- Suttle had unknowingly isolated CroV at least as far back as 1995. It would be another decade before other scientists realized that giant viruses even existed, and his lab spent years trying to study CroV. Over the years, he would start a student on the project. They would defrost a sample from storage. The CroV would grow and then mysteriously stop growing. “A couple years later, someone would come along to try again,” Suttle says. It wasn’t until DNA-sequencing technology improved and Matthias Fischer joined the lab that they finally figured out what was going on: Their samples also contained virophages that were “killing” the giant viruses. Freezing destroyed most of the virophages, but each time someone took the giant viruses out of the freezer and grew them in the lab, the virophages would start replicating and attacking the giant viruses. Suttle and Fischer published a paper describing all this in 2011.
The virophages that Suttle found could also explain the origin of another strange genetic phenomenon. Transposons, or “jumping genes,” are DNA sequences that can move around within the genomes of living organisms. The DNA sequences of the virophages that Suttle and Fischer found were similar to those of a certain type of transposon called Polintons. Virophages are now known to infect giant viruses, which in turn infect host cells such as algae or amoebas. Suttle and Fischer hypothesized that this might have gone one step further in the past: Ancient virophages might have become part of the host cell over time as part of a mutually beneficial arrangement—the virophages killed attacking giant viruses for the host and got a safe place to hang out. Eventually, those virophages became transposons.