Genetics and Gene Mapping of Disease Resistance in Brassica - Translational Genomics for Crop Breeding: Biotic Stress - page 332

parental lines (Ayers and Lelacheur 1972). The
inheritance of clubroot resistance in crosses of
cabbage also showed that the resistance was
determined by two unlinked complementary
genes in cabbage (Voorrips et al.1997). B. napus
is an amphidiploid species and originally evolved
from the interspecific hybridization of the two
parental diploid species, B. rapa and B. oler-
acea . Similar to the reports in the parental diploid
species, a few genes were commonly identified
as conferring clubroot resistance in B. napus
(Manzanares-Dauleux et al. 2000b).

B. napus lines and their corresponding mapping
populations.
In B. rapa , early genetic studies of resis-
tant turnips suggested that dominant race-
specific genes conferred clubroot resistance in
the European fodder turnips. These results were
confirmed through gene mapping in Chinese
cabbage cultivars in which clubroot-resistance
genes were introgressed from the European fod-
der turnips. For example, Kuginuki and col-
leagues (1997) found RAPD markers linked to
a clubroot-resistance gene. Similarly, Suwabe
and colleagues (2003, 2006) used dozens of
simple sequence repeats (SSRs) in B. rapa to
identify the SSR markers linked to two major
clubroot-resistance genes Crr1 and Crr2 .Hirai
and colleagues (2004) added another dominant
clubroot-resistance gene, Crr3, to the list. Addi-
tionally, Crr4 was detected as a QTL that con-
fers resistance to clubroot disease (Suwabe et al.
2006).
Similarly, Matsumoto and colleagues (1998)
identified a major dominant clubroot-resistance
gene locus, CRa, on linkage group LG3 of an
RFLP genetic map of B. rapa . Another major
locus, CRb in B. rapa, conferring resistance to
P. brassicae, was mapped and closely linked
molecular markers to this major locus were
developed (Piao et al. 2004). More recently,
Sakamoto (2008) detected two major clubroot-
resistance gene loci, CRk and CRc, on linkage
groups R3 and R2 in B. rapa . CRk was mapped
to the region similar to where Crr3 was located,
suggesting that CRk and Crr3 might be the same
gene or different genes in the same genomic
region of B. rapa .

Gene Mapping of Clubroot Resistance

As genomics research advances, molecular
markers become increasingly useful tools for
studying clubroot resistance. In B. oleracea ,Fig-
dore and colleagues (1993) associated RFLP
markers with a major clubroot-resistance locus
in broccoli. Grandclement and Thomas (1996)
used RAPD markers to identify two QTLs for
clubroot-resistance genes in kale. Voorrips and
colleagues (1997) constructed a genetic map
with AFLP markers that was used to identify
two clubroot-resistance QTLs, pb-3 and pb4, in
cabbage. Similarly, a genetic map constructed
with several types of molecular markers and four
single-spore isolates was used to identify nine
genomic regions anchoring clubroot-resistance
genes in kale (Rocherieux et al. 2004).
In B. napus , Landry and colleagues (1992)
mapped two clubroot-resistance loci to P. bras-
sicae race 2 isolates. Manzanares-Dauleux and
colleagues (2000a) mapped a major gene and
QTL that conferred resistance to clubroot dis-
ease in B. napus . In another report, seven differ-
ent isolates were used to detect nineteen QTLs
in one B. napus DH-mapping population and all
QTLs were found to be race specific (Werner
et al. 2008). Additionally, the results in this
report indicated that the broad resistance in the
diploid parental line of B. oleracea disappeared
in newly resynthesized B. napus lines, a result
of epistatic interactions when clubroot-resistant
B. oleracea was used to produce these synthetic

Comparative Genomics in Clubroot
Resistance

Comparative mapping of clubroot-resistance
genes in Chinese cabbage has progressed dra-
matically because B. rapa is a diploid species
and its whole genome sequence is available
(Wang et al. 2011). According to the previous
description, eight dominant clubroot-resistance