Tourists visiting the town of Klosterneuburg in eastern Austria often head for the 12th century monastery or the nearby memorial to author Franz Kafka. Virologists and evolutionary biologists, however, may one day pay homage to the town’s sewage treatment plant, which has yielded a genome that appears to be from the most cell-like viruses yet. These oddities challenge the controversial hypothesis that so-called giant viruses are descendants of a vanished group of cellular organisms—a fourth domain of life. Instead, the study argues, these outsized viruses have more pedestrian origins.
“I found [the work] very convincing,” says environmental virologist Matthias Fischer of the Max Planck Institute for Medical Research in Heidelberg, Germany. “Based on the data available now, I would not put my money on the fourth domain hypothesis.”
Most viruses are much smaller than cells and need few genes because they replicate by co-opting the machinery of their hosts. Certain bird and pig viruses, for example, get by with just two genes, compared with nearly 4400 genes in a common strain of the intestinal bacterium Escherichia coli. Because viruses cannot reproduce independently and lack other hallmarks of cellular organisms, biologists have typically blackballed them from the club of life.