20 years of research on giant viruses

Abstract

Some twenty years ago, the discovery of the first giant virus, Acanthamoeba polyphaga mimivirus (now mimivirus bradfordmassiliense species), paved the way for the discovery of more than 10 new families of protist-infecting DNA viruses with unexpected diversity in virion shape and size, gene content, genome topology and mode of replication. Following their brief description, we examine how the historical concepts of virology have held up in the light of this new knowledge. Although the initial emphasis was on the gigantism of the newly described viruses infecting amoebae, the subsequent discovery of viruses with intermediate virion and genome sizes gradually re-established a continuum between the smallest and largest viruses within the phylum Nucleocytoviricota.

Fig. 1. Images of virions.

A Icosahedral virions structures determined by cryo-electron microscopy. PBCV-1 capsid (adapted from EMD-5384 with permission), faustovirus outer (EMD-8144) and inner (EMD-8145) capsids, melbournevirus capsid adapted from EMD-37169 with permissions, pacmanvirus capsid (adapted with permission from ref. ), medusavirus capsid (adapted form EMD-32073 with permission), CroV capsid (adapted from EMD-8748 with permission), mimivirus capsid (adapted with permission fromEMD-5039). B Scanning electron microscopy images of some non-symmetrical virions. Pithovirus, pandoravirus mollivirus and tupanvirus (courtesy from J. Abrahão).

Fig. 2. Distribution of viral genome sizes, illustrating the emergence of a continuum between giant and large dsDNA viruses.

*Proposed family names not yet validated by ICTV.