Biology subjects

Fischer, M. G.

Publications and source records attributed to Fischer, M. G..

9 recordsLinked to original sources

bioRxiv2026

Single-cell-resolved natural giant virus infections reveal conserved protist immunity

Single-cell eukaryotes (protists) are key players in microbial ecosystems. Yet, our understanding of how they interact with viruses in nature and how antiviral defenses shape these relationships remains limited. Using a single-cell multi-omics approach, we mapped associations between giant viruses and their hosts and identified active giant-virus infections in an alpine lake protist community. In naturally infected dinoflagellates, infection was accompanied by the selective induction of viperin, an ancient antiviral enzyme conserved from bacteria to humans. We also observed virus-encoded homologs of immunity-related genes whose expression correlated with viperin expression, raising the possibility of a role in viral counter-defense. These findings reveal an ecological dimension of protist immunity and provide a framework for tracing the host-virus arms race across divergent cellular lineages.

microbiology↗

bioRxiv2025

Strain-level diversity of giant viruses infecting chlorarachniophyte algae in the subtropical North Pacific

Giant DNA viruses are ubiquitous among unicellular eukaryotes and occur in marine, freshwater, and terrestrial environments. Despite intense metagenomic data mining, their strain-level diversity remains largely unexplored. Here we introduce a model system comprising four isolates of a giant virus called ChlorV, which infects marine microalgae of the class Chlorarachniophyceae (Rhizaria) from station ALOHA, Hawaii. The ChlorV genomes are 469 kbp to 493 kbp long and encode approximately 400 proteins, at least 106 of which are present n purified virions. Although the four viral genomes are highly syntenic, they differ by several insertions and deletions that often encode methyltransferases. Interestingly, we found that some of these methyltransferase genes correlated with specific DNA methylation patterns in the ChlorV strain Our study describes the first giant viruses infecting the eukaryotic supergroup Rhizaria and demonstrates how viral strain-level variation in gene content and epigenetic features may affect eco-evolutionary processes in marine microalgae.

microbiology↗

bioRxiv2023

Amazing structural diversity of giant virus-like particles in forest soil

Large DNA viruses of the phylum Nucleocytoviricota infect diverse eukaryotic hosts from protists to humans, with profound consequences for aquatic and terrestrial ecosystems. While nucleocytoviruses are known to be highly diverse in metagenomes, knowledge of their capsid structures is restricted to a few characterized representatives. Here, we visualize giant virus-like particles (VLPs, diameter >0.2 {micro}m) directly from the environment using transmission electron microscopy. We found that Harvard Forest soils contain a higher diversity of giant VLP morphotypes than all hitherto isolated giant viruses combined. These included VLPs with icosahedral capsid symmetry, ovoid shapes similar to pandoraviruses, and bacilliform shapes that may represent novel viruses. We discovered giant icosahedral capsids with structural modifications that had not been described before including tubular appendages, modified vertices, tails, and capsids consisting of multiple layers or internal channels. Many giant VLPs were covered with fibers of varying lengths, thicknesses, densities, and terminal structures. These findings imply that giant viruses employ a much wider array of capsid structures and mechanisms to interact with their host cells than is currently known. We also found diverse tailed bacteriophages and filamentous VLPs, as well as ultra-small cells. Our study offers a first glimpse of the vast diversity of unexplored viral structures in soil and reinforces the potential of transmission electron microscopy for fundamental discoveries in environmental microbiology.

microbiology↗

bioRxiv2023

Evolution of virus and virophage facilitates persistence in a tripartite microbial system

Tripartite biotic interactions are inherently complex, and the strong interdependence of species and high levels of exploitation can make these systems short-lived and vulnerable to extinction. The persistence of species depends then on the balance between exploitation and avoidance of exploitation of the resource beyond the point where sustainable exploitation is no longer possible. We used this general prediction to test the potential for long-term persistence in a recently discovered tripartite microbial system in which a eukaryotic host is preyed upon by a giant virus that is in turn parasitized by a virophage. Host and virophage may benefit from this interaction because the virophage reduces the harmful effects of the giant virus on the host population over time and the virophage can survive integrated into the host genome when giant viruses are scarce. Here, we grew hosts in the presence and absence of the giant virus and virophage over ~280 host generations. We found that the three players persisted, but that the beneficial effect of the virophage for the host population diminished over time. We further tested whether the level of exploitation and replication evolved in the giant virus and/or virophage population over the course of the experiment and whether the changes were such that they avoid overexploitation. We found that the giant virus evolved towards lower replication levels and the virophage towards increased replication but decreased giant virus exploitation. These changes are predicted to facilitate persistence by lowering giant virus and host exploitation and consequently reducing the protective effect of the virophage.

evolutionary biology↗

bioRxiv2021

proovframe: frameshift-correction for long-read (meta)genomics

Long-read sequencing technologies hold big promises for the genomic analysis of complex samples such as microbial communities. Yet, despite improving accuracy, basic gene prediction on long-read data is still often impaired by frameshifts resulting from small indels. Consensus polishing using either complementary short reads or to a lesser extent the long reads themselves can mitigate this effect but requires universally high sequencing depth, which is difficult to achieve in complex samples where the majority of community members are rare. Here we present proovframe, a software implementing an alternative approach to overcome frameshift errors in long-read assemblies and raw long reads. We utilize protein-to-nucleotide alignments against reference databases to pinpoint indels in contigs or reads and correct them by deleting or inserting 1-2 bases, thereby conservatively restoring reading-frame fidelity in aligned regions. Using simulated and real-world benchmark data we show that proovframe performs comparably to short-read-based polishing on assembled data, works well with remote protein homologs, and can even be applied to raw reads directly. Together, our results demonstrate that protein-guided frameshift correction significantly improves the analyzability of long-read data both in combination with and as an alternative to common polishing strategies. Proovframe is available from https://github.com/thackl/proovframe.

bioinformatics↗

bioRxiv2021

MOSGA 2: Comparative genomics and validation tools

Due to the highly growing number of available genomic information, the need for accessible and easy-to-use analysis tools is increasing. To facilitate eukaryotic genome annotations, we created MOSGA. In this work, we show how MOSGA 2 is developed by including several advanced analyses for genomic data. Since the genomic data quality greatly impacts the annotation quality, we included multiple tools to validate and ensure high-quality user-submitted genome assemblies. Moreover, thanks to the integration of comparative genomics methods, users can benefit from a broader genomic view by analyzing multiple genomic data sets simultaneously. Further, we demonstrate the new functionalities of MOSGA 2 by different use-cases and practical examples. MOSGA 2 extends the already established application to the quality control of the genomic data and inte-grates and analyzes multiple genomes in a larger context, e.g., by phylogenetics.

bioinformatics↗

bioRxiv2020

Endogenous virophages populate the genomes of a marine heterotrophic flagellate

Virophages can parasitize giant DNA viruses and may provide adaptive anti-giant-virus defense in unicellular eukaryotes. Under laboratory conditions, the virophage mavirus integrates into the nuclear genome of the marine flagellate Cafeteria burkhardae and reactivates upon superinfection with the giant virus CroV. In natural systems, however, the prevalence and diversity of host-virophage associations has not been systematically explored. Here, we report dozens of integrated virophages in four globally sampled C. burkhardae strains that constitute up to 2% of their host genomes. These endogenous mavirus-like elements (EMALEs) separated into eight types based on GC-content, nucleotide similarity, and coding potential and carried diverse promoter motifs implicating interactions with different giant viruses. Between host strains, some EMALE insertion loci were conserved indicating ancient integration events, whereas the majority of insertion sites were unique to a given host strain suggesting that EMALEs are active and mobile. Furthermore, we uncovered a unique association between EMALEs and a group of tyrosine recombinase retrotransposons, revealing yet another layer of parasitism in this nested microbial system. Our findings show that virophages are widespread and dynamic in wild Cafeteria populations, supporting their potential role in antiviral defense in protists.

microbiology↗

bioRxiv2020

Visualization of giant virus particles using BONCAT labeling and STED microscopy

Giant DNA viruses of the phylum Nucleocytoviricota are being increasingly recognized as important regulators of natural protist populations. However, our knowledge of their infection cycles is still very limited due to a lack of cultured virus-host systems and molecular tools to study them. Here, we apply bioorthogonal noncanonical amino acid tagging (BONCAT) to pulse label the marine heterotrophic flagellate Cafeteria burkhardae during infection with the lytic giant virus CroV. In absence of CroV, we report efficient incorporation of the L-methionine analog L-azidohomoalanine (AHA) into newly synthesized proteins of the methionine prototrophic C. burkhardae. During CroV infection, AHA was predominantly found in viral proteins, and single CroV virions were imaged with stimulated emission depletion (STED) super-resolution microscopy. CroV particles incorporated AHA with 95-100% efficiency while retaining their infectivity, which makes BONCAT/STED a powerful tool to study viral replication cycles in this ecologically relevant marine bacterivore. SignificanceGiant DNA viruses are the dominant class of protist-infecting viruses, yet the vast majority of described giant virus-protist systems remain uncultured. One of the better studied cultured systems is composed of the stramenopile Cafeteria burkhardae (previously C. roenbergensis), the giant Cafeteria roenbergensis virus (CroV) and the virophage mavirus. C. burkhardae is a widespread marine phagotrophic protist that plays an important role in regulating bacterial populations. In addition to being grazed upon by larger zooplankton, C. burkhardae populations are controlled by the lytic giant virus CroV. In turn, CroV is parasitized by the virophage mavirus that increases host population survival in the presence of CroV and forms a mutualistic symbiosis with its host. Despite being of fundamental ecological and evolutionary interest, this tripartite host-virus-virophage system suffers from a lack of molecular tools. Here, we show that CroV particles can be fluorescently labeled and imaged by super-resolution microscopy. To achieve this we established robust procedures for analyzing protist and viral populations and implemented the use of bioorthogonal noncanonical amino acid tagging (BONCAT) in a marine unicellular flagellate.

microbiology↗

bioRxiv2019

Four high-quality draft genome assemblies of the marine heterotrophic nanoflagellate Cafeteria roenbergensis

The heterotrophic stramenopile Cafeteria roenbergensis is a globally distributed marine bacterivorous protist. This unicellular flagellate is host to the giant DNA virus CroV and the virophage mavirus. We sequenced the genomes of four cultured C. roenbergensis strains and generated 23.53 Gb of Illumina MiSeq data (99-282 x coverage per strain) and 5.09 Gb of PacBio RSII data (13-54 x coverage). Using the Canu assembler and customized curation procedures, we obtained high-quality draft genome assemblies with a total length of 34-36 Mbp per strain and contig N50 lengths of 148 kbp to 464 kbp. The C. roenbergensis genome has a GC content of ~70%, a repeat content of ~28%, and is predicted to contain approximately 7857-8483 protein-coding genes based on a combination of de novo, homology-based and transcriptome-supported annotation. These first high-quality genome assemblies of a Bicosoecid fill an important gap in sequenced Stramenopile representatives and enable a more detailed evolutionary analysis of heterotrophic protists.

genomics↗