Biology subjects

Valent, B.

Publications and source records attributed to Valent, B..

15 recordsLinked to original sources

bioRxiv2026

Evolution of supernumerary chromosomes in wheat blast fungal pathogens

A genome of Pyricularia oryzae (synonym Magnaporthe oryzae), the fungus that causes blast disease on diverse grass species, has seven core chromosomes and may contain supernumerary mini-chromosomes. The P. oryzae Triticum (PoT) pathotype is the phylogenetic lineage responsible for devastating epidemics of wheat blast disease. Genomic analysis of wheat blast field isolates from the initial outbreak in 1985 in Brazil through recent field isolates in South America revealed dynamic presence and structure of mini-chromosomes. Two "earliest" field isolates representing founder lineages for the Triticum pathotype contain similar mini-chromosomes. Another PoT founder isolate from 1986 and 37 out of 39 Triticum field isolates collected between 1986 and 1992 lack mini-chromosomes. Mini-chromosomes present in the founder strains each contain two copies of the PWT7 wheat blast avirulence gene, and PWT7 was lost from subsequent early strains through mini-chromosome loss. Almost all PoT field isolates from 2005 to 2020 have regained mini-chromosomes in which PWT7 sequences have been replaced by other sequences. Telomere-to-telomere assemblies of 11 mini-chromosomes identified two major mini-chromosome types in the South American PoT population, and demonstrated significant within-mini-chromosome sequence alterations as well as recombination with other mini-chromosomes or core chromosome ends. Additionally, our data indicate horizontal mini-chromosome transfer between Pyricularia species, resulting in nearly identical genomic fragments shared between P. oryzae and Pyricularia pennisetigena isolates in the PWT4 avirulence gene region. Our genomic analysis depicts the dynamic mini-chromosome compartment in the diverse South American Triticum field population through time, indicating important roles for mini-chromosomes in pathogen adaptation and pathogenicity.

evolutionary biology↗

bioRxiv2026

Distinct modes of sequence evolution and epigenetic modifications underpin the origins of Starship-mediated variation in Pyricularia fungal plant pathogens

Fungal pathogens display remarkable variation in genome content and organization that directly impacts their survival and host interactions. Although numerous models have been proposed to explain the origins of this variation, they generally fail to explain or predict the mechanisms that generate the genome variation observed in natural populations. Starships are a recently discovered group of giant fungal transposons that carry dozens of genes as cargo and horizontally transfer both within and between species. Here, we identify the features of a newly defined "Starship compartment" in the major fungal plant pathogen Pyricularia oryzae. We test the hypothesis that the Starship compartment makes distinct contributions to fungal genome evolution by explicitly comparing its transferability, mutability, and epigenetic modifications with those of the canonical core and accessory compartments. To enable this, we developed an updated and user-friendly version of the annotation tool stargraph for the comprehensive annotation of Starships and Starship-like regions. Using this approach, we identified two distinct families of Starships and related Starship-like regions in P. oryzae that differ in their activity, impacts on genome organization, modes of sequence evolution, and epigenetic modifications. Elements from the more active family exhibit higher rates of structural variation than all other genomic compartments in the predominantly clonal isolates infecting rice. Both families of Starships encode specific suites of known effector sequences that contribute to plant disease and Starship activity accounts for avirulence gene turnover, which suggests that evolutionary change within the Starship compartment may subsequently impact the evolution of plant-fungal interactions. Starships from the more active family have repeatedly transferred across the Pyricularia genus and tend to be depleted in heterochromatic histone modifications and repeat-induced point mutations. However, contrasting histone modification profiles in this family suggests a genomic conflict between silencing or maintaining Starship activity. Our findings demonstrate that variation in the mode of sequence diversification and epigenetic modification within the Starship compartment underpins the impacts of these giant transposons on fungal genome evolution. We argue for the explicit consideration of not only the Starship compartment but of element-specific dynamics when investigating the evolution of host-fungal interactions.

genomics↗

bioRxiv2024

Highly dynamic supernumerary mini-chromosomes in a Magnaporthe oryzae strain

Magnaporthe oryzae (syn. Pyricularia oryzae), the causative agent of devastating crop diseases, exhibits remarkable genomic plasticity that contributes to its adaptability and pathogenicity. Individual M. oryzae strains may contain supernumerary mini-chromosomes, which are dispensable and highly repetitive. Here, we explored the stability of two mini-chromosomes of a Lolium strain isolated in the US, TF05-1, in which one mini-chromosome contains sequences nearly identical to the mini-chromosome of the wheat isolate B71 from Bolivia. The discordance of their phylogenetic relationships based on genomic polymorphisms in core chromosomes and polymorphisms in mini-chromosomes indicated horizontal transfer of the mini-chromosome. Karyotyping analysis and genome sequencing analysis found variation in numbers and sizes of mini-chromosomes among asexual monoconidial progeny of TF05-1. Rearrangement within mini-chromosomes occurred frequently in the TF05-1 progeny. We characterized an intrachromosomal rearrangement presumably mediated by a palindrome repeat. The rearrangement resulted in a 300-kb deletion and a 900-kb duplication. The susceptibility to structural variation in mini-chromosomes may be associated with repetitive features and the high activity of transposable elements in mini-chromosomes, in which many intact retrotransposons were recently inserted, largely unmethylated, and likely have yet to be silenced by fungal genome defense mechanisms such as repeat-induced point mutation.

genomics↗

bioRxiv2023

Using recurrent neural networks to detect supernumerary chromosomes in fungal strains causing blast diseases

The genomes of the fungus Magnaporthe oryzae that causes blast diseases on diverse grass species, including major crop plants, have indispensable core-chromosomes and may contain one or more additional supernumerary chromosomes, also known as mini-chromosomes. The mini-chromosome is speculated to play a role in fungal biology, provide effector gene mobility, and may transfer between strains. To understand and study the biological function of mini-chromosomes, it is crucial to be able to identify whether a given strain of M. oryzae possesses a mini-chromosome. In this study, we applied recurrent neural network models, more specifically, Bidirectional Long Short-Term Models (Bi-LSTM), for classifying DNA sequences as core-or mini-chromosomes. The models were trained with sequences from multiple available core- and mini-chromosome assemblies. The trained model was then used to predict the presence of the mini-chromosome in a global collection of M. oryzae isolates using short-read DNA sequences. The model predicted that the mini-chromosome was prevalent in M. oryzae isolates, including those isolated from rice, wheat, Lolium and many other grass species. Interestingly, 23 recent wheat strains collected since 2005 all carried the mini-chromosome, but none of nine early strains collected before 1991 had the mini-chromosome, indicating the preferential selection for strains carrying the mini-chromosome in recent years. Based on the limited sample size, we found the presence of the mini-chromosome in isolates of pathotype Eleusine was not as high as isolates of other pathotypes. The deep learning model was also used to identify assembled sequence contigs that were derived from the mini-chromosome and partial regions on core-chromosomes potentially translocated from a mini-chromosome. In summary, our study has developed a reliable method for categorizing DNA sequences and showcases an application of recurrent neural networks in the field of predictive genomics.

genomics↗

bioRxiv2023

Natural genomic variation in rice blast genomes is associated with specific heterochromatinmodifications

Genome organization in eukaryotes exhibits non-random patterns tied to transcription, replication, and chromatin. However, the driving forces across these processes, and their impacts on genome evolution remain unclear. To address this, we analyzed sequence data from 86 Magnaporthe oryzae isolates, a globally important plant pathogenic fungus, alongside new reference genomes, to investigate DNA sequence variation and the epigenome. Histone modification profiles were obtained through genome-wide chromatin immunoprecipitation-sequencing of the four new reference strains, which revealed that repressive histone marks (H3K27me3, H3K9me3) were associated with SNP and INDEL frequency. Densely grouped SNPs were found to reside in heterochromatin, often outside transposable elements, highlighting the link between heterochromatin and DNA variation. Even when controlling for selection, silent SNP frequency/kb was higher in H3K27me3-associated genes. Effector genes, key to pathogenicity, also displayed this trend. Comparing the reference strains, euchromatic regions were often syntenic, while heterochromatic regions trended towards non-syntenic. Heterochromatin emerged as a major factor associated with diverse DNA variations in M. oryzae populations, even when accounting for selective pressure. This underscores heterochromatins pivotal role in shaping genetic diversity in these mainly asexually reproducing fungi.

evolutionary biology↗

bioRxiv2022

A Re-evaluation of phylogenomic data reveals that current understanding in wheat blast population biology and epidemiology is obfuscated by oversights in population sampling

Wheat blast, caused by the Triticum lineage of Pyricularia oryzae (PoT), is a serious disease that first emerged in Brazil and quickly spread to neighboring countries. The recent appearance of this disease in Bangladesh and Zambia highlights a need to understand the population biology and epidemiology of the disease so as to mitigate pandemic outbreaks. Current knowledge in these areas is largely based on analyses of wheat blast isolates collected in Brazil, and their comparison with isolates from non-wheat, endemic grasses. Those studies concluded that wheat blast is caused by a highly diverse P. oryzae population that lacks host specificity and, as a result, undergoes extensive gene flow with populations infecting non-wheat hosts. Additionally, based on genetic similarity between wheat blast and isolates infecting Urochloa species, it was proposed that the disease originally emerged via a host jump from this grass, and the widespread use of Urochloa as a pasture grass likely plays a central role in wheat blast epidemiology. Inconsistencies with earlier phylogenetic studies prompted us to re-analyze the Brazilian data in the context of a comprehensive, global, phylogenomic dataset. We now show that the seminal studies failed to sample the P. oryzae populations normally found on endemic grasses and, instead, repeatedly sampled PoT and P. oryzae Lolium (PoL) members that happened to be present in these hosts. The resulting lack of accurate and representative information about the grass-infecting populations in Brazil means that current conclusions about wheat blasts evolution, population biology and epidemiology are unsubstantiated and could be equivocal.

microbiology↗

bioRxiv2022

Rapid mini-chromosome divergence among fungal isolates causing wheat blast outbreaks in Bangladesh and Zambia

Global wheat production is seriously threatened by the filamentous fungal pathogen, Magnaporthe oryzae, causing wheat blast disease. The pathogen was first identified in South America and recently spread across continents to Bangladesh (South Asia) and Zambia (South-central Africa). M. oryzae strains closely related with a South American field isolate B71 was found to have caused the wheat blast outbreaks in South Asia and Africa. Here, we studied the genetic relationship among isolates found on the three continents. Using an improved reference genome for B71 and whole genome sequences of isolates from Bangladesh, Zambia, and South America, we found strong evidence to support that the outbreaks in Bangladesh and Zambia were caused by the introductions of genetically separated isolates. Structural variation analysis using whole genome short-read sequencing data indicate all isolates closely related to B71 maintained at least one supernumerary mini-chromosome and, interestingly, some Zambian isolates contain more than one mini-chromosome. Long-read sequencing and de novo genome assemblies of two Zambian isolates show that both contain a mini-chromosome similar to the B71 mini-chromosome, although pervasive structural variation exists among them. Genome assemblies also provide evidence that one Zambian isolate carries an additional mini-chromosome that is highly divergent from the B71 mini-chromosome. Our findings show that while the core genomes of the multiple introductions are highly similar, the mini-chromosomes have undergone marked diversification. The maintenance of the mini-chromosome during the multiple introductions, and the rapid sequence and structural variation suggests the mini-chromosomes may serve important virulence or niche adaptation roles under diverse environmental conditions.

genomics↗

bioRxiv2021

Clathrin-mediated Endocytosis Facilitates Internalization of Magnaporthe oryzae Effectors into Rice Cells

Fungi and oomycetes deliver effectors into living plant cells to suppress defenses and control plant processes needed for infection. Little is known about the mechanism by which these pathogens translocate effector proteins across the plasma membrane into the plant cytoplasm. The blast fungus Magnaporthe oryzae secretes cytoplasmic effectors into a specialized biotrophic interfacial complex (BIC) before translocation. Here we show that cytoplasmic effectors within BICs are packaged into vesicles that are occasionally observed in the host cytoplasm. Live cell imaging with fluorescently-labeled rice showed that effector vesicles colocalize with plant plasma membrane and with clathrin light chain-1, a marker for clathrin-mediated endocytosis (CME). Inhibition of CME using Virus-Induced Gene Silencing (VIGS) and chemical treatments results in cytoplasmic effectors in swollen BICs lacking vesicles. In contrast, fluorescent marker co-localization, VIGS and chemical inhibitor studies failed to support a major role for clathrin-independent endocytosis in effector vesicle formation. Localization studies of two novel effectors, and of known effectors after CME inhibition, indicate that cytoplasmic effector translocation occurs underneath appressoria before invasive hyphal growth. Taken together, this study provides evidence that cytoplasmic effector translocation is mediated by clathrin-mediated endocytosis in BICs and suggests a role for M. oryzae effectors in co-opting plant endocytosis.

plant biology↗

bioRxiv2021

Recombination of standing variation in a multi-hybrid swarm drove adaptive radiation in a fungal pathogen and gave rise to two pandemic plant diseases

Adaptive radiations fuel speciation and are characterized by rapid genetic diversification and expansion into new ecological niches. Historically, these processes were believed to be driven by selection on novel mutations but genomic analyses now indicate that standing variation and gene flow often have prominent roles. How "old" variation is combined, however, and its resulting genetic architecture within newly-adapted populations is not well understood. We reconstructed a recent radiation in the fungus, Pyricularia oryzae, that spawned a population pathogenic to eleven grass genera, and caused two new plant diseases: wheat blast - already a serious threat to global agriculture - and gray leaf spot of ryegrasses. We show that the new population evolved in a multi-hybrid swarm using only the standing variation that was present in seven individuals from five distinct, host-specialized lineages. Sexual and parasexual recombination within the swarm reassorted key host-specificity factors and generated more diversity in possibly just a few weeks than existing lineages had accumulated over hundreds to thousands of years. We suggest that the process was initiated by sexual opportunity arising when a fertile fungal strain was imported into Brazil on Urochloa introduced as forage for beef production; and we further contend that the host range expansion was largely fortuitous, with host selection playing little, if any, role in driving the process. Finally, we believe that our findings point to an overlooked role for happenstance in creating situations that allow organisms to skirt rules that would normally hold evolution in check. Commercial Endorsement DisclaimerMention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. Equal Opportunity/Non-Discrimination StatementUSDA is an equal opportunity provider and employer.

evolutionary biology↗

bioRxiv2021

CRISPR-Cas12a induced DNA double-strand breaks are repaired by locus-dependent and error-prone pathways in a fungal pathogen

CRISPR-Cas mediated genome engineering has revolutionized functional genomics. However, basic questions remain regarding the mechanisms of DNA repair following Cas-mediated DNA cleavage. We developed CRISPR-Cas12a ribonucleoprotein genome editing in the fungal plant pathogen, Magnaporthe oryzae, and found frequent donor DNA integration despite the absence of long sequence homology. Interestingly, genotyping from hundreds of transformants showed that frequent non-canonical DNA repair outcomes predominated the recovered genome edited strains. Detailed analysis using sanger and nanopore long-read sequencing revealed five classes of DNA repair mutations, including single donor DNA insertions, concatemer donor DNA insertions, large DNA deletions, deletions plus donor DNA insertions, and infrequently we observed INDELs. Our results show that different error-prone DNA repair pathways resolved the Cas12a-mediated double-strand breaks (DSBs) based on the DNA sequence of edited strains. Furthermore, we found that the frequency of the different DNA repair outcomes varied across the genome, with some tested loci resulting in more frequent large-scale mutations. These results suggest that DNA repair pathways provide preferential repair across the genome that could create biased genome variation, which has significant implications for genome engineering and the genome evolution in natural populations.

microbiology↗

bioRxiv2020

Estimating Assembly Base Errors Using K-mer Abundance Difference (KAD) Between Short Reads and Genome Assembled Sequences

Genome sequences provide genomic maps with a single-base resolution for exploring genetic contents. Sequencing technologies, particularly long reads, have revolutionized genome assemblies for producing highly continuous genome sequences. However, current long-read sequencing technologies generate inaccurate reads that contain many errors. Some errors are retained in assembled sequences, which are typically not completely corrected by using either long reads or more accurate short reads. The issue commonly exists but few tools are dedicated for computing error rates or determining error locations. In this study, we developed a novel approach, referred to as K-mer Abundance Difference (KAD), to compare the inferred copy number of each k-mer indicated by short reads and the observed copy number in the assembly. Simple KAD metrics enable to classify k-mers into categories that reflect the quality of the assembly. Specifically, the KAD method can be used to identify base errors and estimate the overall error rate. In addition, sequence insertion and deletion as well as sequence redundancy can also be detected. Therefore, KAD is valuable for quality evaluation of genome assemblies and, potentially, provides a diagnostic tool to aid in precise error correction. KAD software has been developed to facilitate public uses.

bioinformatics↗

bioRxiv2020

A hierarchical transcriptional network controls appressorium-mediated plant infection by the rice blast fungus Magnaporthe oryzae

Rice blast is a pervasive and devastating disease that threatens rice production across the world. In spite of its importance to global food security, however, the underlying biology of plant infection by the blast fungus Magnaporthe oryzae remains poorly understood. In particular, it is unclear how the fungus elaborates a specialised infection cell, the appressorium, in response to surface signals from the rice leaf. Here, we report the identification of a network of temporally co-regulated transcription factors that act downstream of the Pmk1 mitogen-activated protein kinase pathway to regulate gene expression during appressorium-mediated plant infection. We have functionally characterised this network of transcription factors and demonstrated the operation of a hierarchical transcriptional control system. We show that this tiered regulatory mechanism involves Pmk1-dependent phosphorylation of the Hox7 homeobox transcription factor, which represses hyphal-associated gene expression and simultaneously induces major physiological changes required for appressorium development, including cell cycle arrest, autophagic cell death, turgor generation and melanin biosynthesis. Mst12 then regulates gene functions involved in septin-dependent cytoskeletal re-organisation, polarised exocytosis and effector gene expression necessary for plant tissue invasion.

microbiology↗

bioRxiv2018

Magnaporthe oryzae SMO1 encodes a Ras GTPase-activating protein required for spore morphology, appressorium function and rice blast disease

The pathogenic life cycle of the rice blast fungus Magnaporthe oryzae involves a series of morphogenetic changes, essential for its ability to cause disease. The smo mutation was identified more than twenty-five years ago and affects the shape and development of diverse cell types in M. oryzae, including conidia, appressoria and asci. All attempts to clone the SMO1 gene by map-based cloning and/or complementation, have failed over many years. Here, we report the identification of SMO1 by a combination of bulk segregant analysis and comparative genome analysis. SMO1 encodes a GTPase-activating protein (GAP), which regulates Ras signalling during infection-related development. Targeted deletion of SMO1 results in abnormal, non-adherent conidia, impaired in their production of spore tip mucilage. Smo1 mutants also develop smaller appressoria, with a severely reduced capacity to infect rice plants. SMO1 is necessary for organisation of microtubules and for septin-dependent remodelling of the F-actin cytoskeleton at the appressorium pore. Smo1 physically interacts with components of the Ras2 signaling complex, and a range of other signalling and cytoskeletal components, including the four core septins. SMO1 is therefore necessary for regulation of RAS activation required for conidial morphogenesis and septin-mediated plant infection.

microbiology↗

bioRxiv2018

Effector Gene Reshuffling Involves Dispensable Mini-chromosomes in the Wheat Blast Fungus

Newly emerged wheat blast disease is a serious threat to global wheat production. Wheat blast is caused by a distinct, exceptionally diverse lineage of the fungus causing rice blast disease. To understand genetic diversity in wheat-infecting strains, we report a near-finished reference genome of a recent field isolate generated using long read sequencing and a novel scaffolding approach with long-distance paired genomic sequences. The genome assemblage includes seven core chromosomes and sequences from a dispensable mini-chromosome that harbors effector genes normally found on the ends of core chromosomes in other strains. No mini-chromosomes were observed in an early field strain, and two mini-chromosomes from another field isolate each contain different effector homologous genes and core chromosome end sequences. The mini-chromosome is highly repetitive and is enriched in transposons occurring most frequently at core chromosome ends. Additionally, transposons in mini-chromosomes lack the characteristic signature for inactivation by repeat-induced point (RIP) mutation genome defenses. Our results, collectively, indicate that dispensable mini-chromosomes and non-dispensable core chromosomes undergo divergent evolutionary trajectories, and mini-chromosomes and core chromosome ends are coupled as a mobile, fast-evolving effector compartment in the wheat pathogen genome.\n\nSignificance statementThe emerging blast disease on wheat is proving even harder to control than the ancient, still-problematic rice blast disease. Potential wheat resistance identified using strains isolated soon after disease emergence are no longer effective in controlling recent aggressive field isolates from wheat in South America and South Asia. We report that recent wheat pathogens can contain one or two highly-variable conditionally-dispensable mini-chromosomes, each with an amalgamation of effector sequences that are duplicated or absent from pathogen core chromosome ends. Well-studied effectors found on different core chromosomes in rice pathogens appear side-by-side in wheat pathogen mini-chromosomes. The rice pathogen often overcomes deployed resistance genes by deleting triggering effector genes. Localization of effectors on mini-chromosomes, which are unstably transmitted during growth, would accelerate pathogen adaptation in the field.

genomics↗

bioRxiv2017

Gene flow between divergent cereal- and grass-specific lineages of the rice blast fungus Magnaporthe oryzae

Delineating species and epidemic lineages in fungal plant pathogens is critical to our understanding of disease emergence and the structure of fungal biodiversity, and also informs international regulatory decisions. Pyricularia oryzae (syn. Magnaporthe oryzae) is a multi-host pathogen that infects multiple grasses and cereals, is responsible for the most damaging rice disease (rice blast), and of growing concern due to the recent introduction of wheat blast to Bangladesh from South America. However, the genetic structure and evolutionary history of M. oryzae, including the possible existence of cryptic phylogenetic species, remain poorly defined. Here, we use whole-genome sequence information for 76 M. oryzae isolates sampled from 12 grass and cereal genera to infer the population structure of M. oryzae, and to reassess the species status of wheat-infecting populations of the fungus. Species recognition based on genealogical concordance, using published data or extracting previously-used loci from genome assemblies, failed to confirm a prior assignment of wheat blast isolates to a new species (Pyricularia graminis tritici). Inference of population subdivisions revealed multiple divergent lineages within M. oryzae, each preferentially associated with one host genus, suggesting incipient speciation following host shift or host range expansion. Analyses of gene flow, taking into account the possibility of incomplete lineage sorting, revealed that genetic exchanges have contributed to the makeup of multiple lineages within M. oryzae. These findings provide greater understanding of the eco-evolutionary factors that underlie the diversification of M. oryzae and highlight the practicality of genomic data for epidemiological surveillance in this important multi-host pathogen.\n\nImportanceInfection of novel hosts is a major route for disease emergence by pathogenic micro-organisms. Understanding the evolutionary history of multi-host pathogens is therefore important to better predict the likely spread and emergence of new diseases. Magnaporthe oryzae is a multi-host fungus that causes serious cereal diseases, including the devastating rice blast disease, and wheat blast, a cause of growing concern due to its recent spread from South America to Asia. Using whole genome analysis of 76 fungal strains from different hosts, we have documented the divergence of M. oryzae into numerous lineages, each infecting a limited number of host species. Our analyses provide evidence that inter-lineage gene flow has contributed to the genetic makeup of multiple M. oryzae lineages within the same species. Plant health surveillance is therefore warranted to safeguard against disease emergence in regions where multiple lineages of the fungus are in contact with one another.

microbiology↗