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Comparing a living cell to a virus is a bit like comparing the Sistine Chapel to a backyard dog house. Lacking the intricate machinery of living cells, viruses represent biology stripped down to an extreme level. They are the true minimalists of the biological world. Nevertheless, the field of virology is brimming with unanswered questions about these architecturally simple, yet mysterious entities. In new research, Arvind Varsani, a molecular virologist at Arizona State University, joins a prestigious international team to explore a particular class of viruses, ferreting out genetic fragments revealing the complexities of viral evolution.
The new study examines the evolutionary dynamics of circular Rep-encoding single-stranded (CRESS) DNA viruses. The findings show that this broad class of single-stranded DNA viruses, which infect all three cellular domains of life, have acquired their genetic components through complex evolutionary processes not traceable to a single ancestral event. Rather, viruses are obsessive borrowers, appropriating genetic material from many sources, including bacterial, archaeal and eukaryotic cells as well as circular parasitic replicons, known as plasmids, and other mobile genetic elements, such as transposons. When a group of mobile elements—like CRESS DNA viruses— arise from more than a single common evolutionary ancestor or ancestral group, they are known as polyphyletic. The phenomenon is common in the viral world.
Such explorations also hold the potential to shed new light on the origins of earth's earliest life, and resolve the question of how cell-based life came to co-exist with the planet's staggering array of viruses (dubbed the virome). "Over the last decade we have been discovering viruses in various ecosystems using metagenomic approaches and as a result populating the CRESS DNA virus databases," Varsani says. "This has paved the way for a global analysis for CRESS DNA viruses yielding insights into the origin of these and other related viruses." "It is remarkable to see all these evolutionary connections between viruses and non-viral selfish replicons, which once were considered to be unrelated," Krupovic says. The results reveal three distinct evolutionary events contributing to the genetic composition of CRESS-DNA viruses. An intriguing kinship appears to exist between CRESS-DNA viruses and rolling circle plasmids found in bacteria, archaea and some eukaryotes. "As a result, the general mechanisms of virus evolution as well as the global organization of the vast viral world start to unravel."
Published July 31 2019 in Nature Communications:
