DNA viruses: the really big ones (giruses).

Department of Plant Pathology, University of Nebraska, Lincoln, Nebraska 68583, USA.
Annual review of microbiology (Impact Factor: 13.02). 10/2010; 64:83-99. DOI: 10.1146/annurev.micro.112408.134338
Source: PubMed

ABSTRACT Viruses with genomes greater than 300 kb and up to 1200 kb are being discovered with increasing frequency. These large viruses (often called giruses) can encode up to 900 proteins and also many tRNAs. Consequently, these viruses have more protein-encoding genes than many bacteria, and the concept of small particle/small genome that once defined viruses is no longer valid. Giruses infect bacteria and animals although most of the recently discovered ones infect protists. Thus, genome gigantism is not restricted to a specific host or phylogenetic clade. To date, most of the giruses are associated with aqueous environments. Many of these large viruses (phycodnaviruses and Mimiviruses) probably have a common evolutionary ancestor with the poxviruses, iridoviruses, asfarviruses, ascoviruses, and a recently discovered Marseillevirus. One issue that is perhaps not appreciated by the microbiology community is that large viruses, even ones classified in the same family, can differ significantly in morphology, lifestyle, and genome structure. This review focuses on some of these differences than on extensive details about individual viruses.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Despite the fact that marine viruses have been increasingly studied in the last decade, there is little information on viral abundance and distribution on a global scale. In this study, we report on a global-scale survey covering the Pacific, Atlantic, and Indian Oceans on viral distribution using flow cytometry. Viruses were stained with the SYBR Green I, which targets only dsDNA viruses. The average viral abundance was 1.10±0.73×107 ml-1 in global surface oceans and decreased from the areas with high chlorophyll concentration (on average, 1.47±0.78×107 ml-1) to the oligotrophic subtropical gyres (on average, 6.34±2.18×106 ml-1). On a large-spatial-scale, viruses displayed significant relationships with both heterotrophic and autotrophic picoplankton abundance, suggesting that viral distribution is dependent on their host cell abundance. Our study provided a basin scale pattern of marine viral distributions and their relationship with major host cells, indicating that viruses play a significant role in the global marine ecosystem.
    PLoS ONE 11/2014; 9(11). DOI:10.1371/journal.pone.0111634 · 3.53 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The family Iridoviridae of the superfamily Megavirales currently consists of five genera. Three of these, Lymphocystivirus, Megalocytivirus and Ranavirus, are composed of species that infect vertebrates, and the other two, Chloriridovirus and Iridovirus, contain species that infect invertebrates. Until recently, the lack of genomic sequence data limited investigation of the evolutionary relationships between the invertebrate iridoviruses (IIVs) and vertebrate iridoviruses (VIVs), as well as the relationship of these viruses to those of the closely related family Ascoviridae, which only contains species that infect insects. To help clarify the phylogenetic relationships of these viruses, we recently published the annotated genome sequences of five additional IIV isolates. Here, using classical approaches of phylogeny via maximum likelihood, a Bayesian approach, and resolution of a core protein tree, we demonstrate that the invertebrate and vertebrate IV species constitute two lineages that diverged early during the evolution of the family Iridoviridae, before the emergence of the four IIV clades, previously referred to as Chloriridoviruses, Polyiridoviruses, Oligoiridoviruses and Crustaceoiridoviruses. In addition, we provide evidence that species of the family Ascoviridae have a more recent origin than most iridoviruses, emerging just before the differentiation between the Oligoiridoviruses and Crustaceoiridovirus clades. Our results also suggest that after emergence, based on their molecular clock, the ascoviruses evolved more quickly than their closest iridovirus relatives.
    Molecular Phylogenetics and Evolution 01/2015; 84. DOI:10.1016/j.ympev.2014.12.013 · 4.02 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Diverse eukaryotes including animals and protists are hosts to a broad variety of viruses with double-stranded (ds) DNA genomes, from the largest known viruses, such as pandoraviruses and mimiviruses, to tiny polyomaviruses. Recent comparative genomic analyses have revealed many evolutionary connections between dsDNA viruses of eukaryotes, bacteriophages, transposable elements, and linear DNA plasmids. These findings provide an evolutionary scenario that derives several major groups of eukaryotic dsDNA viruses, including the proposed order “Megavirales,” adenoviruses, and virophages from a group of large virus-like transposons known as Polintons (Mavericks). The Polintons have been recently shown to encode two capsid proteins, suggesting that these elements lead a dual lifestyle with both a transposon and a viral phase and should perhaps more appropriately be named polintoviruses. Here, we describe the recently identified evolutionary relationships between bacteriophages of the family Tectiviridae, polintoviruses, adenoviruses, virophages, large and giant DNA viruses of eukaryotes of the proposed order “Megavirales,” and linear mitochondrial and cytoplasmic plasmids. We outline an evolutionary scenario under which the polintoviruses were the first group of eukaryotic dsDNA viruses that evolved from bacteriophages and became the ancestors of most large DNA viruses of eukaryotes and a variety of other selfish elements. Distinct lines of origin are detectable only for herpesviruses (from a different bacteriophage root) and polyoma/papillomaviruses (from single-stranded DNA viruses and ultimately from plasmids). Phylogenomic analysis of giant viruses provides compelling evidence of their independent origins from smaller members of the putative order “Megavirales,” refuting the speculations on the evolution of these viruses from an extinct fourth domain of cellular life.
    Annals of the New York Academy of Sciences 02/2015; 1341(1):10-24. DOI:10.1111/nyas.12728 · 4.31 Impact Factor

Full-text (2 Sources)

Available from
May 30, 2014