By Andrea Anderson

Researchers are using comparative genomics to understand the evolution and
epidemiology of a destructive plant virus affecting African maize crops -
and to develop resistance strategies against the virus.
In a paper scheduled to appear in the September issue of the Journal of
General Virology, an international team of researchers sequenced more than
80 maize streak viruses in an effort to understand the evolution of MSV-A -
a strain that damages maize crops in sub-Saharan Africa. The research
uncovered a handful of new grass-adapted MSV strains and provided new
insights into the emergence of the disease-causing strain. Now, researchers
say, this information is being applied to develop maize resistance
strategies.

"Given the frailty of African agriculture and perpetual famine risks with
millions of lives at stake, MSV is actually one of the most important plant
pathogens worldwide," senior author Darren Martin, an infectious disease
researcher at the University of Cape Town, said in a statement. "We wanted
to learn more about how the virus emerged and spread so we can develop new
ways to fight the diseases it causes."

Maize streak disease, caused by the MSV-A strain, is transmitted by
leafhoppers and leads to yellowing and severe disease in African maize
crops. In contrast, four grass-adapted MSV strains - MSV -B to -E - mainly
infect crops such as wheat, rye, barley, and oats, causing only mild damage
in maize.

The researchers suspected that understanding the genetics of these
grass-adapted strains could improve their understanding of MSV-A's biology
and evolution as well. They sequenced the genomes of 83 MSVs from African
grasses in South Africa, Zimbabwe, Mozambique, Nigeria, Uganda, Burundi,
Rwanda, Mali, and La Reunion. In the process, they uncovered six new
strains, dubbed MSV-F to -K.

By combining their own data along with publicly available sequence data for
MSV and non-MSV streak viruses, the team began pinpointing recombination
hotspots in the MSV genome. They found that more than 90 percent of the MSVs
appear to be recombinant, with the most pathogenic MSV-A strain representing
a combination of two apparently harmless grass-adapted strains.

"[E]very 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," Martin noted in a statement. "This
chance recombination event could be the reason MSV has become such a serious
problem."

Even so, Martin told GenomeWeb Daily News, there are limitations to MSV
recombination, with MSV strains apparently undergoing conserved gene
swapping events. "The patterns are evolutionarily constrained," Martin said.
"In the design of resistance strategies, we are specifically taking that
into account."

The patterns of MSV infection were also telling. Overall, the researchers
found significant MSV strain differences in southern Africa, East Africa,
West Africa, and La Reunion. In contrast, the MSV-A strains were relatively
similar in each of these regions. The team's conclusion: the MSV-A strain
may be moving across Africa more quickly than other, less damaging MSV
strains.

That, in turn, may be due to the strain's ability to infect numerous plant
species. Indeed, based on their data, the researchers speculated that MSV-A
might be more promiscuous than anticipated, infecting a greater variety of
grasses than any of the other MSV strains.

Together, that convergence of recombination events, broad host range, and
increased transmission speed appear to have contributed to the emergence of
increasingly pathogenic MSV-A viruses, Martin said.

By collecting and sequencing MSVs from additional parts of Africa, the
researchers hope to learn even more about the nature of MSV recombination,
evolution, and the emergence of pathogenic strains such as MSV-A.

In addition, using their knowledge of MSV recombination traits and host
resistance factors, the researchers are developing new tactics to combat
maize streak disease and curb its spread. "There's very little you can do to
stop a virus from emerging," Martin said. "You can just prepare for it to
emerge."

For instance, Martin said, co-author Dionne Shepherd, a virologist and plant
biotechnologist at the University of Cape Town, is spearheading an effort to
create transgenic maize lines with MSV resistance strategies. Those lines,
which are being developed in conjunction with the South African company
Pannar Seed, are currently being tested and are predicted to be on the
market within three years or so, Martin said.

And although there are regulatory hurdles, he added, genetically modified
crops generally face less opposition in Africa than in other parts of the
world. "Starvation and malnutrition are by far the biggest killer in
Africa," Martin said. "Being a bit squeamish about GM plants and things is
not something that African governments are going to do."
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