Scientists in Japan have discovered a giant virus called ushikuvirus that infects amoebae and provides evidence for the theory that viruses contributed to the evolution of complex cells. Isolated from Lake Ushiku, the virus exhibits unique structural and replication traits linking it to other giant DNA viruses. This finding, published in the Journal of Virology, deepens understanding of viral roles in eukaryotic evolution.
The discovery of ushikuvirus emerged from research led by Professor Masaharu Takemura at the Tokyo University of Science, in collaboration with students Jiwan Bae and Narumi Hatori, as well as Dr. Raymond Burton-Smith and Professor Kazuyoshi Murata from the National Institute of Natural Sciences. The virus was isolated from Lake Ushiku in Ibaraki Prefecture, Japan, and named accordingly. It belongs to a group related to the Mamonoviridae family and infects vermamoeba, a type of amoeba.
Ushikuvirus shares similarities with medusavirus and clandestinovirus, such as an icosahedral capsid covered in short spikes. However, it features distinct elements, including multiple spike structures with unique caps and filament-like extensions. During infection, ushikuvirus causes vermamoeba cells to enlarge and disrupts the host's nuclear membrane to replicate, unlike medusaviruses that use the intact nucleus as a factory. This difference highlights evolutionary adaptations among giant viruses.
The finding bolsters the viral eukaryogenesis hypothesis, proposed by Takemura and Dr. Philip Bell in 2001, which posits that the eukaryotic cell nucleus originated from a large DNA virus infecting an archaeal ancestor. Giant viruses form membrane-surrounded 'virus factories' resembling primitive nuclei, supporting this idea.
"Giant viruses can be said to be a treasure trove whose world has yet to be fully understood. One of the future possibilities of this research is to provide humanity with a new view that connects the world of living organisms with the world of viruses," Takemura stated.
Additionally, studying these viruses could aid in addressing infections from pathogenic amoebae, like those causing amoebic encephalitis. The research appears in the Journal of Virology (2025; 99(12)), supported by JSPS/KAKENHI grant 20H03078.
