The newly identified Medusavirus infects amoebas, causing them to grow a hard shell. (The mythological Medusa turned her victims into stone.) Among other things, it was “ the first giant virus to have been isolated from a heated environment.” From ScienceDaily:
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. Paper. open access) – Genki Yoshikawa, Romain Blanc-Mathieu, Chihong Song, Yoko Kayama, Tomohiro Mochizuki, Kazuyoshi Murata, Hiroyuki Ogata, Masaharu Takemura. Medusavirus, a novel large DNA virus discovered from hot spring water. Journal of Virology, 2019; DOI: 10.1128/JVI.02130-18 More.
Giant viruses have only been known from the past few decades. There is still debate about whether viruses are actually life forms. Surely, there will be many game changers to come. Anyone attempting to compile an evolutionary history of giant viruses would be like the person who writes the history of a major league playoff series after the first game. Without the crystal ball.
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