In ancient giant viruses lies the truth behind evolution of nucleus in eukaryotic cells
Giant viruses, like the recently discovered medusavirus, may hold the key
to deciphering the evolutionary mystery of the eukaryotic nucleus

Date:
September 9, 2020
Source:
Tokyo University of Science
Summary:
An exchange of genetic material that occurred when ancient giant
viruses infected ancient eukaryotic cells could have caused the
nucleus of the eukaryotic cell -- its defining feature -- to form. A
novel evolutionary hypothesis opens doors to new discussions on
the subject, bringing us one giant step closer to the truth.



FULL STORY ========================================================================== Perhaps as far back as the history of research and philosophy goes,
people have attempted to unearth how life on earth came to be. In the
recent decades, with exponential advancement in the fields of genomics, molecular biology, and virology, several scientists on this quest have
taken to looking into the evolutionary twists and turns that have resulted
in eukaryotic cells, the type of cell that makes up most life forms today.


==========================================================================
The most widely accepted theories that have emerged state that the
eukaryotic cell is the evolutionary product of the intracellular
evolution of proto- eukaryotic cells, which were the first complex cells,
and symbiotic relationships between proto-eukaryotic cells and other unicellular and simpler organisms such as bacteria and archaea. But
according to Professor Masaharu Takemura of the Tokyo University of
Science, Japan, "These hypotheses account for and explain the driving
force and evolutionary pressures. But they fail to portray the precise
process underlying eukaryotic nucleus evolution." Prof Takemura cites
this as his motivation behind his recent article published in Frontiers in Microbiology, where he looks into the recent theories that, in addition
to his own body of research, have built up his current hypothesis on
the subject.

In a way, Prof Takemura's hypothesis has its roots in 2001 when, along
with PJ Bell, he made the revolutionary proposal that large DNA viruses,
like the poxvirus, had something to do with the rise of the eukaryotic
cell nucleus.

Prof Takemura further explains the reasons for his inquiry into
the nucleus of the eukaryotic cell as such: "Although the structure,
function, and various biological functions of the cell nucleus have been intensively investigated, the evolutionary origin of the cell nucleus,
a milestone of eukaryotic evolution, remains unclear." The origin of
the eukaryotic nucleus must indeed be a milestone in the development of
the cell itself, considering that it is the defining factor that sets eukaryotic cells apart from the other broad category of cells -- the prokaryotic cell. The eukaryotic cell is neatly compartmentalized into membrane-bound organelles that perform various functions. Among them,
the nucleus houses the genetic material. The other organelles float
in what is called the cytoplasm. Prokaryotic cells do not contain such compartmentalization. Bacteria and archaea are prokaryotic cells.

The 2001 hypothesis by Prof Takemura and PJ Bell is based on striking similarities between the eukaryotic cell nucleus and poxviruses:
in particular, the property of keeping the genome separate in a
compartment. Further similarities were uncovered after the discovery
and characterization of a type of large DNA virus called "giant virus,"
which can be up to 2.5 mym in diameter and contain DNA "encoding"
information for the production of more than 400 proteins. Independent phylogenetic analyses suggested that genes had been transferred between
these viruses and eukaryotic cells as they interacted at various points
down the evolutionary road, in a process called "lateral gene transfer." Viruses are "packets" of DNA or RNA and cannot survive on their own. They
must enter a "host" cell and use that cell's machinery to replicate
its genetic material, and therefore multiply. As evolution progressed,
it appears, viral genetic material became integrated with host genetic
material and the properties of both altered.

In 2019, Prof Takemura and his colleagues made another breakthrough
discovery: the medusavirus. The medusavirus got its name because,
like the mythical monster, it causes encystment in its host; that is,
it gives its host cell a "hard" covering.

Via experiments involving the infection of an amoeba, Prof Takemura and
his colleagues found that the medusavirus harbors a full set of histones,
which resemble histones in eukaryotes. Histones are proteins that keep
DNA strands curled up and packed into the cell nucleus. It also holds a
DNA polymerase gene and major capsid protein gene very similar to those
of the amoeba. Further, unlike other viruses, it does not construct its
own enclosed "viral factory" in the cytoplasm of the cell within which
to replicate its DNA and contains none of the genes required to carry
out the replication process. Instead, it occupies the entirety of the
host nucleus and uses the host nuclear machinery to replicate.

These features, Prof Takemura argues, indicate that the ancestral
medusavirus and its corresponding host proto-eukaryotic cells were
involved in lateral gene transfer; the virus acquired DNA synthesis
(DNA polymerase) and condensation (histones) genes from its host and
the host acquired structural protein (major capsid protein) genes from
the virus. Based on additional research evidence, Prof Takemura extends
this new hypothesis to several other giant viruses as well.

Thus, Prof Takemura connects the dots between his findings in 2019 and
his original hypothesis in 2001, linking them through his and others'
work in the two decades that come in between. All of it taken together,
it becomes clear how the medusavirus is prime evidence of the viral
origin of the eukaryotic nucleus.

He says: "This new updated hypothesis can profoundly impact the study
of eukaryotic cell origins and provide a basis for further discussion
on the involvement of viruses in the evolution of the eukaryotic
nucleus." Indeed, his work may have unlocked several new possibilities
for future research in the field.


========================================================================== Story Source: Materials provided by Tokyo_University_of_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Masaharu Takemura. Medusavirus Ancestor in a Proto-Eukaryotic Cell:
Updating the Hypothesis for the Viral Origin of the
Nucleus. Frontiers in Microbiology, 2020; 11 DOI:
10.3389/fmicb.2020.571831 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200909100308.htm

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