A Student's Guide to Giant Viruses Infecting Small Eukaryotes: From Acanthamoeba to Zooxanthellae

Abstract

The discovery of infectious particles that challenge conventional thoughts concerning "what is a virus" has led to the evolution a new field of study in the past decade. Here, we review knowledge and information concerning "giant viruses", with a focus not only on some of the best studied systems, but also provide an effort to illuminate systems yet to be better resolved. We conclude by demonstrating that there is an abundance of new host-virus systems that fall into this "giant" category, demonstrating that this field of inquiry presents great opportunities for future research.

Keywords: Mimiviridae; giant viruses; nucleocytoplasmic large DNA viruses (NCLDVs).

Figure 1

The scale of giant virus genomes. (A). Genome size vs. rank plot for the largest 100 complete viral genomes as of January 2016 from National Center for Biotechnology Information (NCBI). Data points noted (●) were previously used in discussion by Claverie et al. [3] to define giants viruses as having genomes > 280 kb, open circles (○) represent additional data; (B). Genome size vs. rank order of completed bacterial genomes in NCBI as of January 2016. Sizes are color-coded to match the ranges of giant virus genomes.

Figure 2

Transmission electron micrographs of giant virus particles. (A) Pithovirus, as seen in Michel et al. [22]. Originally identified as a KC5/2 parasite, the image shows the electron dense viral wall consisting of perpendicularly oriented fibers or microtubules (arrows), and a marked ostiole (os) located at the apical end of the cell. Reprinted with permission—original magnification at 85,000×; (B) Megavirus chilensis. Image courtesy of Professors Chantal Abergel and Jean-Michel Claverie.

Figure 2

Transmission electron micrographs of giant virus particles. (A) Pithovirus, as seen in Michel et al. [22]. Originally identified as a KC5/2 parasite, the image shows the electron dense viral wall consisting of perpendicularly oriented fibers or microtubules (arrows), and a marked ostiole (os) located at the apical end of the cell. Reprinted with permission—original magnification at 85,000×; (B) Megavirus chilensis. Image courtesy of Professors Chantal Abergel and Jean-Michel Claverie.