Scientists in Japan have discovered a previously unknown giant virus, providing new insight into this enigmatic category of viruses – and possibly also into the origins of multicellular life.
The virus was found infecting an amoeba in a freshwater pond near Tokyo, researchers report. They named it “ushikuvirus” after the pond, Ushiku-numa, located in Ibaraki Prefecture.
Giant viruses were largely overlooked during the first century of modern virology, with initial discoveries often being misidentified as bacteria due to their size. However, although we barely knew they existed until the last few decades, we have since learned that giant viruses are all around us.
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Viruses in general are considered the most abundant biological entities on Earth and some of the most puzzling. Little is known about the evolutionary history of viruses, and there is still ambiguity about their qualification as living organisms.
Even though they are not alive, viruses clearly have an enormous influence on all forms of life, including us. This includes not only hijacking a host’s cells and causing disease, but also occasionally interfering with its evolution.
Viruses can facilitate the horizontal transfer of genes between living things, and some – known as retroviruses – insert their DNA into the genome of host cells. If this happens in the germline of a host, the viral DNA can be passed on to its offspring.
In fact, ancient retrovirus remnants now comprise up to 8% of the human genome, which has its advantages. Retroviral DNA may have given early vertebrates the ability to make myelin and was key to the evolution of the placenta.
Much earlier, viruses would have triggered an even greater and more mysterious innovation: the evolutionary leap from prokaryotes, or unicellular organisms, to eukaryotes, or multicellular organisms.
Eukaryotic cells typically have a membrane-bound nucleus, representing a “design canvas” from their nucleus-less prokaryotic ancestors. It’s not clear how such a dramatic change occurred, but one intriguing theory suggests that the nuclei were a gift from viruses.
Known as viral eucarogenesis, this idea was first proposed in 2001 by Masaharu Takemura, a molecular biologist at Tokyo University of Science. He suggested that the nucleus of eukaryotic cells arose from a large DNA virus, like a poxvirus, that infected some prehistoric prokaryotes.
Instead of causing trouble, the virus made itself at home in the cell’s cytoplasm, eventually acquiring important genes from its host and gradually moving into a cell nucleus.
This theory gained traction with the discovery in 2003 of giant DNA-containing viruses that form structures called “virus factories” inside host cells. These factories are sometimes enclosed in a membrane and tend to look and function much like the nuclei of eukaryotic cells.
Scientists have since discovered a variety of these giant viruses, including species in the Mamonoviridae family and the closely related clandestinovirus, which infect certain types of amoebae. However, giant viruses are very diverse and difficult to isolate, so a new discovery like ushikuvirus is a big deal.
Takemura is still investigating viral eukaryogenesis a quarter of a century after he introduced the idea and was part of the team that identified and described the ushikuvirus in the new study.
“It can be said that giant viruses are a treasure whose world has not yet been fully understood,” says Takemura. “One of the future possibilities of this research is to provide humanity with a new vision that connects the world of living organisms with the world of viruses.”
Ushikuvirus infects amoebae known as vermamoeba (Vermamoeba vermiformis), a habit it shares with clandestinovirus; while the shape and surface of the stinging capsid resemble those of medusaviruses.
However, it stands out from other giant viruses. They force their host cells to grow abnormally large, for example, and capsid tips have unique caps and fibrous structures.
Instead of retaining the nucleus of a host cell and replicating inside, as clandestinoviruses and medusaviruses do, ushikuvirus instead forms a viral factory and destroys the host’s nuclear membrane.
These similarities and differences can be vital clues, helping scientists piece together the evolutionary history of giant viruses. Takemura and his colleagues hope to learn how and why these viruses diversified so much, as well as what role they played in the rise of eukaryotes like us.
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“The discovery of a new virus related to Mamonoviridae, ‘ushikuvirus,’ which has a different host, is expected to increase knowledge and stimulate discussions on the evolution and phylogeny of the Mamonoviridae family,” says Takemura.
“As a result, it is believed that we will be able to approach the mysteries of the evolution of eukaryotic organisms and the mysteries of giant viruses,” he says.
The study was published in Journal of Virology.