Why not Y naught

Summary

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

Findings worth citing

Finding 1 — Across 10,754 surveyed plant and animal species (excluding multi-sex-chromosome systems), 67% have XX/XY, ~28% have XX/XO, and only a single species is known to have a YO/WO univalent sex-specific chromosome system.

Excluding 1453 species with multiple sex chromosomes likely due to sex chromosome-autosome fusions, 7191 (67%) of the remaining 10,754 species exhibit XX/XY systems, and 2994 have XX/XO systems — p. 75

Why this is citable: Provides the quantitative baseline for how common different sex chromosome systems are across eukaryotes — a citable synthesis for anyone making claims about the prevalence or rarity of particular sex determination systems.

Counter / limitation: The dataset is compiled from heterogeneous karyotype databases with known taxonomic biases (especially under-sampling of female heterogamety, which the paper itself acknowledges is ‘undoubtedly an under-estimate’); the percentages therefore reflect sampling effort as much as biological reality. Additionally, the finding text states ‘~28% have XX/XO’ but 2994/10754 = ~27.8%, and the paper rounds to 27% in the text, not 28%.

Topics: sex_chromosome_evolution, karyotype_database

Finding 2 — Sexually antagonistic selection favoring fusions between autosomes carrying SA loci and univalent Y/W chromosomes would convert YO/WO systems into XY or ZW systems, providing a mechanism for their instability.

If SA loci are present on autosomes, fusions with a univalent Y or W chromosome would be favored if they create a linkage between the SA locus and the sex-determining locus — p. 76

Why this is citable: Provides a theoretical mechanism — drawing on Charlesworth & Charlesworth (1980) and van Doorn & Kirkpatrick (2007) — for why YO/WO systems should be transitory: SA-driven autosome fusions with the univalent Y or W would convert them into conventional XY or ZW systems. Citable for arguments about sex chromosome turnover and the instability of univalent sex chromosomes.

Counter / limitation: This is a verbal/theoretical argument that synthesizes existing SA theory without new quantitative modeling; the relative strength of SA-driven fusion versus genetic drift in small populations is not formally estimated, and empirical support is limited to the XX/XO-to-XY case in Polyneoptera (Sylvester et al. 2020) rather than direct evidence from YO/WO systems.

Topics: sex_chromosome_evolution, sexually_antagonistic_selection, y_naught_asymmetry

Question copied. Paste it into the NotebookLM tab.