Drift drives the evolution of chromosome number I: The impact of trait transitions on genome evolution in Coleoptera

Summary

Ingested 2026-04-21. 4 findings extracted and verified.

Findings worth citing

Finding 1 — Across 12 Coleoptera clades, those with two Ne-reducing traits had mean fusion rates of 0.05–0.11 in Polyphaga compared with below 0.025 in medium and high Ne clades, and Calathus (low Ne) showed rates more than ten times higher than other Adephaga clades.

In Polyphaga, the mean rate of fusions in low N e clades ranges from 0.05 to 0.11. In con- trast in medium and high Ne clades mean rates were all below 0.025. In Adephaga, only one clade is classified as low N e (Calathus). However, Calathus exhibits strikingly high rates compared with the other four clades. All rates for Calathus were more than ten times higher than the average of the other Adephaga clades. — p. 179

Why this is citable: Provides a quantitative, clade-level comparison showing the magnitude by which reduced Ne elevates karyotype evolution rate — a central citable empirical claim for drift-based models of chromosome evolution.

Counter / limitation: Ne classes were assigned categorically from four proxy traits without direct Ne estimation, and some clades (e.g. Calathus) had as few as 15–16 species, so results may be driven by a few outlier taxa rather than true clade-wide rates.

Topics: selection_and_drift, karyotype_evolution_overview, coleoptera_genomics

Finding 2 — Phylogenetic model-based estimates of chromosome evolution rate are not correlated with traditional scaled-variance estimates derived from fossil ages (Kendall’s τ = 0.11, P = 0.76).

Notably, we find no correlation between the scaled variance- based estimates of karyotype evolution and the phyloge- netic model-based rate estimates for the nine clades with overlapping analyses (τ = 0.11, P-value = 0.76). — p. 179

Why this is citable: This directly challenges the validity of decades of scaled-variance studies of karyotype evolution, making it an important methodological citation for comparative chromosome evolution work.

Counter / limitation: The comparison is based on only nine clades with available fossil data, and the authors themselves note that poor fossil records in insects may penalize the scaled-variance method more than other taxa — so the lack of correlation may not generalize.

Topics: karyotype_evolution_overview, chromosome_number_evolution

Finding 3 — The Coleoptera karyotype dataset shows Adephaga autosome counts range from 3 to 34 (mean 15.57) with a bimodal distribution peaking at 11 and 18, while Polyphaga ranges from 1 to 35 (mean 10.63) with a single mode at 9 autosomes accounting for 29% of species.

In Adephaga, the number of autosomes ranged from 3 to 34 (mean = 15.57 ± 0.14), while in Polyphaga, the range was from 1 to 35 (mean = 10.63 ± 0.06). Polyphaga exhibits a single mode of nine autosomes, accounting for 952 species or 29% of all Polyphaga records. Conversely, Adephaga is bimodal, with concentrations at 11 and 18 autosomes accounting for 276 and 242 species or 23% and 20%, respectively — p. 176

Why this is citable: Provides baseline descriptive statistics on karyotype distribution in the two major beetle suborders that downstream studies of chromosome number optima and coleopteran karyotype evolution can cite.

Counter / limitation: Sampling is uneven across families — family variance in chromosome number correlates with record count (r = 0.41), indicating that rare or understudied families are not adequately represented in these summaries.

Topics: chromosome_number_optima, coleoptera_genomics, karyotype_database

Finding 4 — In Carabidae, wingless lineages show higher rates of chromosome number increase than winged lineages, with the credible interval for the fission rate difference entirely positive (0.005 to 0.044).

In contrast, the credible interval for ΔR fission was en- tirely positive (0.005 to 0.044; Fig. 3A). — p. 176

Why this is citable: This quantitative result from the simpler model (fusions and fissions only, no whole-genome duplication) directly supports the claim that winglessness — a proxy for reduced Ne — accelerates fission rates in Carabidae, grounding citations about drift-driven chromosome number increase in beetles. Note that the complex model including whole-genome duplication does not show the same fission signal, so this result is model-dependent.

Counter / limitation: Only 136 of 1,065 Carabidae species could be included due to overlap between karyotype, phylogeny, and trait data, and wing scoring was sometimes imputed from genus-level frequencies, which could bias rate estimates.

Topics: karyotype_evolution_overview, selection_and_drift, coleoptera_genomics

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