10.1017/s0016672300014051

Summary

Ingested 2026-04-22. 2 findings extracted and verified.

Findings worth citing

Finding 1 — A necessary condition for selection to favor a centric fusion between a sex chromosome and an autosome is that alleles at the autosomal locus are maintained at different frequencies in the two sexes — a fusion is neutral if the selected locus has equal allele frequencies in males and females.

a necessary condition for selection to favour a fusion between an autosome and a sex chromosome is that the alleles at the autosomal locus are maintained by selection at different frequencies in the two sexes. This result, of course, depends on the assumptions that the fusions are not themselves associated with any effect on fitness, nor subject to distorted segregation in their favour in heterozygotes. — p. 210

Why this is citable: This finding identifies the precise genetic condition (a selectively maintained sex difference in allele frequencies at the autosomal locus) under which sex-chromosome–autosome fusions are favored, correcting White’s (1957) verbal model which incorrectly invoked heterozygote advantage in the heterogametic sex as the key requirement. Directly relevant to discussions of the evolutionary origins of neo-sex chromosome systems.

Counter / limitation: The result depends on the assumption of no intrinsic fitness cost to the fusion itself and no segregation distortion; the authors themselves acknowledge that centric fusions must involve some loss of chromosomal material, making the no-fitness-cost assumption ‘unlikely to be generally true,’ which could reduce or eliminate the selective advantage in real populations.

Topics: sex_chromosome_evolution, karyotype_evolution_overview, selection_and_drift

Finding 2 — The initial rate of increase of a rare Y-autosome fusion is approximately three times that of an X-autosome fusion under equivalent parameter values.

the initial rate of increase of a rare Y-autosome fusion is about three times that of an X-autosome fusion, with the same parameter values. — p. 205

Why this is citable: This quantitative asymmetry between Y- and X-autosome fusions is a direct theoretical prediction about relative fixation rates, useful for interpreting comparative karyotypic data. The paper itself tests this prediction against available data and finds no empirical support for Y-fusion predominance, making the finding useful as both a theoretical benchmark and a cautionary example of a model prediction that does not match observations.

Counter / limitation: The threefold difference is derived under idealized conditions (no recombination between locus and centromere, no segregation disturbance, infinite population size), and the paper’s own survey of empirical data in Drosophila and mammals finds roughly equal numbers of X- and Y-autosome fusions with no predominance of Y fusions, suggesting the predicted asymmetry is not realized in practice.

Topics: sex_chromosome_evolution, karyotype_evolution, selection_theory