Size doesn’t matter… for giant viruses!

Giant viruses or ‘Giruses’ in the marine environment are often a hot topic of debate when it comes to their origin and phylogenetic histories. Characterised by their large genome size, they change our understanding of the full potential of virus particles. They infect a wide variety of hosts, and have often been shown to acquire genes from a diverse source of these hosts, showing unique properties suggesting a possible fourth domain of life.

This study focuses on Aureococcus anophagefferens, a unicellar microalgae associated with devastating brown tide blooms, and the interactions in which its virus, named A. anophagefferens virus (AaV), can play a part in their modulation. AaV was isolated in 2002 and kept in constant culture. DNA was extracted and the genome sequenced and annotated to full depth with Illumina^TM technology and Prodigal webserver. AaV was shown to be a double stranded DNA virus, with 88.3% coding density typical of large dsDNA viruses, thought to be of the emerging clade, Megaviridae. While AaV has only 370920 bp, making it is the smallest of this group, it shares nine out of the ten genes typically associated with the NCLDV group suggesting the strong likely hood of its phylogenetic position. This smaller genome also suggests that size isn’t everything when it comes to the Megaviridae clade as ancestors of AaV were most likely smaller than those in the present study, accumulating genome size with gene duplication and from the host and other organism along the evolutionary pathway.

BLASTp searches identified at least 13 genes in AaV thought to originate in the host organism showing evidence of horizontal gene. Several novel genes were also found to the NCLDV group, and of particular interest are the genes of eukaryotic origin. The eukaryotic RNA polymerase subunits present in AaV are thought to be involved in modulating the transcriptional state of the host and are utilised in virus maturation. Eight tRNA genes were found in the AaV genome, which suggest independent protein synthesis in the virus, a characteristic found commonly in the giant virus group.

The genome analysis of AaV also showed a phylogenetic link between Phycodna and Mimiviridae, as genes from both clades were present suggesting a common ancestor between the two divergent groups. AaV is the second virus of the Megaviridae to infect a photosynthetic host, suggesting the potential for more giant viruses with photosynthetic hosts to soon be discovered.

This study provides a substantial outlook into the dynamics and functioning of the Megaviridae family in terms of genomics and phylogenetic evolution suggesting the breadth and perhaps the gaps of knowledge we have when it comes to giant viruses. The fact that Mimiviridae is defined by more than just genome size and the fact that AaV is only the second of the Megaviridae to infect a photosynthetic host shows the potential of this area for development and future research.

Moniruzzaman, M., LeCleir, G., Brown, C., Gobler, C., Bidle, K., Wilson, W. and Wilhelm, S. (2014). Genome of brown tide virus (AaV), the little giant of the Megaviridae, elucidates NCLDV genome expansion and host–virus co-evolution. Virology, 466-467, pp.60-70.