Coleoptera genomics

Current understanding

Beetles (Coleoptera) are the most species-rich animal order, yet chromosome-level reference genomes remain scarce and Y chromosomes are almost entirely uncharacterized at the sequence level. New work on the endangered long-armed scarab Cheirotonus formosanus (Scarabaeidae: Euchirinae), the jewel scarab Chrysina gloriosa (Scarabaeidae: Rutelinae), and the southern pine beetle Dendroctonus frontalis (Curculionidae: Scolytinae) each provide chromosome-level reference assemblies, advancing understanding of karyotype conservation and sex-chromosome biology in the order. Complementing genomic assembly work, comparative cytogenetic surveys across Adephaga reveal that meiotic mechanism — specifically, whether males undergo achiasmatic meiosis — is a key predictor of sex-chromosome stability. At the macroevolutionary scale, a drift-based framework offers a population-genetic explanation for why chromosome-number evolution rates differ so dramatically across beetle clades.

Supporting evidence

Genome assemblies across Coleoptera. High-quality reference genomes are now available for three beetle species spanning two suborders. The C. formosanus assembly recovered 10 primary large scaffolds (9 autosomes plus an X), consistent with the ancestral coleopteran karyotype of 2n=20 (Chien et al. 2026). The Chrysina gloriosa genome spans 642 MB across 454 scaffolds with a scaffold N50 of 72 MB; the 10 largest scaffolds capture 98% of the genome at BUSCO 95.5% (A reference quality genome 2024, Finding 1). In bark beetles (Curculionidae), the D. frontalis (SPB) assembly is smaller but similarly dominated by a handful of large scaffolds: 173.7 Mbp total with 97.72% of sequence in eight chromosome-level scaffolds ranging from 12.4 to 42.5 Mbp and a scaffold N50 of 24.8 Mbp, achieving 94.2% BUSCO completeness against Endopterygota orthologues (Genome assembly of the 2024, Finding 1). The shared pattern of a small number of large scaffolds dominating each assembly resonates with conserved karyotypic architecture observed cytogenetically across Coleoptera.

Reduced gene content in Dendroctonus. Comparison of annotated gene sets reveals a striking deficit in bark beetles: the three sequenced Dendroctonus species contain on average ~13,400 genes, roughly 3,600 fewer than the ~17,000 mean across 11 other beetle species (Genome assembly of the 2024, Finding 2). Approximately 2,300 of this difference persists after correcting for transposable-element misannotation that artificially inflates gene counts in non-Dendroctonus assemblies, suggesting a genuine biological reduction in gene repertoire within the genus.

Sex-chromosome identification and synteny. Chromosome-quotient analysis in C. formosanus flagged a 1.1 Mbp scaffold with female:male read-depth ratios near 0, identifying hemizygous Y sequence; one gene model on this scaffold carries a JARID1/KDM5 family domain and is entirely absent from female data, paralleling KDM5D on the mammalian Y (Chien et al. 2026; Chien et al. 2026). In SPB, the putative X chromosome (scaffold 8) shows synteny with scaffold 1 of D. ponderosae, which corresponds to that species’ neoXY system, and the nine ancestral Coleopteran Stevens elements are conserved across SPB, MPB, and Tribolium castaneum (Genome assembly of the 2024, Finding 3).

Sex-chromosome stability and meiotic mechanism. A broader comparative analysis of Adephaga demonstrates that achiasmatic clades retain Y chromosomes at strikingly elevated rates. Among 45 taxa within Trechitae only 3 XO species are observed where 16 are expected; simulation tests confirm these deficits are highly improbable under neutral Y-loss assumptions, implicating achiasmy as a stabilizing force (Blackmon & Demuth 2015, Finding 1).

Karyotype diversity and drift-driven evolution. Adephaga autosome counts range from 3 to 34 (mean 15.57) with a bimodal distribution, while Polyphaga ranges from 1 to 35 (mean 10.63) with a mode at 9 autosomes (Drift drives the evolution 2024, Finding 2). Across 12 Coleoptera clades, those with two Ne-reducing traits show mean fusion rates of 0.05–0.11 compared with below 0.025 in higher-Ne clades; winglessness also predicts elevated fission rates in Carabidae (credible interval 0.005–0.044 entirely positive), consistent with reduced Ne acting as a permissive force for karyotype change (Drift drives the evolution 2024, Finding 1; Drift drives the evolution 2024, Finding 3).

Demographic inference. PSMC trajectories from C. formosanus were nearly identical across platforms, and ROH data from ddRAD (n=46) show segments dominated by short runs (<1 Mbp) consistent with long-term low ancestral Ne rather than recent inbreeding (Chien et al. 2026; Chien et al. 2026).

Contradictions / open disagreements

Assembly completeness. Both C. gloriosa (642 MB vs. ~850 MB flow-cytometry estimate) and SPB (173.7 Mbp vs. 194.7 Mbp flow-cytometry estimate) assemblies are smaller than their cytometric genome sizes, likely reflecting unassembled repetitive content (A reference quality genome 2024, Finding 1; Genome assembly of the 2024, Finding 1). The C. gloriosa assembly also derives from a single female, leaving sex-specific sequences uncharacterized.

Gene-count deficit and annotation methodology. The Dendroctonus gene-count deficit relies on heuristic domain-keyword TE filtering rather than manual curation, so part of the apparent ~3,600-gene gap could reflect pipeline differences rather than genuine biology (Genome assembly of the 2024, Finding 2).

Sex-chromosome identification by read depth alone. Y-linkage in C. formosanus was inferred from read-depth in a single male–female pair without PCR or population-level confirmation (Chien et al. 2026), and the KDM5-like gene was assigned by domain architecture rather than orthology (Chien et al. 2026). Similarly, SPB X-chromosome assignment rests solely on reduced male read coverage without cytogenetic confirmation (Genome assembly of the 2024, Finding 3).

Ne categorization and clade-size limitations. The drift-driven rate differences rely on categorical Ne proxies rather than direct estimates, and some clades contain as few as 15–16 species, potentially inflating variance (Drift drives the evolution 2024, Finding 1). The elevated fission rate in wingless Carabidae is also model-dependent (Drift drives the evolution 2024, Finding 3).

Achiasmy rate assumption. The simulation test for suppressed Y loss assumes a single background Y-loss rate across all Adephaga; if loss rates vary independently of meiotic mechanism, expected counts are biased (Blackmon & Demuth 2015, Finding 1).

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