Y fuse? Sex chromosome fusions in fishes and reptiles.

Summary

Ingested 2026-04-22. 3 findings extracted and verified.

Findings worth citing

Finding 1 — In fishes, 41% of XY species have fused sex chromosomes versus only 5% of ZW species, and in reptiles 33% of XY species have fusions versus only 3% of ZW species.

In fishes, 41% (45/109) of XY species have fused sex chromosomes, whereas only 5% (2/38) of ZW species do (Fisher’s exact test P < 0.001). In reptiles, 33% (40/120) of XY species have fusions, whereas only 3% (6/240) of ZW species do (Fisher’s exact test P < 0.001).

Why this is citable: This quantified comparison grounds the claim that XY lineages accumulate sex chromosome-autosome fusions at dramatically higher rates than ZW lineages, a pattern demanding mechanistic explanation and useful as a comparative benchmark.

Counter / limitation: Fisher’s exact tests ignore phylogenetic non-independence among species, so the statistical significance may be inflated; the phylogenetic MCMC analyses are more rigorous but show slightly weaker support (~98-99% posterior probability) rather than formal p-values.

Topics: sex_chromosome_evolution, karyotype_evolution

Finding 2 — Y-autosome fusions establish at a much higher rate than X-autosome, Z-autosome, or W-autosome fusions in both fishes and squamate reptiles, as confirmed by phylogenetic Markov chain analyses.

We found that Y-A fusions establish at a higher rate than other sex chromosomes, even when accounting for the shared evolutionary history among taxa ( S3 Fig for fish and S4 Fig for squamates).

Why this is citable: This is the central empirical finding of the paper, confirmed by phylogenetic Markov chain analyses (MCMC mapped onto fish and squamate trees) that account for shared evolutionary history, providing the key pattern that all subsequent theoretical modeling attempts to explain.

Counter / limitation: The analysis relies on karyotypically detectable multiple sex chromosome systems; X-A fusions generate neo-Y chromosomes that could be lost without detection (due to masking in the hemizygous sex), potentially inflating the apparent Y-A excess by making X-A fusions appear more ephemeral than Y-A fusions.

Topics: sex_chromosome_evolution, karyotype_evolution, chromosome_number_evolution

Finding 3 — The most plausible explanation for the excess of Y-autosome fusions is that fusions are slightly deleterious and arise more often in males due to male-biased mutation rates, with the reduced effective population size of Y chromosomes allowing these deleterious fusions to fix more frequently by drift.

The most plausible explanation for the observed data seems to be (a) that fusions are slightly deleterious, and (b) that the mutation rate is male-biased or the reproductive sex ratio is female-biased.

Why this is citable: This is the paper’s primary mechanistic conclusion from its theoretical modeling, connecting male-biased mutation rates and reduced effective population size of the Y to the observed excess of Y-autosome fusions — citable for work on Y chromosome degeneration, mutation rate asymmetry, or structural karyotype evolution in fishes and squamates.

Counter / limitation: The evidence for male-biased mutation specifically in fishes and squamate reptiles (as opposed to mammals) is limited; the paper largely relies on human translocation data to support the male-mutation-bias component, which may not generalize across these diverse taxa.

Topics: sex_chromosome_evolution, karyotype_evolution, selection_and_drift, mutation_rate_bias