🧬 genomics

A chromosome-scale genome assembly of the Swiss Lolium multiflorum ecotype Tremona reveals a scalable method to purge spurious duplications

This study presents a high-quality, chromosome-scale genome assembly of the Swiss *Lolium multiflorum* ecotype Tremona and introduces a scalable tool called ParaLies to effectively purge spurious duplications caused by heterozygosity, thereby significantly improving assembly accuracy for this complex forage species.

Original authors: Piat, L., Herren, G., Grieder, C., Roulin, A. C.

Published 2026-09-05

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CC BY 4.0

Original authors: Piat, L., Herren, G., Grieder, C., Roulin, A. C.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Grasslands are the silent engines of European agriculture, covering nearly half the continent's farmland and providing the essential feed that sustains cattle and dairy herds. For centuries, farmers have managed these fields, but modern breeding has turned them into semi-natural landscapes shaped by human hands. Among the grasses used, Italian ryegrass stands out for its rapid growth and high-quality feed, making it a cornerstone of livestock production. However, improving this grass through breeding is a complex task. Unlike many crops that can be self-pollinated to create uniform lines, Italian ryegrass is naturally outcrossing, meaning it breeds within large, genetically diverse populations. To breed better varieties, scientists need a precise map of the plant's genetic code, a genome assembly, to understand which genes control desirable traits like drought tolerance or disease resistance. Yet, creating such a map for this specific grass has been a formidable challenge because its genetic code is enormous, filled with repeating patterns, and highly variable from one individual to another, often causing computer algorithms to get confused and create false copies of the same genetic regions.

A team of researchers has now overcome these hurdles by producing a high-resolution, chromosome-scale map of the genome for a specific Swiss variety of Italian ryegrass known as Tremona. This variety, collected in 2008 in the Ticino region, has been a workhorse in Swiss breeding programs for decades. The scientists used advanced sequencing technology to read the DNA, but they encountered a common problem: the software initially created a map that was cluttered with "spurious duplications." These are not real, extra copies of genes that evolved over millions of years, but rather artificial duplicates created because the software could not tell the difference between two slightly different versions of the same gene that exist within a single plant. To solve this, the team developed a new computational tool called ParaLies. This tool acts like a meticulous editor, scanning the genetic map to identify and remove these false copies while carefully preserving the genuine, ancient duplicates that are part of the plant's true evolutionary history.

The result of this careful editing is a remarkably clean and accurate genome. Before the correction, the map showed that nearly 17 percent of its essential genes appeared to be duplicated, a sign that the assembly was flawed. After applying the new tool, this number dropped to less than 7 percent, bringing the map in line with what is expected for a healthy, single set of chromosomes. The final assembly spans 2.06 billion building blocks of DNA, organized into seven distinct chromosomes, and captures 94 percent of the plant's total genetic material. It reveals that the genome is dominated by jumping genetic elements, specifically a type called Gypsy, which make up half of the entire sequence and tend to hide in the regions between genes, leaving the gene-rich areas relatively clear. This high-quality reference provides a solid foundation for future breeding efforts, allowing scientists to pinpoint the exact genetic variations that make certain plants more resilient or productive.

Beyond the technical achievement of the map itself, the study offers a fresh look at the genetic identity of the Tremona population. By comparing the DNA of the Tremona plants with other Swiss wild varieties and populations from North America, the researchers found that Tremona is genetically distinct and remarkably uniform. Unlike other populations that show signs of recent mixing or inbreeding, the Tremona group maintains a stable genetic balance, suggesting it has adapted well to its local environment without losing its diversity. This genetic homogeneity, combined with its unique traits, marks it as a valuable and distinct resource for breeders looking to introduce new characteristics into their crops. The study also highlights that while we have made progress with this specific variety, much of the genetic diversity of Italian ryegrass across Europe remains unexplored, pointing to the need for broader studies to fully understand how these plants adapt to different environments.

The development of the ParaLies tool represents a significant step forward for the field, offering a cost-effective and scalable way to clean up genetic maps for other complex, highly variable plants. By demonstrating that high-quality genomes can be assembled using standard sequencing data combined with smart computational filtering, the researchers have provided a blueprint that other breeding programs can follow. This work does not just give us a better map of one grass; it provides a method to ensure that future maps are accurate, free from the confusion of artificial duplicates, and ready to guide the next generation of agricultural innovation. The Tremona genome stands as a testament to what can be achieved when advanced technology is paired with a deep understanding of the biological complexities inherent in the plants that feed the world.

Technical Summary: A Chromosome-Scale Genome Assembly of Lolium multiflorum Ecotype Tremona

Problem Statement
Italian ryegrass (Lolium multiflorum) is a critical temperate forage species in Europe, yet its genomic resources are limited by a large (2.2 Gb), highly repetitive (>70% transposable elements), and highly heterozygous genome. High heterozygosity poses a specific challenge for de novo assembly: unresolved haplotypes often result in "spurious duplications," where allelic regions are incorrectly assembled as separate paralogous sequences. This artifact inflates gene duplication estimates, distorts downstream analyses (e.g., SNP calling, copy number variation), and obscures true evolutionary signals. While hybrid sequencing strategies (combining PacBio, Nanopore, and Hi-C) can produce contiguous assemblies, they remain prohibitively expensive for many breeding programs. Furthermore, existing L. multiflorum assemblies often retain high levels of redundancy even after standard curation.

Methodology
The authors assembled the genome of the Swiss L. multiflorum ecotype "Tremona," collected in Ticino in 2008 and utilized in Swiss breeding programs.

Sequencing and Initial Assembly: The team generated 120.60 Gb of PacBio HiFi reads (~54.8× coverage). They performed de novo assembly using hifiasm, selecting the best-balanced pseudo-haplotype (hap1) based on BUSCO completeness.
Initial Curation: The assembly underwent curation using the PurgeGrass module of the PhaseGrass pipeline to remove haplotigs, followed by reference-guided scaffolding against the L. multiflorum "GULF" assembly using RagTag.
Development of ParaLies: To address residual spurious duplications that persisted after initial curation (16.91% duplicated BUSCOs), the authors developed ParaLies, a post-assembly tool.
- Principle: ParaLies distinguishes between true evolutionary paralogs and haplotype-induced artifacts by analyzing synonymous divergence ( $K_s$ ), read depth, and shared polymorphism.
- Logic: True paralogs (ancient duplications) exhibit high $K_s$ values, independent read depth, and low shared polymorphism. Spurious duplications (recent allelic splits) show near-zero $K_s$ , depleted read depth (as reads are split between copies), and high shared polymorphism.
- Implementation: The tool uses a lightweight Python pipeline that operates on structural annotations. It classifies intra-genomic syntenic blocks based on a user-defined $K_s$ threshold (set at 0.2) and auxiliary signals (normalized depth < 0.8× or shared polymorphism > 0.1) to identify and excise one copy of the artifactual pair.
Population Genomics: The authors generated Illumina whole-genome resequencing data for 10 Tremona individuals and 10 additional Swiss ecotypes. These were integrated with public data from 94 individuals across six North American populations to perform population structure analysis (PCA, neighbor-joining trees, sNMF), diversity estimation ( $\pi$ , Tajima's $D$ ), and inbreeding coefficient ( $F_{IS}$ ) calculations.

Key Results

Genome Assembly Quality: The final Tremona assembly spans 2.06 Gb across seven pseudo-chromosomes. It achieves a contig N50 of 15.69 Mb and a scaffold N50 of 325.01 Mb. The assembly captures 94% of the estimated genome size and has a consensus quality value (QV) of 74.3.
Effectiveness of ParaLies: Application of ParaLies reduced the duplicated BUSCO rate from 16.91% to 6.72% without significant loss of bona fide genomic content. Single-copy completeness increased to 84.16%. The tool successfully removed 196 Mb of redundant sequence, primarily recent intra-chromosomal blocks, while preserving ancient inter-chromosomal blocks associated with the pan-grass $\rho$ whole-genome duplication.
Genome Architecture: The assembly is 70.22% repetitive, dominated by Gypsy LTR retrotransposons (1.02 Gb). Repeat density is high in pericentromeric regions, while gene density is concentrated on chromosome arms.
Population Genetic Insights:
- Distinctiveness: Tremona forms a genetically homogeneous cluster distinct from other Swiss ecotypes and North American populations, with no evidence of recent bottlenecks or substantial admixture.
- Diversity: Despite its distinctness, Tremona maintains nucleotide diversity ( $\pi = 0.0151$ ) statistically equivalent to most other populations in the panel.
- Demography: Unlike other populations which show negative Tajima's $D$ (indicating potential selection or expansion), Tremona exhibits a Tajima's $D$ indistinguishable from zero, suggesting it is a demographically stable population in Hardy-Weinberg equilibrium.

Significance and Claims
The paper claims that the Tremona genome assembly and the ParaLies tool provide valuable resources for L. multiflorum genomics and breeding.

Scalable Methodology: The authors demonstrate that a scalable, cost-efficient approach using only PacBio HiFi data, combined with the ParaLies post-processing tool, can effectively purge haplotype-induced redundancy in highly heterozygous diploid genomes. This offers an alternative to expensive hybrid sequencing strategies for breeding programs.
Resource Availability: The high-quality, chromosome-scale assembly serves as a new reference for gene discovery, structural variation analysis, and marker-assisted breeding in Italian ryegrass.
Genetic Characterization: The study highlights Tremona as a distinct and potentially valuable source of adaptive variation for Swiss breeding programs, while simultaneously underscoring the need for broader population genomic studies across the European native range to fully resolve patterns of local adaptation and diversity.

The authors remain modest regarding the scope of their findings, noting that while Tremona is distinct, the broader genetic landscape of European L. multiflorum remains largely uncharacterized, and further studies are required to harness this diversity fully.

Technical Summary: A Chromosome-Scale Genome Assembly of Lolium multiflorum Ecotype Tremona

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