News Release

A petrifying virus key to evolution

The newly discovered Medusavirus give new insights on how organisms and viruses co-evolved

Peer-Reviewed Publication

Kyoto University

The Medusavirus

image: This virus was found infecting amoebas in the hot springs of northern Japan. The amoeba forms cysts, and produces a hard protective covering, giving it the name: Medusavirus view more 

Credit: (Kyoto University/Robin Hoshino

Scientists are constantly discovering new species.

A research team consisting of scientists from Kyoto University, Tokyo University of Science, National Institute for Physiological Sciences, and Tokyo Institute of Technology, report in the Journal of Virology the Medusavirus, a unique giant virus that gives pause to current theory on viral evolution.

The name Medusavirus was given for the effect this virus has on its host, Acanthamoeba castellanii. Once infected, the amoeba forms cysts, a phenomenon called encystment. This is a typical response to environments hostile to survival, and leaves the amoeba with a hard, protective covering. Perhaps it was not a coincidence then that Medusavirus was found in the hot springs in northern Japan, the first giant virus to have been isolated from a heated environment.

Along with the location of its discovery, Medusavirus holds a number of distinguishing features compared with other giant viruses. Its DNA codes for all five types of histones, the key proteins that help compact DNA within the nucleus. In fact, no other known virus has all five types. Further, Medusavirus encoded neither RNA polymerase nor DNA topoimerase II, whereas all other giant viruses encode at least one.

These features could explain why the replication of Medusavirus DNA begins and completes in the host nucleus to eventually fill the amoeba nucleus with viral DNA, which again is unlike other giant viruses.

Moreover, the morphology of the capsid surface was unique, in that it was covered with an extraordinary number of spherical-headed spikes. In addition, the amoeba genome encoded several capsid surface proteins.

The existence of histone genes in Medusavirus and capsid protein genes in amoeba suggest lateral gene transfer going both directions -- host-to-virus and virus-to-host.

Overall, the findings suggest that Medusavirus offers a new model for host-virus co-evolution and that the Medusavirus is a new family of large DNA viruses.


The paper "Medusavirus, a novel large DNA virus discovered from hot spring water" appeared on 6 Feburary 2019 in Journal of Virology, with doi: 10.1128/JVI.02130-18

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at both undergraduate and graduate levels is complemented by numerous research centers, as well as facilities and offices around Japan and the world. For more information please see:

Tokyo Institute of Technology (Tokyo Tech)

Tokyo Tech stands at the forefront of research and higher education as the leading university for science and technology in Japan. Tokyo Tech researchers excel in fields ranging from materials science to biology, computer science, and physics. Founded in 1881, Tokyo Tech hosts over 10,000 undergraduate and graduate students per year, who develop into scientific leaders and some of the most sought-after engineers in industry. Embodying the Japanese philosophy of "monotsukuri," meaning "technical ingenuity and innovation," the Tokyo Tech community strives to contribute to society through high-impact research.

The Earth-Life Science Institute (ELSI)

Launched in 2012, ELSI is one of Japan's ambitious World Premiere International research centers, whose aim is to achieve progress in broadly inter-disciplinary scientific areas by inspiring the world's greatest minds to come to Japan and collaborate on the most challenging scientific problems. ELSI's primary aim is to address the origin and co-evolution of the Earth and life.

About WPI

The World Premier International Research Center Initiative (WPI) was launched in 2007 by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) to help build globally visible research centers in Japan. These institutes promote high research standards and outstanding research environments that attract frontline researchers from around the world. These centers are highly autonomous, allowing them to revolutionize conventional modes of research operation and administration in Japan.

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