United Kingdom
August 20, 2008
Source:
Society for General Microbiology
Scientists have discovered why one
of the world’s most important agricultural diseases emerged,
according to research* published in the September issue of the
Journal of General Virology.
Maize streak virus (MSV) causes the main virus disease of
Africa’s most important food crop. By comparing the genome of
the virus to those of its less harmful relatives, scientists
have discovered how and why MSV became a serious pest and spread
so rapidly across Africa.
“Given the fragility of African agriculture and perpetual famine
risks with millions of lives at stake, MSV is actually one of
the most important plant pathogens worldwide,” said Dr Darren
Martin from the University of Cape Town in South Africa. “We
wanted to learn more about how the virus emerged and spread so
we can develop new ways to fight the diseases it causes.”
There are many different strains of MSV but only one of these
causes a severe disease in maize. The others cause relatively
mild infections in other grassy food crops such as wheat, oats
and sugarcane. Scientists have known for some time now that the
“maize adapted” MSV strain is peculiar; all versions of the
virus that have been sampled throughout the entire African
continent are genetically almost identical. The researchers
looked at strains of the virus that infect natural grasses and
compared these to the maize adapted strain. The results show
that the maize adapted strain is even more unique than was
previously thought.
“We found that the maize adapted virus infects a greater variety
of grasses than any of the other MSV strains,” said Dr Martin.
“The virus appears to be spreading around Africa faster than the
other strains too. When we compared the genomes of 11 different
strains of MSV, we discovered that lots of genes had been
exchanged between the strains in a process called recombination.
We also found that every MSV that causes severe disease in maize
has descended from an ancestral virus that was the recombinant
offspring of two relatively harmless wild grass infecting
viruses. This chance recombination event could be the reason MSV
has become such a serious problem.”
All available information suggests that over 100 years ago, two
grass adapted MSVs recombined to produce a new “wide-host range”
strain that could infect a greater variety of other plants than
its parents. This meant the virus could survive the winters in
wild grasses more effectively than its relatives – something
that potentially increased the speed at which it spread across
Africa. The ability of this recombinant strain to infect many
different host species may have also enabled it to rapidly adapt
to maize – a process that today has led to its emergence as an
economically important crop disease.
“Understanding the chain of events that trigger the emergence of
novel pathogens is a major goal of epidemiologists worldwide,”
said Dr Martin. “There is a lot of circumstantial evidence
suggesting how evolutionary processes like recombination might
trigger the emergence of a pathogen but there are actually very
few well documented examples of this having occurred.”
The next step is to carry out rigorous tests to provide more
evidence for the specific biological causes of the emergence of
MSV. The researchers also hope to develop different genetic
strategies to help the crops resist disease. “Comparing
important pathogens with their “agriculturally irrelevant”
relatives can tell us useful stuff about the pathogens,” said Dr
Martin. “Knowing that maize adapted MSVs are unusually mobile I
hope will prompt scientists to test MSV resistance genes against
the large bank of virus strains that we have catalogued from
across Africa.”
“We are currently testing various resistance strategies, some in
an advanced stage of development, using many of the viruses
we’ve sampled. Complex genetic strategies like stacking
resistance genes and targeting different virus components that
cannot be simultaneously exchanged by recombination might
ultimately be the only way to control the disease.”
Research paper in Journal of General Virology, "Recombination,
decreased host specificity and increased mobility may have
driven the emergence of maize streak virus as an agricultural
pathogen", available at
http://jgv.sgmjournals.org/cgi/content/full/89/9/2063. |
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