CIAT parental genotypes
Two CIAT genotypes (CR52A-31 and AR14-10) had
all fourmarker alleles associatedwithCMD2 indicating
that it is the probable source of CMD resistance in these
genotypes. Phenotypic data showed no symptoms for
the disease in these two genotypes. Each of the other six
CIAT lines had one to three marker alleles associated
with CMD2 [three marker alleles in AR15-5; two
marker alleles inCR52A-4, CR 41-10,CR59-4; and one
marker allele in CR52A-25 and AR42-4 (Table 3)].
With the exception of AR42-4 which was susceptible,
the other five genotypes were phenotypically resistant to
the disease.The five genotypes were developed inCIAT
for CMD resistance using the CMD2 gene through
MAS and therefore, CMD2 is the probable source of
resistance in these genotypes. The phenotypic results
confirmed CMD resistance in these five lines. Generally,
CIAT has been deploying CMD2 donor parents as
the source of resistance in Latin American cassava
germplasm and this has remained the main source of
CMD resistance in the AR and CR series of cassava
genotypes as used in this study. Although AR42-4 was
developed using theCMD2 donor parent in its pedigree,
its susceptibility to CMD indicates that the RME-1
marker allele for the CMD2 gene as found in this
genotype from marker analysis was a false positive
which could occur through recombination.
F1 progeny disease reaction
The F1 families showed segregation for the CMD2
gene. Of the total of 684 genotypes initially planted in
the seedling nursery, phenotypic data indicated that
525 individuals were resistant with 159 individuals
being susceptible. The progenies scored with a CMD
severity of 1 and 2, thus showing resistance, were used
in the molecular analysis.
Results indicated that 83 % of the genotypes
showed at least a marker allele for the CMD2 gene
(data not shown as the dataset is too large). If CMD2
was responsible for all the resistance observed in the
F1 families, it would imply that the markers associated
with the gene was 83 % successful in identifying
resistant genotypes in the phenotypically selected
resistant individuals analysed with molecular markers.
A total of 88 individuals (17 %) had all four marker
alleles associated with the CMD2 gene. A total of 179
individuals had between one to three marker alleles
each associated with CMD2. A total of 91 individuals
had no marker allele associated with the CMD2 gene.
Discussion
The CIAT genotypes, in addition to other important
traits which they possess, such as high dry matter
content and yield, also served as donor parent for the
CMD2 gene in these crosses. The F1 families were
therefore expected to show segregation for the CMD2
gene. Although 684 individuals were initially planted,
phenotypic data indicated that 525 individuals were
resistant with 159 individuals being susceptible. The
progenies used in the molecular analysis were those
selected for CMD resistance based on phenotypic
scoring and should be expected to express marker
alleles or corresponding bands for the CMD2 gene.
Rabbi et al. (2014) used a high density single
nucleotide polymorphism (SNP) map in a bi-parental
mapping population segregating for the dominant
monogenic resistance to CMD, and found a single
locus with large effect, which is probably the CMD2
locus. In their study they found nearly half of the
progeny and the female parent to be resistant to CMD,
with the remainder of the F1 individuals showing
disease symptoms ranging from mild to severe.
The results of this study indicated that 83 % of the
genotypes showed at least one marker allele for the
CMD2 gene. The CMD markers identified were
around 2–8 cM away from the CMD2 locus, so the
markers are not on the gene, which means that
recombination could occur, causing a deviation from
a 100 % marker-trait association (Okogbenin et al.
2007). Therefore selection is more likely to be
accurate if there are multiple markers. One allele
linked to the gene is suggestive that the gene is present,
but not always. If CMD2 was responsible for all the
resistance observed in the F1 families, it would imply
that the markers associated with the gene was 83 %
successful in identifying resistant lines in the phenotypically
selected resistant individuals analysed with
molecular markers. Generally, CMD2 markers have
been found to be 68 % effective in MAS, but this is in
populations that were not pre-selected phenotypically.
So even though CMD2 is a dominant gene, false
positives can occur. The gene will confer resistance
when expressed either in the homozygous or heterozygous
state, although the resistance is better in the
CIAT parental genotypes
Two CIAT genotypes (CR52A-31 and AR14-10) had
all fourmarker alleles associatedwithCMD2 indicating
that it is the probable source of CMD resistance in these
genotypes. Phenotypic data showed no symptoms for
the disease in these two genotypes. Each of the other six
CIAT lines had one to three marker alleles associated
with CMD2 [three marker alleles in AR15-5; two
marker alleles inCR52A-4, CR 41-10,CR59-4; and one
marker allele in CR52A-25 and AR42-4 (Table 3)].
With the exception of AR42-4 which was susceptible,
the other five genotypes were phenotypically resistant to
the disease.The five genotypes were developed inCIAT
for CMD resistance using the CMD2 gene through
MAS and therefore, CMD2 is the probable source of
resistance in these genotypes. The phenotypic results
confirmed CMD resistance in these five lines. Generally,
CIAT has been deploying CMD2 donor parents as
the source of resistance in Latin American cassava
germplasm and this has remained the main source of
CMD resistance in the AR and CR series of cassava
genotypes as used in this study. Although AR42-4 was
developed using theCMD2 donor parent in its pedigree,
its susceptibility to CMD indicates that the RME-1
marker allele for the CMD2 gene as found in this
genotype from marker analysis was a false positive
which could occur through recombination.
F1 progeny disease reaction
The F1 families showed segregation for the CMD2
gene. Of the total of 684 genotypes initially planted in
the seedling nursery, phenotypic data indicated that
525 individuals were resistant with 159 individuals
being susceptible. The progenies scored with a CMD
severity of 1 and 2, thus showing resistance, were used
in the molecular analysis.
Results indicated that 83 % of the genotypes
showed at least a marker allele for the CMD2 gene
(data not shown as the dataset is too large). If CMD2
was responsible for all the resistance observed in the
F1 families, it would imply that the markers associated
with the gene was 83 % successful in identifying
resistant genotypes in the phenotypically selected
resistant individuals analysed with molecular markers.
A total of 88 individuals (17 %) had all four marker
alleles associated with the CMD2 gene. A total of 179
individuals had between one to three marker alleles
each associated with CMD2. A total of 91 individuals
had no marker allele associated with the CMD2 gene.
Discussion
The CIAT genotypes, in addition to other important
traits which they possess, such as high dry matter
content and yield, also served as donor parent for the
CMD2 gene in these crosses. The F1 families were
therefore expected to show segregation for the CMD2
gene. Although 684 individuals were initially planted,
phenotypic data indicated that 525 individuals were
resistant with 159 individuals being susceptible. The
progenies used in the molecular analysis were those
selected for CMD resistance based on phenotypic
scoring and should be expected to express marker
alleles or corresponding bands for the CMD2 gene.
Rabbi et al. (2014) used a high density single
nucleotide polymorphism (SNP) map in a bi-parental
mapping population segregating for the dominant
monogenic resistance to CMD, and found a single
locus with large effect, which is probably the CMD2
locus. In their study they found nearly half of the
progeny and the female parent to be resistant to CMD,
with the remainder of the F1 individuals showing
disease symptoms ranging from mild to severe.
The results of this study indicated that 83 % of the
genotypes showed at least one marker allele for the
CMD2 gene. The CMD markers identified were
around 2–8 cM away from the CMD2 locus, so the
markers are not on the gene, which means that
recombination could occur, causing a deviation from
a 100 % marker-trait association (Okogbenin et al.
2007). Therefore selection is more likely to be
accurate if there are multiple markers. One allele
linked to the gene is suggestive that the gene is present,
but not always. If CMD2 was responsible for all the
resistance observed in the F1 families, it would imply
that the markers associated with the gene was 83 %
successful in identifying resistant lines in the phenotypically
selected resistant individuals analysed with
molecular markers. Generally, CMD2 markers have
been found to be 68 % effective in MAS, but this is in
populations that were not pre-selected phenotypically.
So even though CMD2 is a dominant gene, false
positives can occur. The gene will confer resistance
when expressed either in the homozygous or heterozygous
state, although the resistance is better in the
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