Revisiting evolutionary trajectories and the organization of the Pleolipoviridae family

Global Distribution and Diversity of Haloarchaeal pL6-Family Plasmids

Australian isolates of Haloquadratum walsbyi, a square-shaped haloarchaeon, often harbor small cryptic plasmids of the pL6-family, approximately 6 kb in size, and five examples have been previously described. These plasmids exhibit a highly conserved gene arrangement and encode replicases similar to those of betapleolipoviruses. To assess their global distribution and recover more examples for analysis, fifteen additional plasmids were reconstructed from the metagenomes of seven hypersaline sites across four countries: Argentina, Australia, Puerto Rico, and Spain. Including the five previously described plasmids, the average plasmid size is 6002 bp, with an average G+C content of 52.5%. The tetramers GGCC and CTAG are either absent or significantly under-represented, except in the two plasmids with the highest %G+C. All plasmids share a similar arrangement of genes organized as outwardly facing replication and ATPase modules, but variations were observed in some core genes, such as F2, and some plasmids had acquired accessory genes. Two plasmids, pCOLO-c1 and pISLA-c6, shared 92.7% nt identity despite originating from Argentina and Spain, respectively. Numerous metagenomic CRISPR spacers matched sequences in the fifteen reconstructed plasmids, indicating frequent invasion of haloarchaea. Spacers could be assigned to haloarchaeal genera by mapping their associated direct repeats (DR), with half of these matching Haloquadratum. Finally, strand-specific metatranscriptome (RNA-seq) data could be used to demonstrate the active transcription of two pL6-family plasmids, including antisense transcripts.

Pleomorphic viruses establish stable relationship with marine hyperthermophilic archaea

Non-lytic viruses with enveloped pleomorphic virions (family Pleolipoviridae) are ubiquitous in hypersaline environments across the globe and are associated with nearly all major lineages of halophilic archaea. However, their existence in other ecosystems remains largely unknown. Here, we show that evolutionarily-related viruses also infect hyperthermophilic archaea thriving in deep-sea hydrothermal vents. Archaeoglobus veneficus pleomorphic virus 1 (AvPV1), the first virus described for any member of the class Archaeoglobi, encodes a morphogenetic module typical of pleolipoviruses, including the characteristic VP4-like membrane fusion protein. We show that AvPV1 is a non-lytic virus chronically produced in liquid cultures without substantially affecting the growth dynamics of its host with a stable virus-to-host ratio of ~1. Mining of genomic and metagenomic databases revealed broad distribution of AvPV1-like viruses in geographically remote hydrothermal vents. Comparative genomics, coupled with phylogenetic analysis of VP4-like fusogens revealed deep divergence of pleomorphic viruses infecting halophilic, methanogenic, and hyperthermophilic archaea, signifying niche separation and coevolution of the corresponding virus-host pairs. Hence, we propose a new virus family, "Thalassapleoviridae," for classification of the marine hyperthermophilic virus AvPV1 and its relatives. Collectively, our results provide insights into the diversity and evolution of pleomorphic viruses beyond hypersaline environments.

Distribution and Implications of Haloarchaeal Plasmids Disseminated in Self-Encoded Plasmid Vesicles

Even though viruses and plasmids are both drivers of horizontal gene transfer, they differ fundamentally in their mode of transfer. Virus genomes are enclosed in virus capsids and are not dependent on cell-to-cell contacts for their dissemination. In contrast, the transfer of plasmids most often requires physical contact between cells. However, plasmid pR1SE of Halorubrum lacusprofundi is disseminated between cells, independent of cell-cell contacts, in specialized membrane vesicles that contain plasmid proteins. In this study, we searched for pR1SE-like elements in public databases and a metagenomics dataset from Australian salt lakes and identified 40 additional pR1SE-like elements in hypersaline environments worldwide. Herein, these elements are named apHPVs (archaeal plasmids of haloarchaea potentially transferred in plasmid vesicles). They share two sets of closely related proteins with conserved synteny, strongly indicating an organization into different functional clusters. We find that apHPVs, besides transferring themselves, have the potential to transfer large fragments of DNA between host cells, including virus defense systems. Most interestingly, apHPVs likely play an important role in the evolution of viruses and plasmids in haloarchaea, as they appear to recombine with both of them. This further supports the idea that plasmids and viruses are not distinct but closely related mobile genetic elements.