Viruses within viruses
From the biomedical journal Proceedings of the National Academy of Sciences (PNAS), and brought to my attention via io9.com, a truly alarming story! A patient was diagnoses with keratitis of the conjunctiva, or an eye infection, that had been brought about because she had washed her contact lenses with tap water diluted contact lens solution, and because she was wearing her lenses 2 months past their expiration date. Never a good combination as she ended up with an infection, however it was easily treatable before permanent damage occurred to her eyes.
Researchers were interested in the microorganisms cultured from the contaminated contact lens solution, and found significant numbers of the single-celled eukaryote Acanthamoeba polyphaga. This organism is associated with serious disease acquired via the fecal-oral route of transmission, however it is quite rare and only affects individuals with an underlying immunodeficiency. In this case, the disease was exclusively superficial. The researchers at the University of Nebraska further identified the presence of a virus within the Acanthamoeba cells from the contact lens solution, which was determined to be a member of the Lentille virus family. This result on its own was interesting, as there are a number of significant diseases of humans, where major virulence factors in the etiologic agent of the disease is transferred between virulent and avirulent strains by a viral intermediary.
What was really interesting, and makes this story noteworthy, was when the researchers examined the Lentille virus genome, they identified the integration of a mini virus termed a virophage which they called “Sputnik-2.” A bacteriophage is a virus that infects a bacterium, and so the term “virophage” refers to a virus that infects another virus. Sputnik-2 was only able to reproduce in Acanthamoeba cells which had been infected with the Lentille virus. And then to make the story even more recursive, the Sputnik-2 virophage was found to contain mobile genetic elements termed transpovirons.
The take home message from the case study is this: many factors contribute to the movement of genes through populations, and drive evolution. Genetic exchange via sexual reproduction is the most apparent mechanism in eukaryotic biology. We have mentioned in class that genetic exchange can be driven in bacteria, specifically via the structures we called “pili”, particularly with regards to the movement of antibiotic resistance genes between bacteria. It has long been known that viruses of bacteria are also critical for the exchange of genetic elements including antibiotic resistance as well, and it should come as no surprise that similar methods would be found in eukaryotes too. What is novel in this is that genetic elements are also exchanged between populations of viruses as well.