In a recent article in the journal Cell Host & Microbe, scientists have
discovered the first clear evidence of a multicomponent virus that can infect
animal cells. Ranging back into the 1970’s, examples of multicomponent viruses,
those that have their genome separated into multiple distinct segments, have
been found commonly within both plants and fungi.1 Typically, most
animal viruses package their entire genome into a single virus particle, which
alone has the potential to infect cells and replicate itself within them.
Multicomponent viruses, however, have numerous unique particles that
collectively make up the viral genome, and at least two or more of these different
types of particles are required for successful infection and replication within
cells. While many scientists believe that this segmentation may allow for the
virus to have more control over genome expression and variation, a higher rate
of infection is logically required so that all of the various particles may be
successfully transmitted into the cell. Presumably, this requirement may be one
reason why multicomponent viruses are previously unknown to animals, as
compared to more stationary plants and fungi. In this study from the U.S Army
Medical Research Institute of Infectious Diseases, two new examples of
multicomponent pathogens were discovered, one named Guaico Culex virus (GCXV),
isolated from mosquitoes in Central and South America, and one isolated from
red colobus monkeys in Uganda, a variant of the Jingmen Tick virus (JMTV). The
discovery of these multicomponent viruses and their ability to infect animals
has interesting significance both in human health and in the complex and
relatively unknown evolutionary history of viruses.
In order to determine the
multicomponent nature of these viruses, the researchers sequenced isolates from
the mosquitoes and found five distinct particles for GCXV, although the fifth
particle was only identified in some of the samples.
Figure 1: Identification of distinct viral particles
for GCXV in infected cells2
Interestingly, when these particles were transfected into
cells in various combinations, it was found that only segments one through four
were necessary for infection and replication, while segment five was not
necessary and did not seem to provide any significant improvement in
infectivity.2 This was shown through the use of viral plaque
formation, which observes the regions of cell death within a cell culture that
are caused by viral infection. Whereas in a non-segmented virus, plaques can be
formed in the presence of just a single particle, in multicomponent viruses, all
of the necessary particles are required in order for these plaques to form. For
the JMTV isolates, four distinct segments were found, and were present in all
of the samples isolated from both the red colobus monkey and from ticks.2
Ultimately, the sequencing and transfecting of these segments supports the
multicomponent nature of these viruses, with the JMTV variant remarkably
representing the first known example of a multicomponent virus infecting
primates. While the extent of multicomponent animal viruses is yet unknown, the
existence of this primate-infecting virus alone suggests that there may be
other segmented animal viruses, which may have relevant applications in human
and animal health as research expands and develops.
The discovery of these viruses also
has significance evolutionarily, as both of these multicomponent viruses, GCXV
and the JMTV variant, fit well with a recently classified group of segmented
viruses called “jingmenviruses”. Viruses in this category are all segmented into
at least four different virus particles, have been found in ticks and other
insects across China and South America, and are highly similar (80-90%) to GCXV
and the JMTV variant on a genetic level.3 From this information the
researchers reasonably suspect that all of the jingmenviruses share a segmented
common ancestor. Numerous structural differences among these viruses, however, along
with certain genetic differences, suggest incredible diversity within the
clade, much of which is yet to be discovered and classified. Another major
finding was that GCXV and other jingmenviruses are genetically similar to the
unsegmented category of viruses called flaviviruses, although whether the
evolutionary pathway diverged from an unsegmented or a segmented common ancestor
is yet unknown. While viral evolution is highly complex and unknown to a large extent,
the information provided by these newly discovered multicomponent viruses is
another important piece in the evolutionary puzzle.
Figure 2: Potential evolutionary relationships and
pathways between Jingmenvirus and Flavivirus2
In summary, this study reported the
discovery of two new multicomponent viruses, including the first identified example
of a virus of this type that can infect animals. Both evolutionarily and health
wise, the discovery of these segmented viruses has important implications for
future research, as scientists continue to connect together the evolutionary
pathways of viruses and work to counter infectious diseases in humans. Ultimately,
this study was just the first insight into this new class of viruses, and
further examples of animal-infecting multicomponent viruses are needed to
determine their prevalence among animals. Based on the isolates extracted from
the red colobus monkey, identification of other animal-infecting multicomponent
viruses may be found in other monkeys in Africa and South America, especially
in places with high numbers of common jingmenvirus vectors like ticks and
mosquitoes. Additionally, further examples of these viruses may aid in
determining the evolutionary relationship of the Jingmenvirus and Flavivirus clades,
particularly through the use of bioinformatics and comparisons of their genomes.
Overall, the incredible variation of viruses ranges well beyond our knowledge,
and identification of novel classes of viruses such as these are exciting
developments in a field with much left to discover.
1 Fulton, W.R.
(1980). Biological significance of multicomponent viruses. Annual Review Phytopathology 18, 131-146.
2 Ladner, J.T.,
Wiley, M.R., Beitzel, B., Kramer, L.D., Tesh, Robert B., Palacios, G., et al.
(2016). A multicomponent animal virus isolated from mosquitoes. Cell Host & Microbe 20, 357-367.
3 Shi,
M., Lin, X.D., Vasilakis, N., Tian, J.H., Li, C.X., Chen, L.J., Eastwood, G.,
Diao, X.N., Chen, M.H., Chen, X., et al. (2015). Divergent viruses discovered
in arthropods and vertebrates revise the evolutionary history of the Flaviviridae and related viruses. J. Virology 90, 659–669.
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