Unlike the tiny viruses that cause the common cold, Ushikuvirus belongs to the Nucleocytoviricota phylum—a group of “giant viruses” that rival bacteria in size and complexity. According to the team led by Takemura and Bae, in Michigan (US), this virus represents a significant leap in our understanding of viral evolution.
With a genome exceeding 666,000 base pairs and nearly 800 genes, Ushikuvirus possesses genetic machinery that viruses are not supposed to have. Bae et al. (2025), in the Journal of Virology, noted that it builds a sophisticated “viral factory” inside its host, the amoeba Vermamoeba vermiformis, essentially terraforming the cellular environment.
The discovery has added significant weight to the Viral Eukaryogenesis Hypothesis. This theory, famously championed by Philip J.L. Bell (2001), suggests that the nucleus of our own cells—the “brain” that holds our DNA—may have evolved from a giant virus that infected an ancient, simple cell billions of years ago.
Researchers observed that Ushikuvirus systematically destroys the host’s nuclear membrane to establish its own factory. This behaviour provides a potential blueprint for how a viral “factory” could have eventually stabilised into a permanent cellular nucleus.
Ushikuvirus features a unique “cap” structure on its surface, with some caps displaying fibrous filaments. These structures, are thought to help the virus anchor onto its host, showcasing an evolutionary complexity usually reserved for higher life forms.
Like its relative, the Medusavirus, Ushikuvirus carries genes for histones—proteins that humans use to package DNA. Masaharu Takemura (2020) suggests this implies a massive exchange of genetic material occurred between viruses and complex cells at the dawn of biological history.
While Ushikuvirus is closely related to the Mamonoviridae family, Bae argues that it possesses distinct features that may represent a key taxon for elucidating the evolution of this entire virus group.
Interestingly, the infection cycle of Ushikuvirus is significantly longer than its cousins. Researchers observed that this prolonged interaction allows the virus more time to manipulate host machinery, further blurring the line between parasite and permanent cellular resident.
If viruses played such a foundational role in building the “complex” cells of Earth, it changes our search for astrobiology. We are no longer just looking for simple bacteria; we are looking for the viral architects that might have built the “brains” of cells across the cosmos.
If the Viral Eukaryogenesis hypothesis is correct, we are not just the descendants of ancient bacteria. We may be the living legacy of a prehistoric viral infection that never left. As we study Ushikuvirus, we are effectively looking into a biological mirror at our own distant ancestors.
