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Genetic mapping
of a dominant gene conferring resistance to the cassava mosaic disease (CMD)
M.
Fregene, A.O. Akano, A.G.O. Dixon, C. Mba & E. Barrera
The
cassava mosaic disease (CMD) is the most important disease of cassava in
Africa, and a potential threat to the crop in Latin America where the
disease is still not known but the vector has recently been found. Host
plant resistance is the principal method of control, and was first
identified in 3rd back cross derivatives of an inter-specific
cross between cassava and M. glaziovii.
It is thought to be polygenic with a recessive component.
Recently, several Nigerian cassava landraces have been
identified that show very high resistance to CMD.
Classical genetic analysis and molecular genetic mapping revealed
a single dominant gene control of resistance.
A bulk segregant analysis (BSA) approach was used to identify two
simple sequence repeat (SSR) markers linked to the CMD resistance gene.
The SSR marker is located on linkage group R of the male
parent-derived molecular genetic map. The gene has been designated CMD2
and is flanked by one SSR marker and a RFLP marker at 2 and 9 cM
respectively. The second
SSR marker shows no recombination with the CMD2.
The use of markers linked to CMD2
for marker-assisted breeding of CMD resistance to increase the
efficiency of breeding in Africa and Latin America is discussed. Efforts
are also ongoing to clone CMD2 and genes expressed downstream that are the molecular basis of
resistance using positional cloning and the serial analysis of gene
expression.
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