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bioRxiv2019

The chromosomal-level genome assembly and comprehensive transcriptomes of Chinese razor clam (Sinonovacula constricta) with deep-burrowing life style and broad-range salinity adaptation

BackgroundThe Chinese razor clam, Sinonovacula constricta, is one of the commercially important marine bivalves with deep-burrowing lifestyle and remarkable adaptability of broad-range salinity. Despite its economic impact and representative of the less-understood deep-burrowing bivalve lifestyle, there are few genomic resources for exploring its unique biology and adaptive evolution. Herein, we reported a high-quality chromosomal-level reference genome of S. constricta, the first genome of the family Solenidae, along with a large amount of short-read/full-length transcriptomic data of whole-ontogeny developmental stages, all major adult tissues, and gill tissues under salinity challenge.\n\nFindingsA total of 101.79 Gb and 129.73 Gb sequencing data were obtained with the PacBio and Illumina platforms, which represented approximately 186.63X genome coverage. In addition, a total of 160.90 Gb and 24.55 Gb clean data were also obtained with the Illumina and PacBio platforms for transcriptomic investigation. A de novo genome assembly of 1,340.13 Mb was generated, with a contig N50 of 689.18 kb. Hi-C scaffolding resulted in 19 chromosomes with a scaffold N50 of 57.99 Mb. The repeat sequences account for 50.71% of the assembled genome. A total of 26,273 protein-coding genes were predicted and 99.5% of them were annotated. Phylogenetic analysis revealed that S. constricta diverged from the lineage of Pteriomorphia at approximately 494 million years ago. Notably, cytoskeletal protein tubulin and motor protein dynein gene families are rapidly expanded in the S. constricta genome and are highly expressed in the mantle and gill, implicating potential genomic bases for the well-developed ciliary system in the S. constricta.\n\nConclusionsThe high-quality genome assembly and comprehensive transcriptomes generated in this work not only provides highly valuable genomic resources for future studies of S. constricta, but also lays a solid foundation for further investigation into the adaptive mechanisms of benthic burrowing mollusks.

genomics↗

bioRxiv2019

Measuring genome sizes using read-depth, k-mers, and flow cytometry: methodological comparisons in beetles (Coleoptera)

Measuring genome size across different species can yield important insights into evolution of the genome and allow for more informed decisions when designing next-generation genomic sequencing projects. New techniques for estimating genome size using shallow genomic sequence data have emerged which have the potential to augment our knowledge of genome sizes, yet these methods have only been used in a limited number of empirical studies. In this project, we compare estimation methods using next-generation sequencing (k-mer methods and average read depth of single-copy genes) to measurements from flow cytometry, the gold standard for genome size measures, using ground beetles (Carabidae) and other members of the beetle suborder Adephaga as our test system. We also present a new protocol for using read-depth of single-copy genes to estimate genome size. Additionally, we report flow cytometry measurements for five previously unmeasured carabid species, as well as 21 new draft genomes and six new draft transcriptomes across eight species of adephagan beetles. No single sequence-based method performed well on all species, and all tended to underestimate the genome sizes, although only slightly in most samples. For one species, Bembidion haplogonum, most sequence-based methods yielded estimates half the size suggested by flow cytometry. This discrepancy for k-mer methods can be explained by a large number of repetitive sequences, but we have no explanation for why read-depth methods yielded results that were also strikingly low.

genomics↗

bioRxiv2019

Genomes of Symbiodiniaceae reveal extensive sequence divergence but conserved functions at family and genus levels

Dinoflagellates of the family Symbiodiniaceae (Order Suessiales) are predominantly symbiotic, and many are known for their association with corals. The genetic and functional diversity among Symbiodiniaceae is well acknowledged, but the genome-wide sequence divergence among these lineages remains little known. Here, we present de novo genome assemblies of five isolates from the basal genus Symbiodinium, encompassing distinct ecological niches. Incorporating existing data from Symbiodiniaceae and other Suessiales (15 genome datasets in total), we investigated genome features that are common or unique to these Symbiodiniaceae, to genus Symbiodinium, and to the individual species S. microadriaticum and S. tridacnidorum. Our whole-genome comparisons reveal extensive sequence divergence, with no sequence regions common to all 15. Based on similarity of k-mers from whole-genome sequences, the distances among Symbiodinium isolates are similar to those between isolates of distinct genera. We observed extensive structural rearrangements among symbiodiniacean genomes; those from two distinct Symbiodinium species share the most (853) syntenic gene blocks. Functions enriched in genes core to Symbiodiniaceae are also enriched in those core to Symbiodinium. Gene functions related to symbiosis and stress response exhibit similar relative abundance in all analysed genomes. Our results suggest that structural rearrangements contribute to genome sequence divergence in Symbiodiniaceae even within a same species, but the gene functions have remained largely conserved in Suessiales. This is the first comprehensive comparison of Symbiodiniaceae based on whole-genome sequence data, including comparisons at the intra-genus and intra-species levels.

genomics↗

bioRxiv2019

RepeatModeler2: automated genomic discovery of transposable element families

The accelerating pace of genome sequencing throughout the tree of life is driving the need for improved unsupervised annotation of genome components such as transposable elements (TEs). Because the types and sequences of TEs are highly variable across species, automated TE discovery and annotation are challenging and time-consuming tasks. A critical first step is the de novo identification and accurate compilation of sequence models representing all the unique TE families dispersed in the genome. Here we introduce RepeatModeler2, a new pipeline that greatly facilitates this process. This new program brings substantial improvements over the original version of RepeatModeler, one of the most widely used tools for TE discovery. In particular, this version incorporates a module for structural discovery of complete LTR retroelements, which are widespread in eukaryotic genomes but recalcitrant to automated identification because of their size and sequence complexity. We benchmarked RepeatModeler2 on three model species with diverse TE landscapes and high-quality, manually curated TE libraries: Drosophila melanogaster (fruit fly), Danio rerio (zebrafish), and Oryza sativa (rice). In these three species, RepeatModeler2 identified approximately three times more consensus sequences matching with >95% sequence identity and sequence coverage to the manually curated sequences than the original RepeatModeler. As expected, the greatest improvement is for LTR retroelements. The program had an extremely low false positive rate when applied to simulated genomes devoid of TEs. Thus, RepeatModeler2 represents a valuable addition to the genome annotation toolkit that will enhance the identification and study of TEs in eukaryotic genome sequences. RepeatModeler2 is available as source code or a containerized package under an open license (https://github.com/Dfam-consortium/RepeatModeler, https://github.com/Dfam-consortium/TETools). SignificanceGenome sequences are being produced for more and more eukaryotic species. The bulk of these genomes is composed of parasitic, self-mobilizing transposable elements (TEs) that play important roles in organismal evolution. Thus there is a pressing need for developing software that can accurately identify the diverse set of TEs dispersed in genome sequences. Here we introduce RepeatModeler2, an easy-to-use package for the curation of reference TE libraries which can be applied to any eukaryotic species. Through several major improvements over the previous version, RepeatModeler2 is able to produce libraries that recapitulate the known composition of three model species with some of the most complex TE landscapes. Thus RepeatModeler2 will greatly enhance the discovery and annotation of TEs in genome sequences.

genomics↗

bioRxiv2025

Genome assembly of Spinifex sericeus provides insights into repetitive content and haplotype variation in a dioecious coastal grass

Spinifex sericeus (beach spinifex) is a perennial, dioecious grass native to coastal ecosystem of Oceania that has an important role in dune stability. We present a near telomere-to-telomere, haplotype-resolved nuclear genome alongside complete organelle genomes of S. sericeus. The nuclear genome is highly repeat-rich, dominated by long terminal repeat (LTR) retrotransposons. Comparative analysis of the two haplotypes reveals extensive structural variation and high levels of duplication that are suggestive of active and expanding LTRs that may drive genome evolution and expansion in this species. The chloroplast genome exhibits heteroplasmy, having two distinct circular configurations spanning 138 kbp each. The complex mitochondrial genome consists of a large linear component spanning 1.85 Mbp and a small circular 134 kbp component. These genomic resources provide a foundation for advancing research on sex determination and stress adaptation in grasses, and well as practical applications in crop improvement and genetically informed coastal dune restoration. Significance statementThe reference genome of Spinifex sericeus provides a foundational resource for understanding adaptations to the harsh coastal dune environment, including saline conditions, extreme temperatures, and drought. The genome enables comparative studies within Poaceae, where current reference genomes are dominated by crop species, and supports conservation and restoration efforts for coastal ecosystem management.

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bioRxiv2026

Assessing Hardy-Weinberg Equilibrium in T2T-aligned 1000 Genomes Project

Quality control of markers in genome-wide association studies often includes testing for Hardy-Weinberg equilibrium (HWE). However, this is usually implemented in a homogeneous population without stratifying by sex. Previous work indicates sex-based selection at numerous autosomal loci in cohorts with active recruitment. Sex chromosome sequences can also interfere with autosomal SNPs. We examined genome-wide sex-specific HWE deviations across populations in the telomere-to-telomere (T2Tv2)-aligned high-coverage whole genome sequence of the 1000 Genomes Project data of 2,490 individuals. Our analysis was restricted to bi-allelic SNPs with non-missing genotypes and MAF>=5% in both sexes of the five super-populations. We employed a robust allele-based approach for HWE testing, which enabled the quantification of directional deviations from HWE. A second-order omnibus meta-analysis combining results from the five super-populations and both sexes revealed that 0.9% autosomal SNPs exhibited a significant deviation from HWE at p<5e-8. Most of these deviations were found to be associated with genomic features relating to poor sequence quality. Filtering results to reliable genomic regions yielded 255 autosomal and 1 NPR X chromosomal SNPs, of which 140 autosomal SNPs also showed significant heterogeneity across populations but not across sexes. 8 SNPs in a 15-bp region on chr14 showed excess heterozygosity in both sexes of the AFR (African) super-population. We also generated a well-performing multivariate predictor of HWD (deviation from HWE) using multiple sequence features, which could be combined with HWD estimates in future studies to select SNPs that deviate from HWE due to technical rather than biological reasons. Author SummaryWe conducted a specific quality control test, which compares the observed and expected genotype counts, on an updated version of the 1000 Genomes Project whole genome sequence data generated on [~]2500 individuals. We first performed this analysis by grouping the data by ancestry and sex. We then combined and contrasted the group results. We found that most regions that differed between observed and expected counts overlapped regions of the genome which are difficult to sequence using current short read technology. In the remaining regions we found an interesting cluster of SNPs in a single ancestry, where there is a gross excess of heterozygous genotypes. GWASes typically use a standard strict threshold for this quality control test for genotyping arrays to remove SNPs. Here we suggest a more nuanced approach that is applicable to whole genome sequence data.

genomics↗

bioRxiv2026

Identification of two genomic cryptotypes of Plasmodium malariae in Africa

Plasmodium malariae is a neglected human malaria parasite that causes persistent, often asymptomatic infections and remains difficult to diagnose. Despite being generally associated with lower prevalence and severity than other malaria parasites, P. malariae represents a significant public health concern, particularly in Africa, but also as a zoonosis in South America with monkey-adapted Plasmodium brasilianum. Plasmodium malariae and P. brasilianum population genetic structure, evolutionary history, and adaptive potential remain poorly understood, largely due to the historical scarcity of whole-genome data. By screening 226 monkey samples from two Latin American countries, we identified 20 Plasmodium-positives across multiple primate species, highlighting the persistence of this parasite in sylvatic transmission cycles. We also investigated the evolutionary history and genetic diversity of P. malariae using whole-genome sequencing data. By combining 79 newly sequenced genomes with 248 publicly available genomes, we analyzed a filtered dataset comprising 179 P. malariae, two P. brasilianum, and two P. malariae-like genomes. Population structure analyses revealed the presence of two genetically distinct but recombining clusters across African P. malariae populations. These clusters occur across multiple African countries at varying frequencies, without clear geographic segregation. Genome-wide scans of genetic differentiation and selection further identified numerous cluster-specific signatures of adaptation, including loci putatively involved in interactions with human hosts and mosquito vectors. Our results provide the first evidence for fine-scale population substructure within African P. malariae and reveal ongoing adaptive processes that may contribute to its persistence and transmission. By uncovering previously unrecognized genetic diversity and selection patterns, this study highlights the importance of population genomic approaches for understanding the evolutionary dynamics of this neglected malaria parasite. Author summaryHuman malaria is most often associated with few well-studied parasites (Plasmodium falciparum and Plasmodium vivax), while other species have received far less attention. One such species is Plasmodium malariae, which often causes long-lasting infections with few or no symptoms. Because infections are difficult to detect, the true impact of this parasite is likely underestimated, particularly in Africa, where it is widespread. A closely related form, known as Plasmodium brasilianum, infects monkeys in South America with transmission between humans and wild animals. In our study, we analyzed the parasite genomes from infected people and monkeys to better understand the genetic diversity of these parasites, their population structure, and the molecular evidence of adaptation. We found evidence that P. brasilianum continues to circulate in several wild monkey species in South America. By analyzing nearly 300 genomes, we discovered that African P. malariae consists of two genetically distinct but recombining groups that coexist across the continent. These groups differ in specific regions of their genomes, including genes likely involved in interactions with human hosts and mosquito vectors. Our results reveal cryptic genetic diversity and structure with evidence of ongoing adaptation. These findings highlight the importance of including neglected parasites in malaria surveillance and control efforts.

genomics↗

bioRxiv2026

A gap-free, telomere-to-telomere genome assembly for the Caenorhabditis briggsae reference strain AF16

The nematode Caenorhabditis elegans was the first metazoan to have its genome completely sequenced and assembled. Since that time, researchers have continuously updated the reference genome and manually curated its approximately 20,000 genes. The closely related species, Caenorhabditis briggsae, has served as a comparative model because of its similar morphology, mode of reproduction, and patterns of intra-species genetic variation. However, the genomic resources for C. briggsae lag behind C. elegans, hindering comparative genomics studies between the species. Decades of experimentation have been performed in the AF16 reference strain genetic background, so we obtained high-coverage long-read sequencing and high-throughput chromosome conformation capture data to create an updated reference genome for an isogenic derivative of AF16, named CGC2. The CGC2 genome is vastly improved relative to the existing AF16 assemblies, with no unplaced sequence, no gaps, and telomere-to-telomere contiguity. To provide genomic resources for CGC2, we exploited deep RNA-seq libraries from all developmental stages to predict protein-coding gene annotations comparable in accuracy and completeness to the existing AF16 gene models. We also performed lift-over of 108 validated insertion-deletion variants to the updated coordinate system of the CGC2 genome to facilitate future mappings of mutations. In summary, we present an updated reference genome for the canonical AF16 reference strain with improved genomic resources to enable high-quality intra- and inter-species comparative studies.

genomics↗

bioRxiv2026

Gigabase-scale deletion scanning of the human genome

What fraction of the human genome is essential for cellular viability? To date, essentiality in human cells has been mapped almost exclusively at the level of individual open reading frames (ORFs). Whether noncoding regions and broader architectural features of the genome are required for human cells to remain viable, and where the boundaries of any such regions lie, remains largely unexplored. Here we introduce Shred-seq, which couples Type I-C CRISPR-Cas3-mediated deletions with phage polymerase-based genotyping to enable large-scale deletion scans of the human genome. Shred-seq leverages thousands of genomically integrated, mapped target sites as launchpads for Cas3 to initiate unidirectional deletions ranging in size from hundreds of base pairs (bp) to hundreds of kb. Breakpoints are directly captured at high resolution by in vitro or in situ transcription from flanking phage polymerase promoters, enabling bulk or single-cell phenotyping, respectively. In this proof-of-concept, we generate and genotype 36,257 independent deletions originating from 9,604 Cas3 launchpads, individually spanning 100 bp to 500 kb, collectively covering 461 Mb (14% of the human genome), and totaling to 2.55 Gb of deleted sequence ([~]10-fold coverage of these regions). Surviving deletions are depleted not only for essential protein-coding genes, but also for active, conserved and mutation-constrained non-coding sequences, directly quantifying purifying selection across both coding and noncoding intervals. Deletion length distributions further enable annotation-agnostic fine mapping of essential region boundaries and establish an empirical lower bound on the fraction of the human genome required for cellular viability. Finally, we demonstrate compatibility with single-cell RNA-seq (scRNA-seq), enabling direct linkage of specific deletions to transcriptional phenotypes. Together, Shred-seq provides a scalable platform for the systematic dissection of genome architecture and noncoding function, as well as for generating training data for predictive and generative models of genome structure-function, analogous to the roles that conventional Perturb-seq screens play for ORFs. By enabling the empirical delimitation of the genomic content required for human cellular viability, Shred-seq may also open a path to the construction of a minimal human genome.

genomics↗

bioRxiv2026

Chromosome-level Genome Assembly of the Potato Leafhopper Empoasca fabae (Hemiptera: Cicadellidae)

The potato leafhopper, Empoasca fabae (Harris, 1841), is a highly polyphagous, migratory insect pest of eastern North America that feeds on more than 200 herbaceous and woody plant species, causing substantial losses to forage and field crops. Despite its agricultural and ecological importance, no genome has been available for this species. Here, we present the first chromosome-level genome assembly of E. fabae, generated from Oxford Nanopore long reads, Illumina short reads, and Omni-C proximity-ligation data. The final assembly spans 908 Mb across 132 scaffolds, with 99.8% of the assembly captured in ten chromosome-length scaffolds (nine autosomes and an X chromosome) with a scaffold N50 of 96.2 Mb. The assembly is highly complete, recovering 92.4% of conserved hemipteran single-copy orthologs, and is composed of 47.6% repetitive sequence, dominated by long terminal repeat retrotransposons and unclassified elements. Read-depth comparison between male and female individuals supports assignment of a single sex-linked chromosome, consistent with an XO sex-determination system. BRAKER3 gene annotation predicted 31,406 protein-coding genes after retaining the longest isoform per locus. Comparative genome analysis against the two closest related Typhlocybinae species with genomes available, Matsumurasca onukii and Hebata decipiens, revealed extensive chromosome-scale collinearity, while defining a shared core gene repertoire. This reference genome provides a foundation for comparative and population genomic studies and for investigating genetic traits in this economically important crop pest species. SIGNIFICANCELeafhoppers (Cicadellidae) are among the most diverse families of plant-feeding insects, but chromosome-level genomes remain scarce, particularly for mesophyll-feeding members of the subfamily Typhlocybinae. The potato leafhopper, Empoasca fabae, is an unusually polyphagous crop and migratory pest of major importance across North America. Here, we provide the first chromosome-level genome assembly for this species. This chromosomal reference reveals broad synteny with two related Typhlocybinae relatives. This assembly will serve as a critical resource, enabling further comparative genomics, population genomics, and functional studies of host-plant adaptation in a significant agricultural crop pest species.

genomics↗

bioRxiv2026

Integrated pangenome and population genomics reveal selection on standing genetic variation driving fiber flax-linseed divergence

Flax (Linum usitatissimum L.) has been domesticated for dual end uses as linseed and fiber flax, yet the genomic basis of morphotype divergence remains unclear. Here, we constructed a morphotype-resolved pangenome by integrating three newly generated near telomere-to-telomere genome assemblies with 14 previously published ones. Despite substantial variation in assembly size, driven primarily by DNA transposons, gene content was highly conserved, with little evidence for significant morphotype-specific gene presence-absence variation. Population genomic analyses of 407 accessions revealed that fiber flax had reduced nucleotide diversity, extended linkage disequilibrium, and a more compact population structure relative to linseed, consistent with stronger selection and a narrower genetic base. Genome-wide differentiation was heterogeneous and concentrated in discrete regions. Integration of FST, nucleotide diversity ratios, Tajimas D, and genome-wide association signals identified morphotype-enriched genomic blocks distributed across the genome. Many candidate regions are primarily supported by directional shifts in nucleotide diversity rather than extreme differentiation, indicating selection on standing genetic variation. Genome-wide association analyses identified 1,712 unique quantitative trait nucleotides (QTNs), with predominantly small effect sizes and strong enrichment in gene-proximal regions, consistent with a polygenic architecture. Overall, fiber flax traits tend to be controlled by fewer loci with moderate-to-large effects, whereas linseed traits exhibit a more diffuse genetic architecture. Patterns of Tajimas D further support non-classical selection dynamics, with predominantly positive values in linseed and localized negative values in fiber flax, consistent with selection on standing genetic variation. Together, our results suggest that flax morphotype divergence is driven primarily by selection on pre-existing allelic variation within a conserved gene repertoire. This study provides a comprehensive framework linking genome structure, population genomics, and trait architecture, and highlights the importance of standing genetic variation as a key resource for flax breeding and improvement.

genomics↗

bioRxiv2026

The genome of the avian malaria parasite Haemoproteus majoris (lineage WW2) and its relationship to other Plasmodium species

Avian malaria parasites form a highly prevalent and genetically diverse group within the haemosporidians, yet they have long been overlooked relative to their human- and rodent-infecting counterparts. Among these, parasites of the genus Haemoproteus (Haemosporida, Haemoproteidae) are widespread and prevalent blood parasites of birds, transmitted by louse flies (Hippoboscidae) and biting midges (Ceratopogonidae). Recent phylogenomic analyses place Haemoproteus parasites at the root of the haemosporidian tree, making genomic data from these taxa essential for understanding the evolutionary origins of malaria parasites. To date, only two avian Plasmodium and one avian Haemoproteus genomes have been sequenced. We present the first assembled genome of Haemoproteus majoris (lineage WW2), a common blood parasite of passerine birds. As avian erythrocytes are nucleated, parasite DNA was enriched by FACS-based sorting to discriminate and isolate the parasite from host cell nuclei prior to whole-genome amplification. The genome was assembled using Nanopore long-read sequencing and polished with Illumina short-reads, yielding 145 contigs with a total assembly size of 23.9Mb and a G+C content of 27.85%. Genome annotation identified 5501 protein-coding genes, 69 non-coding RNA genes, and 57 long terminal repeat retrotransposons (LRT-RTs), including one full-length element. This genomic resource represents a critical step towards elucidating the evolutionary history and genomic architecture of avian malaria parasites. SIGNIFICANCE STATEMENTWe present the first assembled genome of Haemoproteus majoris (lineage WW2), a prevalent and generalist avian malaria parasite. Taxonomic resolution of this genus is difficult as there are few distinct morphological differences among closely-related species. This genome provides a valuable resource for studying the evolution within the Haemoproteus genus and to elucidate the evolutionary history of malaria parasites.

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bioRxiv2026

Three annotated tiger beetle genomes (Coleoptera, Adephaga, Cicindelidae)

Advances and accessibility to next-generation technology provide opportunities to sequence non-model organisms. Despite this increase in whole-genome sequencing data, annotation and the production of reference genomes remain limited. Reference genomes are a critical tool for a variety of studies in evolutionary biology, functional genomics, and conservation genetics. Tiger beetles (Cicindelidae) are a diverse and globally distributed family of beetles that serve as bioindicator taxa and flagship species for insect conservation. Here, we report highly complete, contiguous, and annotated genome assemblies representing draft reference genomes for three species of tiger beetle spanning the phylogeny. These draft reference genomes are for Audouins night-stalking tiger beetle, Omus audouini; the montane giant tiger beetle, Amblycheila baroni; and the western red-bellied tiger beetle, Cicindelidia sedecimpunctata. Article SummaryTiger beetles are a charismatic group with [~]3000 species distributed globally. Despite their popularity among insect enthusiasts and their role as bioindicators of ecosystem health, the group currently lacks a reference genome. This article outlines genome assembly and annotation for three tiger beetle species that span evolutionary relationships within the lineage. Quality control analyses show that the assemblies are reference quality and demonstrate high contiguity, completeness, and accuracy. The resulting draft genome annotations will be a valuable resource for scientific endeavors and allow for continued research on tiger beetles and their allies.

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bioRxiv2026

A Practical Framework for Constructing Population-Specific and Alternate-Contig-Aware Genome References: A case study of Vietnam

Current studies in human genomics typically rely on the standard genome reference GRCh38 which is known to be biased toward populations of European ancestry and therefore has limitations when applied to other populations. Although various graph-based pangenome references were constructed for several populations to deal with this bias, their usage in practice is currently still limited compared to linear genome references. Here we present a framework for constructing a population-specific genome reference using GRCh38 as backbone with alternate-contig awareness to enhance genomic data analysis in the target population. We demonstrated the advantages of our framework using both public and in-house Vietnamese whole-genome sequencing (WGS) datasets. Genomic variants derived from high-coverage WGS data of the 1000 Vietnamese Genomes Project (VN1K) were imported into our framework to build a Vietnamese-specific Genome Reference (VGR). VGR was then compared to GRCh38 in read alignment and variant calling using high-coverage WGS data of 99 Vietnamese individuals (KHV) from the 1000 Genomes Project (1kGP). Using Omni array genotyping data from 99 KHV samples as an independent benchmark, we found that VGR improved variant-calling precision and reduced false-positive calls compared to GRCh38. Our framework could be easily used for other populations as long as they have a variant database similar to VN1K. Our code is publicly available at github.com/VinGenome/VGR

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bioRxiv2026

Genome reshuffling as a route to specialization? Chromosome-level insights from the bark beetle symbiont genus Geosmithia.

Evolution of genome architecture is increasingly recognized as a major driver of fungal adaptation. However, because most studies have focused on plant and human pathogens, the genomic mechanisms underlying adaptation beyond pathogenic lifestyles remain poorly understood. Here, we establish the bark beetle-associated fungal genus Geosmithia as a model for studying genome evolution during ecological transitions. Geosmithia species occupy diverse ecological niches ranging from generalists, facultative symbionts of multiple beetle vectors and host tree species to more specialized conifer specialists and obligate nutritional symbionts of ambrosia beetles. Using chromosome-level genome assemblies of eleven species representing independently evolved ecological strategies, we investigated how genome architecture, repetitive DNA, and gene family evolution contribute to this ecological diversification. Comparative genomics revealed pervasive intra- and interchromosomal rearrangements, even between recently diverged species, demonstrating that extensive chromosome restructuring can accumulate over short evolutionary timescales. Breakpoints frequently occurred in gene-rich rather than repeat-rich regions, contrasting with the transposable element-associated patterns commonly described in fungi. Hi-C analyses revealed a typical Rabl chromosome organization but highly unusual centromeres that are repeat-poor, gene-rich, transcriptionally active, and conserved as syntenic blocks across species. Further, centromere identity may be determined by a highly divergent CenH3 variant. Despite similar genome sizes, specialists had increased repetitive DNA content and widespread gene family contractions, consistent with metabolic streamlining during ecological specialization. Our results demonstrate that extensive structural genome evolution can occur in gene-dense fungal genomes and establish Geosmithia as a powerful model for studying the interplay between genome evolution and ecological diversification.

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bioRxiv2026

Population genomics of Actinobacillus pleuropneumoniae reveals lineage-associated K:O antigen and Apx toxin diversity

Actinobacillus pleuropneumoniae (APP) is the causative agent of porcine pleuropneumonia, causing high mortality and morbidity and substantial economic losses in the swine industry. Nineteen capsular (K) serotypes, defined by variation in capsular polysaccharide (CPS) biosynthesis loci, ten O serotypes, defined by variation in lipopolysaccharide (LPS) biosynthesis loci, characteristic Apx toxin profiles, and genomic diversity have been increasingly described; however, relationships among genomic lineage, surface antigen genotype, and Apx toxin repertoire remain incompletely resolved. Here, we analyzed 328 APP genomes, including 153 newly sequenced isolates and 175 public genomes from 21 countries. Core-genome SNP phylogeny and FastBAPS identified 23 genomic lineages. Nineteen capsular K types and 10 O-antigen types formed 26 K:O genotypes, of which 24 were restricted to a single lineage. Adjusted Wallace analysis showed greater concordance of K:O genotype with lineage than K type alone (0.881 versus 0.759), and with ApxI, ApxII and ApxIII genotype (0.949 versus 0.787). ApxIVA related sequence was detected in all genomes, while 56 complete loci revealed recurrent length variation within the N-terminus or the RTX repeat rich region and an additional divergent ApxIVA related homolog. In conclusion, our comprehensive population genomic study gives a more informative picture of APP genomics and virulence-associated diversity, providing a useful genomic basis for future diagnostic test and vaccine development.

genomics↗

bioRxiv2026

The Braarudosphaera bigelowii genome reveals cellular remodeling around a nitrogen-fixing organelle

Braarudosphaera bigelowii hosts the nitrogen-fixing cyanobacterial organelle UCYN-A, or the nitroplast, but the absence of a reference genome has limited investigation of the genomic features accompanying this association. Here, we present nuclear and organellar genome resources for the cultured B. bigelowii strain FR-21. The assembly comprises a 472.1-Mb nuclear genome together with representative nitroplast, chloroplast, and mitochondrial genomes of 1.49 Mb, 107 kb, and 44 kb, respectively. The four genomic compartments have strongly differentiated GC compositions, consistent with their distinct evolutionary origins. The large nuclear genome does not appear to reflect a corresponding expansion of protein-coding capacity. Repeat annotation indicates that approximately 77% of the nuclear assembly is repetitive, leaving approximately 108 Mb of nonrepetitive sequence containing roughly 16,000 predicted protein-coding genes. Phylogenomic analysis places B. bigelowii within the expected diversity of haptophytes, indicating that its unusual diazotrophic lifestyle is not associated with an anomalous phylogenetic position. Comparative analysis of nitrogen metabolism genes reveals extensive remodeling of host nitrogen acquisition and assimilation. B. bigelowii lacks major components required for nitrate and nitrite assimilation and has reduced capacity to use urea, cyanate, and several internally recycled nitrogen sources. In contrast, ammonium transport and the glutamine synthetase-glutamate synthase pathway are retained. This combination suggests reduced flexibility in nitrogen use and a host metabolism increasingly centered on ammonium supplied by the nitroplast. B. bigelowii also encodes an expanded set of flavodoxin and cryptochrome genes. Eleven of 14 flavodoxin candidates and 21 of 39 cryptochrome candidates contained motifs linked to nitroplast targeting. Together with previous evidence that some of these proteins are abundantly imported into the nitroplast, these expansions point to host-encoded roles in nitroplast redox metabolism and diel regulation. These genome resources establish a foundation for investigating nitroplast protein import, host-organelle metabolic integration, and the genomic consequences of acquiring an intracellular nitrogen-fixing compartment. They further show that the evolution of the B. bigelowii-UCYN-A relationship involved substantial remodeling of genome architecture and nitrogen metabolism in both partners.

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bioRxiv2026

The chromosome-scale genome of the drought-adapted Caatinga tree Spondias tuberosa reveals a conserved, unduplicated architecture within Anacardiaceae

Spondias tuberosa (umbu) is a tree endemic to the semi-arid Caatinga of north-eastern Brazil, adapted to recurrent drought and high temperatures. Although genomes of mango (Mangifera indica), pistachio (Pistacia vera) and cashew (Anacardium occidentale) are available, comparative analyses of genome evolution across Anacardiaceae remain scarce. We aimed to generate a chromosome-scale reference genome for S. tuberosa and use it to examine genome architecture, meiotic recombination and gene family evolution within the family. We assembled the genome of a single individual using PacBio HiFi and Omni-C (Hi-C) data, mapped meiotic crossovers by single-nucleus RNA sequencing of pollen, and compared the genome with those of six other Sapindales species through orthology, synteny, synonymous substitution (Ks) and gene family expansion/contraction analyses within a dated phylogenomic framework. The assembly anchors 288 Mb onto 16 chromosomes (98.8% BUSCO completeness) and contains 24,232 protein-coding genes, 52.01% repetitive DNA and 1.78 million heterozygous SNPs (0.62%). We identified 1,740 crossovers in 153 pollen nuclei (4.01 cM.Mb1), which were depleted in repeat-rich pericentromeric regions. Ks analyses revealed a whole-genome duplication shared by Mangifera and Anacardium that is absent from S. tuberosa, which retains only the ancient {gamma} triplication. The 97 gene families expanded in S. tuberosa, mostly through tandem duplication, include receptor-like kinases, glutathione S-transferases and disease-resistance proteins, consistent with roles in environmental sensing and stress tolerance. Genome structure has remained highly conserved across Sapindales, except in the mango cashew lineage, which underwent extensive rearrangements following a whole-genome duplication. As sister to the remaining Anacardiaceae sampled and with an unduplicated, largely collinear architecture, S. tuberosa provides a reference for comparative mapping and for transferring genomic information to economically important woody relatives.

genomics↗