Genome of Phaeocystis globosa virus PgV-16T highlights the common ancestry of the largest known DNA viruses infecting eukaryotes

Abstract

Large dsDNA viruses are involved in the population control of many globally distributed species of eukaryotic phytoplankton and have a prominent role in bloom termination. The genus Phaeocystis ( Haptophyta , Prymnesiophyceae ) includes several high-biomass-forming phytoplankton species, such as Phaeocystis globosa , the blooms of which occur mostly in the coastal zone of the North Atlantic and the North Sea. Here, we report the 459,984-bp-long genome sequence of P. globosa virus strain PgV-16T, encoding 434 proteins and eight tRNAs and, thus, the largest fully sequenced genome to date among viruses infecting algae. Surprisingly, PgV-16T exhibits no phylogenetic affinity with other viruses infecting microalgae (e.g., phycodnaviruses), including those infecting Emiliania huxleyi , another ubiquitous bloom-forming haptophyte. Rather, PgV-16T belongs to an emerging clade (the Megaviridae) clustering the viruses endowed with the largest known genomes, including Megavirus, Mimivirus (both infecting acanthamoeba), and a virus infecting the marine microflagellate grazer Cafeteria roenbergensis . Seventy-five percent of the best matches of PgV-16T–predicted proteins correspond to two viruses [Organic Lake phycodnavirus (OLPV)1 and OLPV2] from a hypersaline lake in Antarctica (Organic Lake), the hosts of which are unknown. As for OLPVs and other Megaviridae, the PgV-16T sequence data revealed the presence of a virophage-like genome. However, no virophage particle was detected in infected P. globosa cultures. The presence of many genes found only in Megaviridae in its genome and the presence of an associated virophage strongly suggest that PgV-16T shares a common ancestry with the largest known dsDNA viruses, the host range of which already encompasses the earliest diverging branches of domain Eukarya.

Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology

“…Finally the Pithovirus genome is predicted to encode a mere 467 proteins, much less than the 2,500 predicted proteins of Pandoravirus salinus. The Pithovirus particle appears to be out of proportion with its gene content compared with other DNA viruses such as the Phaeocystis globosa virus packing a similar number of genes into an icosahedral capsid 150 nm in diameter (150-fold less in volume) (19).…”

Chimeric viruses blur the borders between the major groups of eukaryotic single-stranded DNA viruses

“…1 and Supplementary Data 2), suggesting that such classification scheme might not prove to be practical. (4) and tombusviruses (48). Whiskers correspond to the R ggplot library geom_boxplot function default paramers (for upper whisker: the highest value that is within 1.5*Inter-Quartile Range of the hinge, and for lower whisker: the smallest value that is within 1.5*Inter-Quartile Range of the hinge).…”

“…At any rate, the association of different CHIV contigs with two different types of eukaryotes raises an intriguing possibility that unicellular eukaryotes serve as hosts for at least some CHIVs. Interestingly, we identified a close homologue (AET73220; E ¼ 4e À 29, 35% identity) of CHIV12 RC-Rep (but not the CP) encoded in the genome of a giant double-stranded DNA virus, PgV-12T, infecting Phaeocystis globosa 48 , a photosynthetic unicellular algae. It has been recently demonstrated that satellite viruses and transposons integrate into the genome of the Lentille virus, a relative of mimiviruses 49 .…”

Comparative Genomics of Chrysochromulina Ericina Virus and Other Microalga-Infecting Large DNA Viruses Highlights Their Intricate Evolutionary Relationship with the Established Mimiviridae Family

“…Prior to defining these two genera, metagenomic studies had already hinted at the presence of mimivirus relatives in marine environments (6). The successful isolation and characterization of several of these viruses showed that they correspond to smaller icosahedral particles (140 to 180 nm in diameter) packing smaller dsDNA genomes (370 to 475 kb in length) (7)(8)(9). In core gene phylogenies, these viruses clearly cluster with the Mimiviridae, although they appear to constitute a distinct clade ( Fig.…”