counter / limitation: Drosophila’s achiasmatic male meiosis and small chromosome number make it an unusual case; the deficit may reflect Y-chromosome degeneration dynamics specific to this clade rather than a general refutation of sexual antagonism hypotheses.

Topics: sex_chromosome_evolution, fragile_y_hypothesis, karyotype_evolution_overview

Finding 2 — Habronattus jumping spiders show a statistically significant excess of sex chromosome–autosome fusions relative to the null expectation, with the observed 8 SA-fusions out of 10 total yielding p < 10⁻⁵.

P(8 SA/C0 fusions out of 10)¼ X 10 i¼8 X10/C0 i j¼0 10! i!/C1 j!/C1 (10/C0 i/C0 j)!PðSAÞi /C1 PðAAÞj /C1 PðSSÞ10/C0 i/C0 j , 10/C0 5: — p. 3

Why this is citable: Provides a rigorous quantitative null test confirming the long-standing qualitative claim that Habronattus has an excess of SA-fusions, making this the citable source for that result and demonstrating the utility of the null model developed in the paper.

Counter / limitation: The p-value is calculated assuming a single ancestral karyotype (XXO, 2n=26 autosomes) based on the most common state in the ancestral state estimation; integrating over phylogenetic uncertainty in the ancestral karyotype could alter the exact p-value, and the test counts only 10 total fusions, limiting statistical power for finer-grained inference.

Topics: sex_chromosome_evolution, chromosome_fusion, sexual_antagonism

Read the paper

doi.org/10.1098/rsbl.2020.0648