Are viruses viruses, after all?

Abstract

Here, we quickly recapitulated the short history (since 2003) of the giant viruses, the discovery and the progressive characterization of which are deeply shaking the foundation of virology. In the mind of most biologists today, a "virus" remains the most reduced and optimized vehicle to propagate a nucleic acid molecule at the expense of a cellular host, an ultimate parasite at the frontier of (or beyond) the living world. With genome sizes and gene contents larger than many bacteria, as well as particle sizes of the order of half a micron, Mimivirus and Megavirus, collectively referred to as "Megaviridae", have now clearly made the point that being small and simple should no longer be considered fundamental properties of viruses (especially dsDNA viruses), nor a testimony to their evolutionary origin. In his landmark 1957 paper, Lwoff was dismissing any hypothetical continuity between viruses and cellular microbes by his famous aphorism: "viruses are viruses". However, the discovery of increasingly reduced and defective parasitic cellular microorganisms on one hand, and of viruses of increasing complexity, on the other hand, is steadily filling up a gap that was once considered irreducible. Future findings might end up proving Lwoff's sentence too hasty and demonstrate that "viruses are not viruses", after all.