A flea has smaller fleas that on him prey: giant viruses, their parasites and evolutionary networks

摘要

We are used to thinking of viruses as the ultimate parasites, the tiny infectious agents that reproduce within cells. Although this attribute of the obligate intracellular parasitism remains, the distinction between viruses and cellular life forms in terms of size and genomic complexity is gone thanks to the discovery of giant viruses infecting unicellular eukaryotes [1]. The isolation and genome sequencing of the mimivirus, which encompasses more than 1000 genes with a 1-Mb genome, squarely put viruses within the range of genome sizes characteristic of cellular life forms [2]. Subsequent studies have shown that mimiviruses represent an expansive, widespread family of giant viruses [3]. The recent isolation of the colossal pandoraviruses, with their 2.5-Mb genomes, larger than the genome of numerous bacteria and Archaea, and even some parasitic eukaryotes [4], implies that there might be virtually no limit to virus genome growth beyond perhaps physically fitting multiple virus particles into the host cell.