The GC% landscape of the Nucleocytoviricota

Abstract Genomic studies on sequence composition count with many different approaches, such as calculating the proportion of guanine and cytosine within a given sequence (GC% content), which can be associated with a variety of aspects of the organism’s biology. In this context, GC% can reveal clues on virus-host relationship and evolution. Here we present a comprehensive analysis, gene-by-gene, of 61 representatives belonging to the phylum Nucleocytoviricota, viruses with the largest genomes of the known virosphere. Parameters were evaluated considering not only the average CG% of a given viral species compared to the whole phylum, but also to the gene position and phylogenetic history. Our results revealed that although some families present similar GC% among their representatives (e.g., Marseilleviridae), others as Poxviridae, Phycodnaviridae and Mimiviridae have members with discrepant GC%, likely related to the adaptation with their specific biological cycles and specific hosts. Interestingly, some genes at terminal regions or at some genomic clusters present GC% values distinct to the average, suggesting recent acquisition or distinct evolutive pressures. Horizontal gene transfer and the occurrence of potential paralogs were also evaluated considering the genes with most discrepant GC% values, suggesting multiple evolutionary histories. Taken together, to the best of our knowledge, this is the first study that presents a global and gene-by-gene analysis of the distribution and profile of the GC% content within genomes of members of the phylum Nucleocytoviricota, illustrating part of their diversity and identifying potential new targets for future studies. IMPORTANCE Viruses of the phylum Nucleocytoviricota are greatly distinguishable from other viruses in general because of the giant viral particles and their notably larger genomes, among other remarkable characteristics. One of the approaches used for studying organisms is unveiling information from their genomes, as they carry a plethora of information about biological aspects. Genomic studies use a great range of approaches, one of which is the evaluation of the percentage of nucleotides that make the DNA or RNA molecule of an organism’s genome. Herein, we demonstrate how genomes of nucleocytoviruses vary according to nucleotide composition metrics – more specifically, the percentage of guanine and cytosine, known as GC% content. Our results provide evidence of gene groups possibly acquired by horizontal gene transfer or duplication events, thus having multiple evolutionary histories within the same viral taxa. Therefore, we believe that these analyses raise insightful discussions about the evolution of large and giant viruses.