Mimivirus Relatives in the Sargasso Sea

TL;DR

This study identifies and analyzes Mimivirus relatives in the Sargasso Sea, revealing abundant unknown large viruses that could shed light on viral evolution and the origin of eukaryotes.

Contribution

It provides evidence of diverse Mimivirus-like viruses in the environment, expanding understanding of large DNA virus diversity and evolution.

Findings

Many homologues of Mimivirus proteins found in Sargasso Sea sequences

Phylogenetic analyses suggest presence of unknown large viruses related to Mimivirus

Potential for discovering new Mimiviridae through environmental sampling

Abstract

The discovery and genome analysis of Acanthamoeba polyphaga Mimivirus, the largest known DNA virus, challenged much of the accepted dogma regarding viruses. Its particle size (>400 nm), genome length (1.2 million bp) and huge gene repertoire (911 protein coding genes) all contribute to blur the established boundaries between viruses and the smallest parasitic cellular organisms. Phylogenetic analyses also suggested that the Mimivirus lineage could have emerged prior to the individualization of cellular organisms from the three established domains, triggering a debate that can only be resolved by generating and analyzing more data. The next step is then to seek some evidence that Mimivirus is not the only representative of its kind and determine where to look for new Mimiviridae. An exhaustive similarity search of all Mimivirus predicted proteins against all publicly available sequences identified many of their closest homologues among the Sargasso Sea environmental sequences. Subsequent phylogenetic analyses suggested that unknown large viruses evolutionarily closer to Mimivirus than to any presently characterized species exist in abundance in the Sargasso Sea. Their isolation and genome sequencing could prove invaluable in understanding the origin and diversity of large DNA viruses, and shed some light on the role they eventually played in the emergence of eukaryotes.