Biology Reference
In-Depth Information
genes -
Crr1, Crr2, Crr3, Crr4, CRa, CRb
,
CRk,
and
CRc
- in Chinese cabbage, introduced from
different European fodder turnips were mapped
on different linkage groups in work done in Japan
(Hirai et al. 2004; Hirai 2006; Matsumoto et al.
1998; Piao et al. 2004; Saito et al. 2006; Suwabe
et al. 2006). Furthermore, most of the mapped
clubroot-resistance genes were integrated on the
corresponding chromosomes.
Crr3
,
CRa, CRb,
and
CRk
were mapped to chromosome R3, and
it seems that
Crr3
and
CRk
might be allelic,
while
CRa
and
CRb
are located at other genomic
regions of the same chromosome.
Crr1
,
Crr2
,
Crr4,
and
CRc
were mapped to chromosomes
R8, R1, R6, and R2, respectively.
For understanding the complexity of clubroot
resistance in Brassica species, gene cloning will
provide answers to many questions about gene
locations, functions, and interactions between
clubroot-resistance genes and pathogen isolates.
Since
B. rapa
contains dominant Mendelian clu-
broot resistance, it would be expected that the
clubroot-resistance genes would be cloned from
B. rapa
first. Saito and colleagues (2006) fine
mapped clubroot-resistance gene
Crr3
and used
the sequence-tagged site (STS) markers to per-
form comparative genomics with the Arabidop-
sis genome. They suggested that the
Crr3
gene
on R3 in
B. rapa
is located in a genomic loca-
tion sharing sequence similarity to a region of
chromosome 3 in Arabidopsis. However, flank-
ing SCAR markers of
CRb
share sequence simi-
larity to a region of chromosome 4 in Arabidop-
sis, suggesting that
CRb
is located in a position
that is different from that of the
Crr3
gene, since
both the genes were mapped on the R3 chromo-
some (Piao et al. 2008).
been successfully used to control clubroot dis-
ease in Japanese production of Chinese cabbage
for many years. However, after the clubroot-
resistant Chinese cabbage cultivars were in pro-
duction for a few years, it was found that some of
the clubroot-resistant Chinese cabbage cultivars
had become susceptible, suggesting that in Japan
the resistance of some clubroot-resistance genes
had been overcome (Kuginuki et al. 1999). For-
tunately, most mapped clubroot-resistance genes
are located in different chromosomes or in differ-
ent genomic regions of the same chromosome,
which would allow gene pyramiding to combine
multiple clubroot-resistance genes into a culti-
var. Pyramiding of different clubroot-resistance
genes from various sources may facilitate the
development of durable clubroot resistance in
the future.
Marker-assisted selection is essential for
the successful pyramiding of disease-resistance
genes. The abundant repertoire of clubroot-
resistance genes in Brassica species offers some
opportunities to produce clubroot-resistant cul-
tivars. On the other hand, it also poses chal-
lenges to exploiting different clubroot-resistance
sources. In particular, gene introgression from
species to species is often necessary, and multiple
genes are required for good resistance. In inter-
specific gene transfer, the flanking regions of
resistance genes are often incorporated into the
new species as a result of linkage drag, and some
traits in the introgressed lines become problem-
atic, especially in chromosome regions where
few recombination events occur.
For pyramiding multiple resistance genes
where it is not easy to distinguish the phenotypic
differences among individuals with one, two, or
more resistance genes, it becomes difficult to
transfer these genes using conventional breeding.
Actually, most disease-resistance genes interact
in this way, and each gene gives only a relatively
small contribution to the phenotypic variation.
Therefore, molecular marker-assisted selection
(MAS) becomes mandatory in order to trans-
fer multiple resistance genes simultaneously.
Successful Use of Clubroot-Resistance
Genes in Chinese Cabbage
Clubroot-resistance genes from European fodder
turnips have been successfully used to develop
clubroot-resistant Chinese cabbage cultivars in
Japan, and these clubroot
-
resistant cultivars have
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