Viruses have a ubiquitous presence in the world. Their population is estimated to be 1031, 10 times greater than the nonillion (1030) of microbes on the
The predicted hosts for the Klosneuviruses are protists —
«The discovery presents virus evolution for us in new ways, vastly expanding our understanding of how many essential host genes viruses can capture during their evolution," said National Institutes of Health evolutionary and computational biologist Eugene Koonin, a study
Giant Viruses Have a Unique Ability
Scientists have been fascinated by giant viruses since 2003, when a group of French biologists led by Didier Raoult discovered the Mimiviruses. Since then, a handful of other giant virus groups have been found. The unique ability among them to encode proteins involved in translation (typically DNA to RNA to protein) piqued researchers’ interests as to the origin of giant viruses. Since then, two evolutionary hypotheses have emerged. One posits that giant viruses evolved from an ancient cell, perhaps one from an extinct fourth domain of cellular life.
The discovery of Klosneuvirus supports the latter idea, according to Tanja Woyke, DOE JGI Microbial Genomics Program lead and senior author of the paper. «In this scenario, a smaller virus infected different eukaryote hosts and picked up genes encoding translational machinery components from independent sources over long periods of time through piecemeal acquisition," she said.
At first glance, the suite of «cellular» genes in Klosneuvirus seemed to have a common origin, but when analyzing them in detail, the research team observed they came from different hosts. From the evolutionary trees the team built, they noticed that they were acquired by the viruses bit by bit, at different stages in their evolution. The Klosneuvirus genes contained
Taking a New and Unexpected Direction
JGI postdoctoral researcher Frederik Schulz and Woyke unearthed Klosneuvirus while analyzing microcolony sequence data from a wastewater treatment plant sample in Klosterneuburg, Austria. This data was generated under a DOE JGI Community Science Program (CSP) project focused on the diversity of nitrifying bacteria for converting ammonia to nitrate in industrial and sewage waste treatment. «We expected genome sequences of nitrifying bacteria in the microcolony sequence data," Woyke said. «Finding a giant virus genome took the project into a completely new and unexpected, yet very exciting direction.»
When Schulz, the study’s first author, noticed that several of the metagenomes were viral in origin, he and Woyke conducted analyses to determine their source. They found that the Klosneuvirus group came from a novel viral lineage affiliated with Mimiviruses.
«Mining sequence data in DOE JGI’s Integrated Microbial Genomes & Microbiomes (IMG/M) system, which houses thousands of metagenomes, allowed us to find evolutionary relatives of our Klosneuvirus," Schulz said. He notes that while the metagenomic discovery of Klosneuviruses helped answer important evolutionary questions, the actual biological function of the translation system genes remains
And Koonin believes there are more giant viruses waiting to be discovered in metagenomic data. «I’m quite confident that the current record of the genome size of giant viruses will be broken," he says. «We are going to see the real Goliaths of the giant virus world.»