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."
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