Three novel virophage genomes discovered from Yellowstone Lake metagenomes

Virophages are a unique group of circular double-stranded DNA viruses that are considered parasites of giant DNA viruses, which in turn are known to infect eukaryotic hosts. In this study, the genomes of three novel Yellowstone Lake virophages (YSLVs)--YSLV5, YSLV6, and YSLV7--were identified from Yellowstone Lake through metagenomic analyses. The relative abundance of these three novel virophages and previously identified Yellowstone Lake virophages YSLV1 to -4 were determined in different locations of the lake, revealing that most of the sampled locations in the lake, including both mesophilic and thermophilic habitats, had multiple virophage genotypes. This likely reflects the diverse habitats or diversity of the eukaryotic hosts and their associated giant viruses that serve as putative hosts for these virophages. YSLV5 has a 29,767-bp genome with 32 predicted open reading frames (ORFs), YSLV6 has a 24,837-bp genome with 29 predicted ORFs, and YSLV7 has a 23,193-bp genome with 26 predicted ORFs. Based on multilocus phylogenetic analysis, YSLV6 shows a close evolutionary relationship with YSLV1 to -4, whereas YSLV5 and YSLV7 are distantly related to the others, and YSLV7 represents the fourth novel virophage lineage. In addition, the genome of YSLV5 has a G+C content of 51.1% that is much higher than all other known virophages, indicating a unique host range for YSLV5. These results suggest that virophages are abundant and have diverse genotypes that likely mirror diverse giant viral and eukaryotic hosts within the Yellowstone Lake ecosystem.

This study discovered novel virophages present within the Yellowstone Lake ecosystem using a conserved major capsid protein as a phylogenetic anchor for assembly of sequence reads from Yellowstone Lake metagenomic samples. The three novel virophage genomes (YSLV5 to -7) were completed by identifying specific environmental samples containing these respective virophages, and closing gaps by targeted PCR and sequencing. Most of the YSLV genotypes were associated primarily with photic-zone and nonhydrothermal samples; however, YSLV5 had a unique distribution with an occurrence in vent samples similar to that in photic-zone samples and with a higher GC content that suggests a distinct host and habitat compared to other YSLVs. In addition, genome content and phylogenetic analyses indicate that YSLV5 and YSLV7 are distinct from known virophages and that additional as-yet-uncharacterized virophages are likely present within the Yellowstone Lake ecosystem.