For many years, the genesis of complex cells seemed like a resolved narrative, neat and reliably cellular. However, a recently uncovered giant virus from a Japanese freshwater pond is reigniting an even stranger theory: that viruses may have played a role in the formation of the first nuclei of complex forms of life.
The virus, named ushikuvirus, was extracted from Lake Ushiku by a team led by Masaharu Takemura at the Tokyo University of Science. In their article published in the Journal of Virology, the researchers report that this amoeba-infecting giant possesses essential characteristics akin to a small but intriguing faction of viruses that blur the lines between viral and cellular realms.
Ushikuvirus infects a single-celled organism known as vermamoeba and comprises a colossal DNA genome exceeding 660,000 base pairs, containing a total of 784 genes. This substantial size positions it squarely within the so-called giant viruses, a category identified only within the last twenty years, which has compelled scientists to reconsider the capabilities of viruses. Over half of its genes are classified as “ORFans,” indicating they lack known relatives anywhere in biological systems.
Viral Factories That Restructure Cells From Within
What renders ushikuvirus particularly fascinating is its behavior once it infiltrates its host. Rather than merely commandeering cellular processes and rupturing the cell, the virus constructs organized viral factories and ultimately disturbs the host’s nuclear membrane. It establishes its own manufacturing center and alters the cell from within.
This conduct distinguishes ushikuvirus from its closest kin. Medusaviruses and clandestinoviruses replicate within a preserved nucleus. In contrast, ushikuvirus ruptures the nucleus and performs replication in newly created viral factories, a tactic observed in some of the largest known viruses.
From a structural perspective, the virus is also remarkable. High-resolution imaging has unveiled a unique capsid surface adorned with atypical spike-like structures, some equipped with filamentous extensions. These characteristics are absent in related viruses and may clarify why ushikuvirus targets a distinct amoeba host.
Another noteworthy effect emerges at the overall cellular level. Rather than diminishing or lysing, infected amoebae significantly enlarge, sometimes doubling their size. This growth signifies a prolonged, gradual infection cycle. Instead of dramatically eliminating its host, ushikuvirus exits more subtly through exocytosis, gradually releasing new particles while managing the host’s resources over time.
A Disconcerting Possibility Regarding Our Cellular Ancestors
These particulars are significant because ushikuvirus lends credence to the viral eukaryogenesis hypothesis, a concept that Takemura and others have advocated for years. This hypothesis posits that the nucleus of eukaryotic cells emerged from an ancient large DNA virus that permanently established itself within a simpler host. In simpler terms, it implies that one of the hallmark features of complex life may trace back to a protracted viral infection that persisted indefinitely.
Ushikuvirus bolsters this theory by taking an evolutionary intermediary position. It shares foundational genes and structural features with viruses that replicate within the nucleus, yet it also operates like viruses that entirely dismantle the nucleus. This blend suggests potential transitional forms that may have existed early in the evolution of eukaryotes.
“Giant viruses can be considered a treasure trove whose realm has yet to be fully explored. A potential future avenue of this research is to provide humanity with a refreshed perspective that links the domain of living organisms with that of viruses,” explains Masaharu Takemura.
The virus also encodes a complete array of histone proteins, the same molecular spools that organize DNA in eukaryotic cells. While these viral histones may not operate identically to their cellular equivalents, their existence highlights how intricately connected viral and cellular evolution might be.
Aside from evolutionary implications, the discovery bears practical consequences. Some amoebae related to vermamoeba can induce rare but significant infections in humans. Gaining insight into how giant viruses invade and incapacitate these organisms could ultimately pave the way for new methods of controlling them.
For the moment, ushikuvirus achieves something even more profound. It disrupts the reassuring belief that viruses are merely parasites within the tree of life. Instead, it proposes that they may be interwoven into the trunk itself.
Journal of Virology: 10.1128/jvi.01206-25
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