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Molecular Plant Breeding 2012, Vol.3, No.8, 80
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resistance in the variety ‘Kukri’. The resistance of
adult plants of this variety was assessed by growing
plants in open-ended tubes placed in outdoor terraces.
Bulked segregant analysis identified a locus on
chromosome 4B near the semi-dwarfing gene
Rht1
(Wallwork et al., 2004). The second genotype investigated
was the breeding line ‘2-49’. Based on a seedling
assay, two different quantitative trait loci (QTL)
conferring CR resistance were detected from this
genotype. One of them, located on chromosome arm
1DL, explained up to 21% of phenotypic variance.
The other, located on chromosome arm 1AL,
explained up to 10% of the variance (Collard et al.,
2005). The third report was on the genotype
‘W21MMT20’, again based on a seedling assay
(Bovill et al., 2006). These authors reported several
putative QTL conferring CR resistances. However,
none of them reached significant levels in all of the
three assays reported. The most significant QTL was
located on 5D. It reached significant levels in two of
the three trials and explained up to 28.0% of the
phenotypic variance. The other QTL, located on 2D,
reached significant level in only one of the three
assays conducted and it explained 10.2% of the
phenotypic variance. These variable results are not
surprising as accurate CR assessment is notoriously
difficult (Collard et al., 2005). It is of note that none of
the five loci derived from the three different resistant
genotypes co-locate. They were found on five
different chromosomes. However, recent studies have
identified a major QTL on the long arm of chromosome
3B for CR resistance in three different wheat
genotypes (Li et al., 2010a; Ma et al., 2010, Bovill et
al., 2010). The magnitude of the 3B CR QTL identified
in these genotypes explained up to 34.6%, 40.2%, and
46.2% of the phenotypic variance with detection in
each of the trials in the three independent studies.
Although the three genotypes are totally different, one
is a modern wheat variety from USA, another is an
Australian breeding line, and the third is an old
landrace from India; a major QTL was identified in
each of these resources and they were mapped to a
very similar position on 3BL (Li et al., 2010a; Ma et
al., 2010; Bovill et al., 2010).
Compared to bread wheat, genetic studies on CR
resistance in barley is lag behind. However, a major
QTL for CR resistance in barley has recently been
identified from the Chinese landrace ‘TX9425’ which
showed a moderate resistance to CR, markers linked
to this QTL on the long arm of chromosome 3H count
for over 60% of the phenotypic variation in resistance
(Li et al., 2009). The discovery of promising QTLs for
a trait of interest is an important, but primary step in
developing a marker-assisted selection program for
genetic improvement. Ideally, the identified QTLs
should be validated in additional genetic backgrounds
and environments and should not have undesirable
effects on other important traits. Moreover, because
the chromosome 3H in barley has a possible
co-linearity with the group 3 chromosomes in wheat
(Devos and Gale, 1993). It is interesting to know
whether the 3B CR QTL in wheat and the 3H CR QTL
in barley is homoeoalleles having a common origin. If
a homoeoallellic relationships between the two loci
exist, it would be much easier to characterise and
isolate this gene in barley because barley is a diploid,
self-pollinating species for which an experimental
population can be developed with relative ease, and
because barley has relatively few chromosomes,
which can avoid any confounding effects from other
homologous groups as it is in wheat.
The present study was carried out (1) to develop
locus-specific molecular markers closely linked to the
wheat 3B and barley 3H CR locus; (2) to validate the
effect of the two loci in different genetic backgrounds
in wheat and barley respectively for the purpose of
using these major genes in breeding commercial
wheat and barley varieties in the future; and to
understand the possible homoeoallellic relationship
between the wheat 3B and barley 3H CR loci.
1 Results
1.1 Development of more closely linked locus-specific
markers in the regions conferring CR resistance on
chromosomes 3B and 3H
The wheat 3B CR QTL was previously identified
between two DArT markers, which were 9.4 cM apart
(Li et al., 2010a). In order to more precisely determine
the position of the QTL, and identify more closely
linked locus-specific markers that can be used for
validation this QTL in different genetic backgrounds