Demarcando critérios taxonômicos para os chlorovírus por meio de análises genômicas e evolutivas

Abstract

Chloroviruses exhibit an intimate relationship with their hosts, which initially led to a taxonomy based solely on the phenotypic aspect of plaque-forming ability, such as the division between viruses that infect different species of green algae. However, with the isolation of strain-specific viruses in this taxon, such as the discovery of highly specialized viruses capable of completing their replication cycle only in the Syngen2-3 strain of C. variabilis (named OSy-viruses), systematic challenges began to emerge. The assembly of DNA sequences from new isolates allowed the generation of a database with 3 times more complete chlorovirus genomes than previously available. The new database was used to analyze genome size, number of protein-coding sequences and transfer RNAs, GC content, as well as phylogenetic reconstruction, average nucleotide identity comparison, and gene orthology prediction. The analyses revealed novel chloroviruses with the largest genomes sequenced to date, over 400 kbp, and indicated four statistically different genomic features in viruses that infect only the Syngen2-3 strain of the alga when compared to the same features of viruses that infect other strains of the alga. Although syntenic analysis showed that the overall structure of these genomes is highly conserved, there are large regions of dissimilarity in the genomes of viruses PBCV-1 and OSy-NE5 when compared to other genomes. These regions contain genes related to the interaction with the host cell machinery and viral capsid proteins, which provided insights into the evolution of the replicative and structural modules in these giant viruses. Phylogenetic analysis using Nucleocytoviricota hallmark genes revealed that OSy-víruses evolved from NC64A-víruses, possibly emerging as a result of the close relationship with their hosts. This, complemented by nucleotide identity analysis, allowed us to establish criteria for the definition of chlorovirus species, demarcating seven distinct species within our database. Together, our results indicate that chloroviruses are a group with great potential for genomic studies, and the characterization of new viruses is fundamental to filling gaps in the field of algal viruses.