The Discovery of Ushikivirus: A Giant Virus from Japan That Might Unlock Evolutionary Secrets

In a groundbreaking discovery, researchers from Tokyo University of Science have identified a new species of giant virus called Ushikivirus in Ushikunuma, Ibaraki Prefecture, Japan. Characterized by its unique capsid structure and its ability to significantly enlarge its host cells, this virus could hold the key to understanding the evolutionary trajectory of eukaryotic organisms.

What is Ushikivirus?

Ushikivirus is not just another virus; it is part of the family of giant viruses that have garnered attention for their complex structures and significant impact on the cells they infect. This virus has a distinct cap structure on its surface and demonstrates a remarkable cytopathic effect (CPE) by doubling the size of its host, the Vervum amoeba, after infection. By analyzing its genome, researchers established that Ushikivirus shares a close relationship with Medusavirus, another giant virus discovered in 2019, but it also exhibits unique characteristics that set it apart from its relatives.

A Step Toward Unraveling Cellular Evolution

The discovery of Ushikivirus is a significant step in understanding the origins of eukaryotic cells. In recent years, a theory known as the 'viral origin of the nucleus hypothesis' has gained traction, suggesting that ancestors of giant viruses infected and coexisted with prokaryotic organisms, ultimately leading to the formation of the nucleus in eukaryotic cells. This hypothesis gains support with every new finding related to giant viruses, and Ushikivirus is no exception.

Key Features of Ushikivirus

The research team, led by Professor Masaharu Takemura from the Faculty of Liberal Arts and Sciences, Tokyo University of Science, and graduate students Jiwan Pe and Narumi Hatori, found several distinct traits in Ushikivirus that differentiate it from other viruses within its family:
1. Longer Infection Cycle: Ushikivirus exhibits a longer infection cycle than Medusavirus and Clandestinovirus, resulting in a significant cytopathic effect, seen as a dramatic increase in the size of infected Vervum amoeba cells.
2. Unique Capsid Structure: The capsid protein presents a variety of cap structures, some of which possess fibrous properties, which have not been observed in previously known giant viruses.
3. Destruction of Nuclear Membrane: The virus has the ability to form a 'viral factory' inside the host cell, ultimately leading to the disruption of the nuclear membrane during the replication process.

Implications for Evolutionary Research

With a genome length of at least 652,555 base pairs and housing around 784 genes, Ushikivirus exhibits gene sequences that are both unique and similar to those found in other members of the Nucleocytoplasmic Large DNA virus (NCLDV) group. Interestingly, around 58% of its genes are orphan genes, lacking known relatives in existing databases. This highlights the evolutionary significance of Ushikivirus, as it could provide insights into the genetic underpinnings of giant viruses and their interactions with host organisms.

Historical Context

The family of giant viruses, including Ushikivirus, has been a focal point of research since the first discovery of Medusavirus in 2019. Initial studies suggested that these viruses play a vital role in the evolution of eukaryotic organisms. The presence of histone genes in these viruses similarly supports the idea that they might have influenced the diversification and complexity of cellular structures in modern-day eukaryotes.

Conclusion

The identification of Ushikivirus opens new avenues for research into the origins and evolution of complex cellular life. The implications of this discovery not only advance our understanding of virology but also contribute significantly to evolutionary biology. As Professor Takemura noted, the ongoing studies will likely lead to further revelations about the enigmatic relationships between viruses and their hosts, ultimately clarifying the mysteries surrounding the evolution of eukaryotic cells.

This remarkable research has been published online in the Journal of Virology and is a testament to the collaborative efforts aimed at unveiling the complexities of life on Earth.

References

This study received support from the Japan Society for the Promotion of Science (JSPS) and the ExCELLS cooperative research program.

For more information, please contact: