A mycovirus identified from an outbreak fungal species complex reveals a parallel antiviral system

A new mycovirus, CgTV1, has been discovered in a fungal species complex known as Cryptococcus gattii, which is responsible for a significant outbreak in healthy individuals. This discovery is surprising, as no mycoviruses had been found in this particular group of fungi before. The researchers screened 386 different Cryptococcus isolates and found that CgTV1, a double-stranded RNA virus, is present in an environmental isolate of the CGSC.

CgTV1 belongs to the genus Totivirus and contains a genome that encodes a capsid protein and an RNA-dependent RNA polymerase. The researchers observed that CgTV1 establishes a hidden infection within the host, as the presence or absence of the virus does not significantly affect the overall gene activity of the host. This suggests that there are mechanisms in place to maintain a balance between the virus and the host.

Further investigation revealed that an RNA interference (RNAi) pathway plays a crucial role in keeping CgTV1 in check. The CGSC strains studied had to lose the function of two proteins, AGO1 and AGO2, in order for the virus to persist. Additionally, the researchers found a gene called CAI1, which is highly expressed in mycovirus-positive isolates. CAI1 directly recognizes and breaks down the CgTV1 dsRNA, thus preventing excessive viral replication.

The study also showed that two other enzymes, Dicer and CAI1, work together to maintain the balance between the virus and the host. When both Dicer and CAI1 were missing, the researchers observed higher viral loads and more significant changes in the host's gene expression compared to when either factor was missing alone. These findings confirm the presence of a mycovirus in the Cryptococcus gattii species complex and uncover parallel mechanisms that allow for the existence of cryptic mycoviral infections.