Integrated viral elements suggest the dual lifestyle of Tetraselmis spp. polinton-like viruses

Abstract

In this study, we aimed at exploring horizontal gene transfer between viruses and Chlorodendraceae green algae (Chlorophyta) using available genomic and transcriptomic sequences for 20 algal strains. We identified a significant number of genes sharing a higher sequence similarity with viral homologues, thus signaling their possible involvement in HGTs with viruses. Further characterization showed that many of these genes were clustered in DNA regions of several tens to hundreds of kilobases in size, originally belonging to viruses related to known Tetraselmis spp. viruses (TetV and TsV). In contrast, the remaining candidate HGT genes were randomly dispersed in the algal genomes, more frequently transcribed and belonged to large multigene families. The presence of homologs in Viridiplantae suggested that these latter were more likely of algal rather than viral origin. We found a remarkable diversity Polinton-like virus elements inserted in Tetraselmis genomes, all of which were most similar to the Tetrasemis striata virus TsV. The genes of PLV-like elements are transcriptionally inactive with the notable exception of the homologue of the TVSG_00024 gene of TsV whose function is unknown. We suggest that this gene may be involved in a sentinel process to trigger virus reactivation and excision in response to an environmental stimulus. Altogether, these results provide evidence that TsV-related viruses have a dual lifestyle, alternating between a free viral phase (i.e., virion) and a phase integrated into host genomes.

Infection cycle and phylogeny of the Polinton-like virus Phaeocystis globosa virus virophage-14T

“…Numbers of sequences for collapsed clades are shown in parentheses. A complementary phylogenetic tree for the TVS group can be found elsewhere 46 . b.…”

“…The extracted scaffolds were extended with Contig Extender v. 0.1 55 and then polished and gap-filled with pilon v. 1.24 74 . (46), Alveolata (11) and Haptophyta (10).…”