A team of scientists led by virologist Masaharu Takemura at Tokyo University of Science and Hiroyuki Ogata at Kyoto University in Japan have discovered a giant virus that, much like the mythical monster Medusa, can turn almost amoeba to a stone-like cyst. Isolated from a hot spring in Japan and eponymously dubbed Medusavirus, this virus infects a species of amoeba known as Acanthamoeba castellanii and causes it to develop a hard, stony shell.
With the Medusavirus, scientists discovered that DNA replication occurred in the nucleus of the host amoeba and observed evidence of exchange of genetic information between the host and the virus as they coevolved. They also found that the giant virus harbors in its ancient genome some of the complex proteins that make up the building blocks of eukaryotic organisms such as animals, plants, and humans. Understanding the presence of these proteins in the virus’ genome may help scientists tackle some of the hardest questions about our origins. In fact, “genomics research of the giant virus indicates that there is likely a relationship between the Medusavirus and the origin of eukaryotic life,” says Professor Takemura from Tokyo University of Science.
So what’s the relationship?
When the Medusavirus petrifies the amoeba, it does so by hijacking the cell directly from its nucleus. The virus transfers its DNA to initiate replication and uses its own DNA polymerase (enzyme that synthesizes DNA) and histones, but overall, it relies on the host to complete the process. The results of an evolutionary analysis done by the authors suggest that in the evolution tree, the Medusavirus DNA polymerase lies at the origin of the DNA polymerase found in eukaryotes. As one of the authors, Dr Genki Yoshikwa from Kyoto University, puts it, this could mean that our DNA polymerase “probably originated from Medusavirus or one of its relatives.” 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; 93 (8) DOI: 10.1128/JVI.02130-18 More.
As described, the authors’ explanation doesn’t follow. The Medusavirus substituting its own DNA for that of the host is no different from the cuckoo substituting its own offspring for another bird’s in a nest. The fact that the strategy works does not necessarily demonstrate a hereditary relationship between the two species.
Could the complex proteins not have been acquired by horizontal gene transfer? How do we know that ginat viruses are not mostly an outcome of complex life rather than a precursor to it?
The giant viruses probably have a bridging role to play in the history of life but it is surely too soon for simple, easy summations.
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