homozygous form (Okogbenin et al. 2007). Rabbi
et al. (2014) found the CMD2 gene locus on linkage
group 16, with a peak at 69.12 cM, which was close to
the previously reported locus (Akano et al. 2002). The
association decreased on both sides of the peak due to
recombination between the markers and the resistance
gene.
A total of 88 individuals (17 %) had all four marker
alleles associated with the CMD2 gene. In these
individuals, themost probable source ofCMDresistance
is CMD2. A total of 179 individuals had between one to
three marker alleles each associated with CMD2. The
progenitors Dabodabo and TMEII were phenotypically
resistant. TMEII appeared consistent with the CMD2
gene resistance (showing bands present for alleles of this
gene in three markers). However, Dabodabo was less
consistent, showing presence of bands for the corresponding
allele for CMD2 gene resistance in only one of
the four markers and may represent a new possible
source of CMD resistance gene(s). The further use of
Dabodabo in crosses with susceptible CMD parent(s) to
test for new CMDresistance gene in this landracewould
confirm the observation.
The 179 genotypes with between one and three
marker alleles for the CMD2 gene were phenotypically
shown to be resistant to the disease. Since they
resulted mainly from crosses between CMD2 donor
parents (from seven CIAT lines) and landraces (with
TMEII and Dabodabo being CMD resistant), resistance
in these genotypes may result from the CMD2
source alone or from both the CMD2 source and any
other new source of CMD resistance if we assume that
the CMD resistant landrace progenitors (Dabodabo
and TMEII) represent a new source of CMD resistance
genes. Rabbi et al. (2014) used the genetic map to
anchor the seven molecular markers previously
reported to be linked to the dominant gene resistance.
They found all the previously related markers (except
for RME1 and RME4) to be located on scaffolds
occurring in the same region of linkage group 16,
indicating that three of the four markers used in this
study were located in this region. The single locus on
linkage group 16 which explained 74 % of phenotypic
variation was co-located with the SSRY28 marker.
They found a total of 281 SNPs at this locus.
A total of 91 individuals had no marker alleles
associated with theCMD2 gene. This would imply that
either these individuals have resistance different from
the CMD2 gene or that they are false resistant
genotypes. The former could apply to individuals
which are progenies of Dabodabo and TMEII if CMD
resistance in the two genotypes is different from
CMD2 resistance. Further study is still required to
confirm resistance in these two genotypes. The latter
possibility (false resistant genotypes) would be most
probable in individuals with no marker allele for
CMD2 derived from crosses between CIAT genotypes
and the susceptible landraces (Afeb and Tuaka). The
expected source of CMD resistance for such individuals
should be the CMD2 gene from the CIAT donor
parents. As the individuals did not have any allele of
the four markers used for CMD2 selection, thenCMD2
resistance is most probably not present in these
individuals and they may therefore be false resistant
genotypes.MAS has been used successfully in cassava
for studying CMD resistance (Fregene et al. 2007;
Okogbenin et al. 2007).
It should be noted that these materials were
phenotypically evaluated in one season. Under high
CMD infestation, disease evaluation in one season
(1 year) is very efficient. Previous studies (Okogbenin
et al. 2007) have indicated that under high disease
pressure, there is a highly significant correlation in
disease expression between one season data and twoseason
data. However, at locations where disease
pressure is not very high, some susceptible individuals
may escape and could be misclassified, although this is
relatively unusual even in moderate CMD pressure
zones. Disease pressure levels for Fumesua and
Ohawu are extremely high. Hence, the choice of these
two locations was appropriate for classification of
CMD reaction. The greatest impact from MAS will be
realized when breeding systems use high throughput
techniques for large populations for genotyping for
multiple target traits. The advantage would be to
achieve the same breeding progress in a much shorter
time than through conventional breeding alone and
from pyramiding genes of several traits that could not
be readily combined through other means.
To conclude, the use of a dominant resistance gene
such as CMD2 implies that introgression of CMD
resistance is becoming possible in a single or a few
crosses. The available markers can be used to rapidly
screen for resistance without the need to plant
thousands of seedlings. The long term effectiveness
of this gene is not known, and resistance breeding
should probably be augmented with quantitative
resistance from M. glaziovii (Rabbi et al. 2014).
homozygous form (Okogbenin et al. 2007). Rabbi
et al. (2014) found the CMD2 gene locus on linkage
group 16, with a peak at 69.12 cM, which was close to
the previously reported locus (Akano et al. 2002). The
association decreased on both sides of the peak due to
recombination between the markers and the resistance
gene.
A total of 88 individuals (17 %) had all four marker
alleles associated with the CMD2 gene. In these
individuals, themost probable source ofCMDresistance
is CMD2. A total of 179 individuals had between one to
three marker alleles each associated with CMD2. The
progenitors Dabodabo and TMEII were phenotypically
resistant. TMEII appeared consistent with the CMD2
gene resistance (showing bands present for alleles of this
gene in three markers). However, Dabodabo was less
consistent, showing presence of bands for the corresponding
allele for CMD2 gene resistance in only one of
the four markers and may represent a new possible
source of CMD resistance gene(s). The further use of
Dabodabo in crosses with susceptible CMD parent(s) to
test for new CMDresistance gene in this landracewould
confirm the observation.
The 179 genotypes with between one and three
marker alleles for the CMD2 gene were phenotypically
shown to be resistant to the disease. Since they
resulted mainly from crosses between CMD2 donor
parents (from seven CIAT lines) and landraces (with
TMEII and Dabodabo being CMD resistant), resistance
in these genotypes may result from the CMD2
source alone or from both the CMD2 source and any
other new source of CMD resistance if we assume that
the CMD resistant landrace progenitors (Dabodabo
and TMEII) represent a new source of CMD resistance
genes. Rabbi et al. (2014) used the genetic map to
anchor the seven molecular markers previously
reported to be linked to the dominant gene resistance.
They found all the previously related markers (except
for RME1 and RME4) to be located on scaffolds
occurring in the same region of linkage group 16,
indicating that three of the four markers used in this
study were located in this region. The single locus on
linkage group 16 which explained 74 % of phenotypic
variation was co-located with the SSRY28 marker.
They found a total of 281 SNPs at this locus.
A total of 91 individuals had no marker alleles
associated with theCMD2 gene. This would imply that
either these individuals have resistance different from
the CMD2 gene or that they are false resistant
genotypes. The former could apply to individuals
which are progenies of Dabodabo and TMEII if CMD
resistance in the two genotypes is different from
CMD2 resistance. Further study is still required to
confirm resistance in these two genotypes. The latter
possibility (false resistant genotypes) would be most
probable in individuals with no marker allele for
CMD2 derived from crosses between CIAT genotypes
and the susceptible landraces (Afeb and Tuaka). The
expected source of CMD resistance for such individuals
should be the CMD2 gene from the CIAT donor
parents. As the individuals did not have any allele of
the four markers used for CMD2 selection, thenCMD2
resistance is most probably not present in these
individuals and they may therefore be false resistant
genotypes.MAS has been used successfully in cassava
for studying CMD resistance (Fregene et al. 2007;
Okogbenin et al. 2007).
It should be noted that these materials were
phenotypically evaluated in one season. Under high
CMD infestation, disease evaluation in one season
(1 year) is very efficient. Previous studies (Okogbenin
et al. 2007) have indicated that under high disease
pressure, there is a highly significant correlation in
disease expression between one season data and twoseason
data. However, at locations where disease
pressure is not very high, some susceptible individuals
may escape and could be misclassified, although this is
relatively unusual even in moderate CMD pressure
zones. Disease pressure levels for Fumesua and
Ohawu are extremely high. Hence, the choice of these
two locations was appropriate for classification of
CMD reaction. The greatest impact from MAS will be
realized when breeding systems use high throughput
techniques for large populations for genotyping for
multiple target traits. The advantage would be to
achieve the same breeding progress in a much shorter
time than through conventional breeding alone and
from pyramiding genes of several traits that could not
be readily combined through other means.
To conclude, the use of a dominant resistance gene
such as CMD2 implies that introgression of CMD
resistance is becoming possible in a single or a few
crosses. The available markers can be used to rapidly
screen for resistance without the need to plant
thousands of seedlings. The long term effectiveness
of this gene is not known, and resistance breeding
should probably be augmented with quantitative
resistance from M. glaziovii (Rabbi et al. 2014).
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