The fragile Y hypothesis: Y chromosome aneuploidy as a selective pressure in sex chromosome and meiotic mechanism evolution

Summary

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

Findings worth citing

Finding 1 — The fragile Y hypothesis proposes a negative correlation between the size of the recombining region (PAR) and the rate of Y-chromosome aneuploidy in species with chiasmatic meiosis.

The fragile Y hypothesis proposes that in species with chiasmatic meiosis the rate of Y-chromosome aneuploidy and the size of the recombining region have a negative correlation. — p. 942

Why this is citable: This is the core theoretical claim of the paper and is the canonical statement cited by downstream work on sex chromosome turnover, PAR evolution, and meiotic mechanism evolution.

Counter / limitation: The hypothesis is largely supported by correlational phylogenetic patterns and indirect proxies (chromosome number for PAR size); direct measurements of PAR size and aneuploidy rates across species are sparse, so the correlation remains to be rigorously quantified.

Topics: fragile_y_hypothesis, sex_chromosome_evolution

Finding 2 — Achiasmatic clades within Adephaga exhibit significantly fewer XO taxa than expected, with Trechitae showing 3 observed XO species versus 16 expected and Cicindelinae+Colyrinae showing 1 observed versus 6 expected under the overall Adephaga rate of Y loss.

Among the 45 taxa within Trechitae, simulations suggest we should expect 16 XO species; however, only three are observed and only 8% of simulations resulted in three or fewer XO taxa. We find a similar pattern in Cicindelinae þ Colyrinae. Among the 21 taxa, only one is XO but simulations suggest we should expect six and only 5% of simulations results in one or zero XO taxa. — p. 946

Why this is citable: This quantitative within-suborder comparison grounds the claim that alternative segregation mechanisms reduce Y loss rates, central to arguments linking meiotic mechanism evolution to sex chromosome stability.

Counter / limitation: The test assumes a single background rate of Y loss across all Adephaga; if loss rates vary for reasons other than meiotic mechanism (e.g. lineage-specific demography or karyotypic constraints), the ‘expected’ counts are biased and the inference weakens.

Topics: fragile_y_hypothesis, karyotype_evolution_overview, coleoptera_genomics

Finding 3 — Turner syndrome (XO) occurs in approximately 3% of human conceptions and causes about 99% prenatal mortality, indicating a high rate of Y mis-segregation in humans despite recent arguments for long-term Y stability.

TS occurs in /C24 3% of all conceptions, a high frequency for a mutation that acts effectively as a dominant lethal (TS causes 99% prenatal mortality) — p. 948

Why this is citable: This quantitative anchor grounds the paper’s argument that humans already experience high rates of Y mis-segregation pressure, used to challenge recent claims of long-term human Y stability. The 3% conception frequency paired with 99% prenatal mortality illustrates the severe selective cost of ongoing Y aneuploidy in humans.

Counter / limitation: The 3% conception figure comes from older cytogenetic surveys with ascertainment limitations, and Turner syndrome incidence conflates paternal meiotic non-disjunction with maternal errors and post-zygotic X-chromosome loss events, so it only indirectly measures Y mis-segregation rate specifically. The paper itself notes that most TS survivors are mosaic, further complicating interpretation.

Topics: fragile_y_hypothesis, sex_chromosome_evolution

Read the paper

doi.org/10.1002/bies.201500040

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