Biology subjects

Roulin, A. C.

Publications and source records attributed to Roulin, A. C..

10 recordsLinked to original sources

bioRxiv2026

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

Italian ryegrass (Lolium multiflorum) is a key temperate forage species underpinning livestock production in Europe. Genomic resources remain limited by its large (2.2 Gb), repetitive, and highly heterozygous genome. Here, we present a high-quality chromosome-scale genome assembly of the Swiss L. multiflorum ecotype Tremona, collected in 2008 in Ticino, Switzerland, and subsequently incorporated into recurrent breeding cycles in the Swiss breeding program. To address systematic assembly artefacts caused by unresolved haplotypes in our initial PacBio HiFi assembly, we developed ParaLies, a post-assembly tool that identifies and removes artefactual duplications based on sequence divergence while preserving true paralogous gene copies. ParaLies reduced the duplicated BUSCO rate from 16.91% to 6.72% without loss of bona fide genomic content. The resulting assembly has a contig N50 of 15.69 Mb and captures 94% of the expected 2.2-Gb genome size. We further analyzed whole-genome resequencing data from Tremona, additional Swiss ecotypes, and publicly available North American germplasm. Tremona was genetically homogeneous, with no evidence of pronounced recent bottlenecks or substantial within-population structure, and was genetically distinct from the other Swiss ecotypes analyzed. Together, the Tremona genome and ParaLies provide valuable resources for L. multiflorum genomics and breeding and demonstrate a scalable approach for reducing haplotype-induced redundancy in highly heterozygous genomes.

genomics↗

bioRxiv2025

Exodermis lignification impacts lateral root emergence in Brachypodium distachyon

RationaleThe mechanisms controlling lateral root emergence in monocots, particularly the role of the exodermis, are poorly understood. We investigated how natural variation in the Brachypodium distachyon stress response shapes root system architecture by modulating cell wall dynamics. MethodsWe used root tip excision to synchronize lateral root development across natural accessions. The resulting phenotypes were analysed using comparative transcriptomics, biochemical lignin quantification, confocal Raman spectroscopy, and chemical inhibition of lignin biosynthesis. Key ResultsTwo distinct root system architectures, pine tree and fishbone, were identified. The fishbone phenotype results from an emergence-specific defect caused by the premature and intense lignification of the exodermis. This was driven by the transcriptional upregulation of lignin biosynthesis genes and was rescued by a lignin inhibitor. Main ConclusionStress-induced exodermal lignification acts as a mechanical brake on lateral root emergence. This positions the exodermis as a key regulatory hub that integrates environmental cues with developmental programs to control RSA plasticity in grasses.

plant biology↗

bioRxiv2023

Transposition of HOPPLA in siRNA-deficient plants suggests a limited effect of the environment on retrotransposon mobility in Brachypodium distachyon

Long terminal repeat retrotransposons (LTR-RTs) are powerful mutagens regarded as a major source of genetic novelty and important drivers of evolution. Yet, the uncontrolled and potentially selfish proliferation of LTR-RTs can lead to deleterious mutations and genome instability, with large fitness costs for their host. While population genomics data suggest that an ongoing LTR-RT mobility is common in many species, the understanding of their dual role in evolution is limited. Here, we harness the genetic diversity of 320 sequenced natural accessions of the Mediterranean grass Brachypodium distachyon to characterize how genetic and environmental factors influence plant LTR-RT dynamics in the wild. When combining a coverage-based approach to estimate global LTR-RT copy number variations with mobilome-sequencing of nine accessions exposed to eight different stresses, we find little evidence for a major role of environmental factors in LTR-RT accumulations in B. distachyon natural accessions. Instead, we show that loss of RNA polymerase IV (Pol IV), which mediates RNA-directed DNA methylation in plants, results in high transcriptional and transpositional activities of RLC_BdisC024 (HOPPLA) LTR-RT family elements, and that these effects are not stress-specific. This work supports findings indicating an ongoing mobility in B. distachyon and reveals that host RNA-directed DNA methylation rather than environmental factors controls their mobility in this wild grass model. Author summaryLong terminal repeat retrotransposons (LTR-RTs) are major components of plant genomes. Their copy- and-paste replication mechanism allows them to rapidly increase in copy number, with potentially negative effects on host fitness. On the other hand, because they can rewire transcriptional networks and alter phenotypes, their mobility is an important driver of evolution. Ever since their discovery, LTR-RT activity has been linked to stress exposure, suggesting that LTR-RTs modulate the pace of evolution in response to the environment. In this study, we test this hypothesis by harnessing the genetic variation in a set of 320 natural accessions of the Mediterranean grass Brachypodium distachyon originating from diverse habitats. We find little evidence for the importance of stresses in activating B. distachyon LTR-RTs. Instead, we show that the loss of RNA polymerase IV, a component of plant retrotransposon silencing, leads to the activation and transposition of an LTR-RT family that we name HOPPLA. HOPPLA is the first LTR-RT family in B. distachyon shown to transpose in real-time. These findings open up new avenues for studying retrotransposon-mediated evolution in this close relative of staple crops, such as rice and wheat.

genomics↗

bioRxiv2023

Polygenic architecture of flowering time and its relationship with local environments in the grass Brachypodium distachyon.

Synchronizing the timing of reproduction with the environment is crucial in the wild. Among the multiple mechanisms annual plants evolved to sense their environment, the requirement of cold-mediated vernalization is a major process that prevents individuals from flowering during winter. In many annual plants including crops, both a long and short vernalization requirement can be observed within species, resulting in so-called early-(spring) and late (winter)-flowering genotypes. Here, using the grass model Brachypodium distachyon, we explored the link between flowering time-related trait (vernalization requirement and flowering time), environmental variation, and diversity at flowering-time genes by combining measurements under greenhouse and outdoor conditions. These experiments confirmed that B. distachyon natural accessions display large differences regarding vernalization requirements and ultimately flowering time. We underline significant, albeit quantitative effects of current environmental conditions on flowering time-related traits. While disentangling the confounding effects of population structure on flowering time-related traits remains challenging, population genomics analyses indicate that well-characterized flowering-time genes may contribute significantly to flowering time variation and display signs of polygenic selection. Flowering-time genes, however, do not colocalize with GWAs peaks obtained with outdoor measurements, suggesting that additional genetic factors contribute to flowering time variation in the wild. Altogether, our study fosters our understanding of the polygenic architecture of flowering time in a natural grass system and opens new avenues of research to investigate the gene-by-environment interaction at play for this trait. Article SummarySynchronizing the timing of reproduction with the environment is crucial in the wild. We used here the model Brachypodium distachyon to expand our knowledge on the adaptive potential and polygenic architecture of flowering time in wild grasses. While genetic factors play an important role in flowering time variation in our species, we showed that flowering time correlates with environmental variables and is therefore a locally adapted trait.

evolutionary biology↗

bioRxiv2023

A diverse panel of 755 bread wheat accessions harbors untapped genetic diversity in landraces and reveals novel genetic regions conferring powdery mildew resistance

Wheat breeding for disease resistance relies on the availability and use of diverse genetic resources. More than 800,000 wheat accessions are globally conserved in gene banks, but they are mostly uncharacterized for the presence of resistance genes and their potential for agriculture. Based on the selective reduction of previously assembled collections for allele mining for disease resistance, we assembled a trait-customized panel of 755 geographically diverse bread wheat accessions with a focus on landraces, called the LandracePLUS panel. Population structure analysis of this panel based on the TaBW35K SNP array revealed an increased genetic diversity compared to 632 landraces genotyped in an earlier study and 17 high-quality sequenced wheat accessions. The additional genetic diversity found here mostly originated from Turkish, Iranian and Pakistani landraces. We characterized the LandracePLUS panel for resistance to ten diverse isolates of the fungal pathogen powdery mildew. Performing genome-wide association studies and dividing the panel further by a targeted subsetting approach for accessions of distinct geographical origin, we detected several known and already cloned genes, including the Pm2a gene. In addition, we identified 22 putatively novel powdery mildew resistance loci that represent useful sources for resistance breeding and for research on the mildew-wheat pathosystem. Our study shows the value of assembling trait-customized collections and utilizing a diverse range of pathogen races to detect novel loci. It further highlights the importance of integrating landraces of different geographical origins into future diversity studies. Key MessageA bread wheat panel reveals rich genetic diversity in Turkish, Pakistani, and Iranian landraces and novel resistance loci to diverse powdery mildew isolates via subsetting approaches in association studies.

plant biology↗

bioRxiv2020

Migration without interbreeding: evolutionary history of a highly selfing Mediterranean grass inferred from whole genomes

Wild plant populations show extensive genetic subdivision and are far from the ideal of panmixia which permeates population genetic theory. Understanding the spatial and temporal scale of population structure is therefore fundamental for empirical population genetics - and of interest in itself, as it yields insights into the history and biology of a species. In this study we extend the genomic resources for the wild Mediterranean grass Brachypodium distachyon to investigate the scale of population structure and its underlying history at whole-genome resolution. 86 accessions were sampled at local and regional scales in Italy and France, which closes a conspicuous gap in the collection for this model organism. The analysis of 196 accessions, spanning the Mediterranean from Spain to Iraq, suggests that the interplay of high selfing and seed dispersal rates has shaped genetic structure in B. distachyon. At the continental scale, the evolution in B. distachyon is characterized by the independent expansion of three lineages during the Upper Pleistocene. Today, these lineages may occur on the same meadow yet do not interbreed. At the regional scale, dispersal and selfing interact and maintain high genotypic diversity, thus challenging the textbook notion that selfing in finite populations implies reduced diversity. Our study extends the population genomic resources for B. distachyon and suggests that an important use of this wild plant model is to investigate how selfing and dispersal, two processes typically studied separately, interact in colonizing plant species.

evolutionary biology↗

bioRxiv2020

Impact of transposable elements on methylation and gene expression across natural accessions of Brachypodium distachyon.

Transposable elements (TEs) constitute a large fraction of plant genomes and are mostly present in a transcriptionally silent state through repressive epigenetic modifications such as DNA methylation. TE silencing is believed to influence the regulation of adjacent genes, possibly as DNA methylation spreads away from the TE. Whether this is a general principle or a context-dependent phenomenon is still under debate, pressing for studying the relationship between TEs, DNA methylation and nearby gene expression in additional plant species. Here we used the grass Brachypodium distachyon as a model and produced DNA methylation and transcriptome profiles for eleven natural accessions. In contrast to what is observed in Arabidopsis thaliana, we found that TEs have a limited impact on methylation spreading and that only few TE families are associated to a low expression of their adjacent genes. Interestingly, we found that a subset of TE insertion polymorphisms is associated with differential gene expression across accessions. Thus, although not having a global impact on gene expression, distinct TE insertions may contribute to specific gene expression patterns in B. distachyon. Significance statementTransposable elements (TEs) are a major component of plant genomes and a source of genetic and epigenetic innovations underlying adaptation to changing environmental conditions. Yet molecular evidence linking TE silencing and nearby gene expression are lacking for many plant species. We show that in the model grass Brachypodium DNA methylation spreads over very short distances around TEs, with an influence on gene expression for a small subset of TE families.

genomics↗

bioRxiv2019

Diversity, dynamics and effects of LTR retrotransposons in the model grass Brachypodium distachyon

O_LITransposable elements (TEs) are the main reason for the high plasticity of plant genomes, where they occur as communities of diverse evolutionary lineages. Because research has typically focused on single abundant families or summarized TEs at a coarse taxonomic level, our knowledge about how these lineages differ in their effects on genome evolution is still rudimentary.\nC_LIO_LIHere we investigate the community composition and dynamics of 32 long terminal repeat retrotransposon (LTR-RT) families in the 272 Mb genome of the Mediterranean grass Brachypodium distachyon.\nC_LIO_LIWe find that much of the recent transpositional activity in the B. distachyon genome is due to centromeric Gypsy families and Copia elements belonging to the Angela lineage. With a half-life as low as 66 ky, the latter are the most dynamic part of the genome and an important source of within-species polymorphisms. Second, GC-rich Gypsy elements of the Retand lineage are the most abundant TEs in the genome. Their presence explains more than 20 percent of the genome-wide variation in GC content and is associated to higher methylation levels.\nC_LIO_LIOur study shows how individual TE lineages change the genetic and epigenetic constitution of the host beyond simple changes in genome size.\nC_LI

genomics↗

bioRxiv2018

High-throughput genotyping of the spelt gene pool reveals patterns of agricultural history in Europe

Spelt, a close relative of hexaploid bread wheat and a dominant wheat subspecies cultivated in Europe before the 20th century, still plays an important role as a high-value niche product today. Compared to most other cereals, spelt has not been subjected to intensive breeding in the 20th century. Even today, mostly traditional landraces are cultivated on a regional scale. The traditional way of spelt cultivation has limited the extensive exchange of germ plasm and intermixing of genetic material, which makes spelt an ideal crop to study the early agricultural history of cereals in Europe. Here, we unraveled the population structure and agricultural history of spelt based on 22,999 high-quality SNPs obtained by genotyping-by-sequencing on 267 spelt accessions covering the entire cultivation range. SNP markers were aligned to the high-quality bread wheat reference genome, which allowed us to analyze individual subgenomes. Our analyses of genetic variation revealed that bread wheat and spelt are most likely of monophyletic origin, but that European spelt diverged from bread wheat by hybridization with tetraploid emmer wheats. Interestingly, spelt accessions from the Iberian Peninsula formed a separate clade that was distinct from the Central European accessions for all three subgenomes. Demographic modelling indicated that Iberian spelt was introduced into Europe independently from Central European spelt. Our analysis provides a comprehensive assessment of spelt diversity and history. The separate introduction of Iberian spelt is supported by recent molecular evidence of two independent prehistoric migrations of ancient farmers from the Near East into Europe.

plant biology↗

bioRxiv2018

Genome-wide scans of selection highlight the impact of biotic and abiotic constraints in natural populations of the model grass Brachypodium distachyon.

Grasses are essential plants for ecosystem functioning. Quantifying the selective pressures that act on natural variation in grass species is therefore essential regarding biodiversity maintenance. In this study, we investigate the selection pressures that act on two distinct populations of the grass model Brachypodium distachyon without prior knowledge about the traits under selection. We took advantage of whole-genome sequencing data produced for 44 natural accessions of B. distachyon and used complementary genome-wide scans of selection (GWSS) methods to detect genomic regions under balancing and positive selection. We show that selection is shaping genetic diversity at multiple temporal and spatial scales in this species and affects different genomic regions across the two populations. Gene Ontology annotation of candidate genes reveals that pathogens may constitute important factors of positive and balancing selection in Brachypodium distachyon. We eventually cross-validated our results with QTL data available for leaf-rust resistance in this species and demonstrate that, when paired with classical trait mapping, GWSS can help pinpointing candidate genes for further molecular validation. Thanks to a near-base perfect reference genome and the large collection of freely available natural accessions collected across its natural range, B. distachyon appears as a prime system for studies in ecology, population genomics and evolutionary biology.

evolutionary biology↗