The exchange of DNA between ancient giant viruses and ancient biological cells could have been the key to the development of nuclei in eukaryotic cells. Photo credit: Tokyo University of Science
Giant viruses like the recently discovered Medusa virus could hold the key to unlocking the evolutionary mystery of the eukaryotic nucleus.
An exchange of genetic material that occurred when ancient giant viruses infected ancient eukaryotic cells may have resulted in the nucleus of the eukaryotic cell – its defining characteristic – being formed. This is what Professor Masaharu Takemura of the Tokyo University of Science, Japan, suggests in his recent review in the journal Frontiers in Microbiology. His novel evolutionary hypothesis opens doors to new discussions on this topic and brings us a big step closer to the truth.
Perhaps in the history of research and philosophy people have already tried to find out how life on earth came about. Over the past few decades, along this path, with exponential advances in genomics, molecular biology, and virology, several scientists have explored the evolutionary twists and turns that led to eukaryotic cells, the type of cell that most defines life forms today.
The most popular theories that have emerged are that the eukaryotic cell is the evolutionary product of the intracellular evolution of proto-eukaryotic cells, which were the first complex cells, and of symbiotic relationships between proto-eukaryotic cells and other unicellular and simpler ones Organisms such as bacteria and archaea. According to Professor Masaharu Takemura from Tokyo University of Science, Japan, “These hypotheses explain and explain the driving force and evolutionary pressures. But they cannot depict the exact process underlying the evolution of the eukaryotic nucleus. “
Prof. Takemura cites this as his motivation for his recent in Limits in Microbiologywhere he delves into the most recent theories which, in addition to his own research, have put forward his current hypothesis on the subject.
In a way, Prof. Takemura’s hypothesis has its roots in 2001, when he and PJ Bell made the revolutionary proposal so big DNA Viruses like the smallpox virus had something to do with the rise of the eukaryotic nucleus. Prof. Takemura further explains the reasons for his study of the nucleus of the eukaryotic cell as such: “Although the structure, function and various biological functions of the cell nucleus have been extensively studied, the evolutionary origin of the cell nucleus, a milestone in eukaryotic evolution, remains unclear.”
The origin of the eukaryotic nucleus must indeed be a milestone in the development of the cell itself, considering that this is the determining factor that distinguishes eukaryotic cells from the other broad category of cells – the prokaryotic cell. The eukaryotic cell is neatly divided into membrane-bound organelles that fulfill various functions. Among them, the nucleus houses the genetic material. The other organelles float in what is known as the cytoplasm. Prokaryotic cells do not contain such compartmentalization. Bacteria and archaea are prokaryotic cells.
The hypothesis of Prof. Takemura and PJ Bell from 2001 is based on striking similarities between the eukaryotic cell nucleus and smallpox viruses: in particular, the property of keeping the genome separate in one compartment. Further similarities were discovered after the discovery and characterization of a type of large DNA virus called “giant virus”, which can be up to 2.5 µm in diameter and contains DNA-coding information for the production of more than 400 proteins. Independent phylogenetic analysis indicated that genes were transferred between these viruses and eukaryotic cells as they interacted at different points along the evolutionary road in a process known as “lateral gene transfer.”
Viruses are “packages” of DNA or RNA and can’t survive alone They have to enter a “host cell” and use the machinery of that cell to replicate their genetic material and therefore multiply. In the course of evolution, it appears that viral genetic material has been integrated into the host genetic material and the properties of both have changed.
In 2019, Prof. Takemura and his colleagues made another breakthrough: the Medusavirus. Medusavirus got its name because, like the mythical monster, it causes cystization in its host; That is, it gives its host cell a “hard” cover.
Through experiments with the infection of an amoeba, Prof. Takemura and his colleagues found that the Medusavirus contains a complete set of histones that are similar to histones in eukaryotes. Histones are proteins that keep strands of DNA curled up and packed in the nucleus. It also contains a DNA polymerase gene and a major capsid protein gene that are very similar to those found in the amoeba. In addition, unlike other viruses, it does not set up its own closed “virus factory” in the cytoplasm of the cell where its DNA is to be replicated, and it does not contain any of the genes necessary to carry out the replication process. Instead, it occupies the entire host core and uses the host core machinery to replicate.
These features, argues Prof. Takemura, suggest that the ancestral Medusavirus and its corresponding proto-eukaryotic host cells were involved in lateral gene transfer; The virus acquired DNA synthesis (DNA polymerase) and condensation genes (histones) from its host, and the host acquired structural protein genes (major capsid protein) from the virus. Based on additional research results, Prof. Takemura extends this new hypothesis to several other giant viruses.
In this way, Prof. Takemura connects the points between his findings in 2019 and his original hypothesis in 2001 and connects them through his and the work of others in the two decades in between. All in all, it becomes clear how the Medusavirus is the main evidence of the viral origin of the eukaryotic nucleus.
He says: “This new updated hypothesis can profoundly influence the study of the origin of eukaryotic cells and provide a basis for further discussion of the involvement of viruses in the development of the eukaryotic nucleus.” Indeed, his work may have opened several new avenues for future research in this area.
Reference: “Medusavirus ancestor in a proto-eukaryotic cell: Updating the hypothesis for the viral origin of the nucleus” by Masaharu Takemura, September 3, 2020, Limits in Microbiology.
DOI: 10.3389 / fmicb.2020.571831
