Biology Reference
In-Depth Information
polygenic resistance in CIAT before field-testing
in Africa. The lack of pathogen also implies that
cassava in Latin America was not able to evolve
with the virus as in the case of Africa, where
disease pressure has aided evolutionary changes
of the crop with enhanced genetic capacity for
survival, as evidenced in the new sources of
resistance available in the secondary center of
diversity. The dominant gene nature of
CMD2
means that CMD resistance can now easily be
transferred and tracked by molecular markers.
Breeding for resistance to CMD in Latin Amer-
ica, where the disease does not exist, therefore
essentially requires the tools of MAS. The use
of markers for CMD resistance is enhancing the
ability of the CIAT breeding program in Latin
America to breed and select in the absence of the
disease, which is now the case in the New World.
CIAT initiated MAS, using TME3 as the
donor parent for
CMD2.
Progenies of TME3
were established from embryo axes and imported
to CIAT from IITA. They were crossed exten-
sively into elite LA cassava parents. Two sets
of germplasm were developed. The first set con-
sists of F
1
germplasm derived from crosses of
elite CIAT lines with CMD-resistant donor par-
ents, followed by MAS for the
CMD2
gene.
These lines developed from this process were
denoted as CR lines. The second set of geno-
types was obtained by crossing CMD-resistant
lines to backcross derivatives of CIAT lines hav-
ing resistance to cassava green mite (CGM). This
second set combines resistance to CMD and
CGM and these were designated as AR lines.
Seeds harvested from the crosses were germi-
nated in vitro from embryo axes according to
standard protocols for cassava (Fregene et al.
1997; CIAT 2002) to allow testing of these geno-
types in Africa. Each plantlet was multiplied and
allowed to grow for several weeks. Leaves of
all plants were removed for molecular analysis
and the plants were multiplied again to obtain
sufficient plantlets.
DNA
Rec
Frac.
Dist
cM
Marker
Id
Name
(5)
RME-2
(10.2 %)
10.4
(4)
RME-1
(4.5 %)
4.5
(1)
CMD2
(7.1 %)
7.1
(2)
NS158
(5.9 %)
5.9
(3)
SSRY28
Fig. 15.5.
Multiple flanking markers of the
CMD2
gene
normally used for marker-assisted selection. For a color ver-
sion of this figure, please refer to the color plate.
polyacrylamide gel electrophoresis (PAGE), or
agarose gel analysis of SSR markers and SCAR
markers were as described by Mba et al. (2001).
Multiple flanking markers of the
CMD2
gene
(Figure 15.5) were used for the molecular anal-
ysis. After molecular analysis, genotypes that
carry the marker allele associated with
CMD2
were further multiplied to obtain enough plants
for testing in Africa. The flow chart for MAS
used is shown in Figure 15.6. Field evalua-
tions have indicated that RME1 and NS158 were
excellent prediction tools for CMD resistance.
A recent validation study indicates that MAS
efficiency with these markers was around 68%
(Okogbenin et al. 2007). The dominant nature
of the gene and its effectiveness against a wide
spectrum of the viral strains make its deployment
very appealing for protecting cassava against the
CMD threat (Blair et al. 2007).
(b) Broadening of the Germplasm Base
in Africa
The presence of CMD in Africa and India and its
absence in the Americas limits the value of cas-
sava germplasm from the crop's center of diver-
sity in Africa because CMGs potentially carried
from Africa would have a devastating effect on
LA germplasm. This situation has imposed huge
limitations on the utilization of germplasm from
isolation
was
done
by
the
rapid
mini
preparation
method
developed
for
rice
(Noboyuki
et
al.
2000).
PCR
amplification,
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