Auswahl der wissenschaftlichen Literatur zum Thema „Polinton- like viruses“

Virophages are small dsDNA viruses that hijack the machinery of giant viruses during the co-infection of a protist (i.e., microeukaryotic) host and represent an exceptional case of “hyperparasitism” in the viral world. While only a handful of virophages have been isolated, a vast diversity of virophage-like sequences have been uncovered from diverse metagenomes. Their wide ecological distribution, idiosyncratic infection and replication strategy, ability to integrate into protist and giant virus genomes and potential role in antiviral defense have made virophages a topic of broad interest. However, one limitation for further studies is the lack of clarity regarding the nomenclature and taxonomy of this group of viruses. Specifically, virophages have been linked in the literature to other “virophage-like” mobile genetic elements and viruses, including polinton-like viruses (PLVs), but there are no formal demarcation criteria and proper nomenclature for either group, i.e., virophage or PLVs. Here, as part of the ICTV Virophage Study Group, we leverage a large set of genomes gathered from published datasets as well as newly generated protist genomes to propose delineation criteria and classification methods at multiple taxonomic ranks for virophages ‘sensu stricto’, i.e., genomes related to the prototype isolates Sputnik and mavirus. Based on a combination of comparative genomics and phylogenetic analyses, we show that this group of virophages forms a cohesive taxon that we propose to establish at the class level and suggest a subdivision into four orders and seven families with distinctive ecogenomic features. Finally, to illustrate how the proposed delineation criteria and classification method would be used, we apply these to two recently published datasets, which we show include both virophages and other virophage-related elements. Overall, we see this proposed classification as a necessary first step to provide a robust taxonomic framework in this area of the virosphere, which will need to be expanded in the future to cover other virophage-related viruses such as PLVs.

Abstract Background Polintons are large mobile genetic elements found in the genomes of eukaryotic organisms that are considered the ancient ancestors of most eukaryotic dsDNA viruses. Originally considered as transposons, they have been found to encode virus capsid genes, suggesting they may actually be integrated viruses; however, an extracellular form has yet to be detected. Recently, circa 25 Polinton-like viruses have been discovered in environmental metagenomes and algal genomes, which shared distantly related genes to both Polintons and virophages (Lavidaviridae). These entities could be the first members of a major class of ancient eukaryotic viruses; however, owing to the lack of available genomes for analysis, information on their global diversity, evolutionary relationships, eukaryotic hosts, and status as free virus particles is limited. Results Here, we analysed the metaviromes of an alpine lake to show that Polinton-like virus genome sequences are abundant in the water column. We identify major capsid protein genes belonging to 82 new Polinton-like viruses and use these to interrogate publicly available metagenomic datasets, identifying 543 genomes and a further 16 integrated into eukaryotic genomes. Using an analysis of shared gene content and major capsid protein phylogeny, we define large groups of Polinton-like viruses and link them to diverse eukaryotic hosts, including a new group of viruses, which possess all the core genes of virophages and infect oomycetes and Chrysophyceae. Conclusions Our study increased the number of known Polinton-like viruses by 25-fold, identifying five major new groups of eukaryotic viruses, which until now have been hidden in metagenomic datasets. The large enrichment (> 100-fold) of Polinton-like virus sequences in the virus-sized fraction of this alpine lake and the fact that their viral major capsid proteins are found in eukaryotic host transcriptomes support the hypothesis that Polintons in unicellular eukaryotes are viruses. In summary, our data reveals a diverse assemblage of globally distributed viruses, associated with a wide range of unicellular eukaryotic hosts. We anticipate that the methods we have developed for Polinton-like virus detection and the database of over 20,000 genes we present will allow for continued discovery and analysis of these new viral groups.

Abstract Polintons (also known as Mavericks) were initially identified as a widespread class of eukaryotic transposons named for their hallmark type B DNA polymerase and retrovirus-like integrase genes. It has since been recognized that many polintons encode possible capsid proteins and viral genome-packaging ATPases similar to those of a diverse range of double-stranded DNA (dsDNA) viruses. This supports the inference that at least some polintons are actually viruses capable of cell-to-cell spread. At present, there are no polinton-associated capsid protein genes annotated in public sequence databases. To rectify this deficiency, we used a data-mining approach to investigate the distribution and gene content of polinton-like elements and related DNA viruses in animal genomic and metagenomic sequence datasets. The results define a discrete family-like clade of viruses with two genus-level divisions. We propose the family name Adintoviridae, connoting similarities to adenovirus virion proteins and the presence of a retrovirus-like integrase gene. Although adintovirus-class PolB sequences were detected in datasets for fungi and various unicellular eukaryotes, sequences resembling adintovirus virion proteins and accessory genes appear to be restricted to animals. Degraded adintovirus sequences are endogenized into the germlines of a wide range of animals, including humans.

Eukaryotic genomes contain a variety of endogenous viral elements (EVEs), which are mostly derived from RNA and ssDNA viruses that are no longer functional and are considered to be “genomic fossils.” Genomic surveys of EVEs, however, are strongly biased toward animals and plants, whereas protists, which represent the majority of eukaryotic diversity, remain poorly represented. Here, we show that protist genomes harbor tens to thousands of diverse, ~14 to 40 kbp long dsDNA viruses. These EVEs, composed of virophages, Polinton-like viruses, and related entities, have remained hitherto hidden owing to poor sequence conservation between virus groups and their repetitive nature that precluded accurate short-read assembly. We show that long-read sequencing technology is ideal for resolving virus insertions. Many protist EVEs appear intact, and most encode integrases, which suggests that they have actively colonized hosts across the tree of eukaryotes. We also found evidence for gene expression in host transcriptomes and that closely related virophage and Polinton-like virus genomes are abundant in viral metagenomes, indicating that many EVEs are probably functional viruses.

Abstract In this study, we aimed at exploring horizontal gene transfer between viruses and Chlorodendraceae green algae (Chlorophyta) using available genomic and transcriptomic sequences for 20 algal strains. We identified a significant number of genes sharing a higher sequence similarity with viral homologues, thus signaling their possible involvement in HGTs with viruses. Further characterization showed that many of these genes were clustered in DNA regions of several tens to hundreds of kilobases in size, originally belonging to viruses related to known Tetraselmis spp. viruses (TetV and TsV). In contrast, the remaining candidate HGT genes were randomly dispersed in the algal genomes, more frequently transcribed and belonged to large multigene families. The presence of homologs in Viridiplantae suggested that these latter were more likely of algal rather than viral origin. We found a remarkable diversity Polinton-like virus (PLV) elements inserted in Tetraselmis genomes, all of which were most similar to the Tetrasemis striata virus TsV. The genes of PLV elements are transcriptionally inactive with the notable exception of the homologue of the TVSG_00024 gene of TsV whose function is unknown. We suggest that this gene may be involved in a sentinel process to trigger virus reactivation and excision in response to an environmental stimulus. Altogether, these results provide evidence that TsV-related viruses have a dual lifestyle, alternating between a free viral phase (i.e., virion) and a phase integrated into host genomes.

Twenty complete genomes (29–63 kb) and 29 genomes with an estimated completeness of over 90 % (30–90 kb) were identified for novel dsDNA viruses in the Yangshan Harbor metavirome. These newly discovered viruses contribute to the expansion of viral taxonomy by introducing 46 potential new families. Except for one virus, all others belong to the class Caudoviricetes. The exception is a novel member of the recently characterized viral group known as Gossevirus. Fifteen viruses were predicted to be temperate. The predicted hosts for the viruses appear to be involved in various aspects of the nitrogen cycle, including nitrogen fixation, oxidation and denitrification. Two viruses were identified to have a host of Flavobacterium and Tepidimonas fonticaldi, respectively, by matching CRISPR spacers with viral protospacers. Our findings provide an overview for characterizing and identifying specific viruses from Yangshan Harbor. The Gossevirus-like virus uncovered emphasizes the need for further comprehensive isolation and investigation of polinton-like viruses.

Bamfordviruses are arguably the most diverse group of viruses infecting eukaryotes. They include the Nucleocytoplasmic Large DNA viruses (NCLDVs), virophages, adenoviruses, Mavericks and Polinton-like viruses. Two main hypotheses for their origins have been proposed: the ‘nuclear-escape’ and ‘virophage-first’ hypotheses. The nuclear-escape hypothesis proposes an endogenous, Maverick-like ancestor which escaped from the nucleus and gave rise to adenoviruses and NCLDVs. In contrast, the virophage-first hypothesis proposes that NCLDVs coevolved with protovirophages; Mavericks then evolved from virophages that became endogenous, with adenoviruses escaping from the nucleus at a later stage. Here, we test the predictions made by both models and consider alternative evolutionary scenarios. We use a data set of the four core virion proteins sampled across the diversity of the lineage, together with Bayesian and maximum-likelihood hypothesis-testing methods, and estimate rooted phylogenies. We find strong evidence that adenoviruses and NCLDVs are not sister groups, and that Mavericks and Mavirus acquired the rve-integrase independently. We also found strong support for a monophyletic group of virophages (family Lavidaviridae) and a most likely root placed between virophages and the other lineages. Our observations support alternatives to the nuclear-escape scenario and a billion years evolutionary arms-race between virophages and NCLDVs.

Ce travail de thèse a été consacrée à l'étude d'un bassin de culture de microalgues (HRAP) avec l'objectif d'examiner les épisodes de mortalité massive, dont la cause est inconnue. Nous avons testé l'hypothèse que des virus de microalgues pourraient être responsables. Cette étude représente la première tentative d'exploration de la diversité virale dans un HRAP, en même temps que la collecte de données des hôtes potentiels grâce au métabarcodage 18S. L’analyse bioinformatique de métagénomes a permis d’identifier des virus présents dans le HRAP, et la dynamique de leurs populations a été suivie par (RT)-qPCR sur une série d'échantillons d'eau prélevés sur deux ans de culture. Ces virus appartiennent à la famille Marnaviridae (Ordre des Picornavirales ; virus à ARN), les Nucleocytoviricota de la famille Phycodnaviridae et de la famille Mimiviridae, un membre de la famille Lavidaviridae (virophage), et les « polinton-like viruses » (PLVs), tous ayant des associations connues avec les microalgues. Le dernier chapitre de la thèse décrit une étude bioinfomatique des séquences génomiques d’algues vertes unicellulaires du genre Tetraselmis qui a permis d’identifier et de caractériser des formes virales intégrées (i.e., ADN viral inséré dans les chromosomes de l’algue) apparentées au virus Tsv-N1, aux PLVs mis en évidence dans le HRAP, ainsi qu’au virus géant TetV-1. Cette analyse étend nos connaissances sur la diversité des virus des Tetraselmis et la complexité des interactions biologiques et évolutives entre ces partenaires
This thesis is devoted to the study of an industrial scale microalgae culturing system, called a high rate algal pond (HRAP), situated in Palavas-les-Flots, France. The objective of the study was to investigate culture crashes (i.e. microalgae die-offs) occurring in the HRAP, of which the source is unknown. We hypothesized that microalgal viruses were contained within the culture, and could potentially cause or contribute to the microalgae die-offs. We assessed the viral diversity by sequencing both RNA and DNA viromes. Using in silico analyses, putative viruses were identified in the HRAP, and tracked over a series of water samples taken over two years of culturing by (RT)-qPCR methods. A number of putative viruses of microalgae were uncovered. These include key players such as family Marnaviridae, families Phycodnaviridae and Mimiviridae (so-called “giant viruses”), a member of family Lavidaviridae (i.e. a virophage), and polinton-like viruses (PLVs), all with known associations to microalgae. An in-depth exploration of these key players was conducted, and host inferences were made using 18S metabarcoding, coupled with dynamics data from our (RT)-qPCR approach. The results are a comprehensive look at HRAP viruses. Finally, the thesis describes a bioinformatic study of the genomic sequences of unicellular green algae of the genus Tetraselmis to identify and characterize integrated viral forms related to the Tsv-N1 virus, to the PLVs identified in the HRAP, and to the giant TetV-1 virus. This analysis extends our knowledge on the diversity of Tetraselmis viruses