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Molecular Plant Breeding 2012, Vol.3, No.8, 80
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isolates. Although these two genotypes are totally
different, one is a modern wheat variety from USA,
the other is an old landrace from India, only one QTL
was identified in each of these two resources and they
were mapped to a very similar position on 3BL (Li et
al., 2010a; Ma et al., 2010). The major QTL identified
in the third resource ‘W21MMT20’ (an Australian
breeding line) was also located to a position close to
the distal end of chromosome 3B, although the author
of this study failed to detect the QTL in their previous
study using this same resistant resource (Bovill et al.,
2006). However, the 3B major CR locus identified in
the three recent studies indicates that this gene could
be a dawn for wheat breeders in their effort of
attacking crown rot disease and breeding resistant
wheat varieties. Compared to wheat, genetic studies
on CR resistance in barley are lacking. However, a
major QTL for CR resistance in barley has recently
been identified from the cultivar ‘TX9425’, and
markers linked to this QTL on the long arm of
chromosome 3H account for over 60% of the phenotypic
variation in resistance (Li et al., 2009).
Because a significant proportion of the markers used
in our previous QTL study (Li et al., 2009; 2010a)
were DArT markers, it is important to develop closely
linked PCR-based markers, which can be used to
validate the effect of the 3B and 3H CR locus and for
use in breeding programs. Four different wheat
populations and three barley populations were used in
this study to validate the effects of the 3B and 3H CR
locus. With an effect varied from 36.7 to 59.4% in
wheat and 29.4 to 55.2% in barley, the validation
experiments in this study demonstrated that both the
3B and 3H CR locus have a stable and significant
effect in reducing CR severity at different genetic
backgrounds.
The huge effect of a single gene as the
CR locus detected in this study warrant a significant
value of incorporating these two genes in wheat and
barley breeding.
2.2 The possible homoeologous relationships between
the 3B and 3H CR QTL
Wheat and barley are two important grass species that
may have a common gene pool and share a significant
level of synteny (Ahn et al., 1993; Morre et al., 1995).
Findings from these comparative mapping studies
between grass families is of considerable values to
plant breeders, since it means that similar genes in
different plants can be treated as essentially allelic
variants, and the different alleles can be transferred,
more or less at will, from one species to another to
improve or alter the performance of a crop.
The major CR QTL identified in the wheat variety
Ernie and the barley genotype TX9425 have large
effects on enhancing CR resistance, and they are both
located on the long arms of the homoeologous group
3 chromosomes, with the barley CR locus proximal to
the centromere of chromosome 3H (Li et al., 2009)
and the one in wheat close to the distal end of wheat
chromosome 3BL (Li et al., 2010). The possible
homoeologous relationship between these two major
CR loci may exist because chromosome 3H in barley
has been reported to show a virtual collinearity with
wheat chromosome 3B (Devos and Gale, 1993). However,
recent studies investigating microcolinearity in the grass
family has revealed numerous small rearrangements
of gene content, order, and orientation between
different grass genomes (Bennetzen and Ramakrishna,
2002); therefore, the arms of barley chromosomes
may not be a completely identical to the genetically
equivalent of the arms of wheat chromosome.
The low transferability of PCR markers between
wheat and barley as showed in this study make the
comparative mapping between the two crops very
labour extensive and less efficient. However, the
comparative mapping attempt in this study indicated
that the barley 3H CR locus may be a homologous
allele of the 3B CR resistance locus because the
flanking markers of the wheat 3B CR locus were
mapped to a barley genomic region, which co-locate
with the barley 3H CR locus. Moreover, three markers
from a region harbouring a CR major QTL on barley
chromosome 3H were mapped to a region of
chromosome 3B in wheat, which is only 15 cM
away from the CR major locus identified on this
chromosome. A small distance between the wheat CR
locus and the region harbouring barley SSR marker on
chromosome 3B results in a reasonable inference that
the two regions could overlap if more markers were
used in comparative mapping of the regions associated