Genetics and Gene Mapping of Disease Resistance in Brassica - Translational Genomics for Crop Breeding: Biotic Stress - page 335

are located on the B4 and B8 chromosomes of
B. nigra .

Rlm9 formed a resistance gene cluster on link-
age group 10, according to comparative linkage
analysis (Delourme et al. 2004).
As molecular marker development advances,
mapping of blackleg-resistance genes will facil-
itate the detection of similar linkage groups and
chromosomes in different laboratories. Several
mapped genes such as LEM1 , cRLMm, and LmR1
on linkage group 6 in Dr. Rimmer's group were
suggested to be on linkage group N7 of the com-
monly used genetic map (Parkin et al. 1995).
Similarly, all resistance genes mapped on link-
age group 10 in France are believed to be on
the same linkage group N7, and Rlm2 on link-
age group 16 should be on linkage group N10
on the Parkin genetic map (Delourme et al.
2006). Gene cloning has been attempted but
it was found that it is too difficult to clone a
blackleg-resistance gene on linkage group N7
because of the complexity of this chromosome
(Mayerhofer et al. 2005). However, until all
the previously mentioned resistance genes on
linkage group N7 are cloned and sequenced,
their linkage and allelic relationships will remain
undetermined.

Gene Mapping of Blackleg Resistance

Gene mapping allows the detection of individ-
ual disease-resistance genes and the compari-
son of major dominant resistance genes from
different resistance sources. The earlier reports
used very limited numbers of RFLP, AFLP, and
RAPD markers and the linkage groups that were
reported to anchor blackleg-resistance genes
were unique in each publication. A major resis-
tance gene locus in 'Cresor' was mapped as a
quantitative trait locus (QTL), explaining 72% of
phenotypic variation in all tested environments,
suggesting that it might be a dominant resistance
gene in this cultivar (Dion et al. 1995). Ferreira
and colleagues (1995) mapped a single major
locus controlling cotyledon resistance ( LEM1 )
in 'Major' to linkage group 6. RAPD and AFLP
markers were used to pinpoint a major resistance
gene locus Lmr1 and cRLMm in 'Shiralee' and
'Maluka,' respectively (Mayerhofer et al. 1997).
All these three resistance genes, LEM1 , LmR1,
and cRLMm were linked on the same linkage
group N7 (Rimmer 2006).
Genetic interaction between pathogen isolates
and host cultivars was used to name the resis-
tance genes in the host that corresponded to their
avirulence gene in the pathogen (Ansan-Melayah
et al. 1995, 1998). According to the interaction
between L. maculans and hosts, two dominant
resistant genes, Rlm1 in 'Quinta' and Rlm2 in
'Glacier,' were described. In addition to these
two resistance genes, another dominant resis-
tance gene in 'Quinta' was also inferred. Later,
Rlm3 in 'Glacier' was separated from the previ-
ously reported Rlm2 (Balesdent et al. 2006). Sim-
ilarly, Rlm4 was identified as linked with Rlm1 ,
but mapped to different positions in 'Quinta.'
Rlm4 also was detected in 'Net Jeuf' (Balesdent
et al. 2001). In addition, Rlm7 in a breeding line
23.1.1 and Rlm9 in 'Yudal' were mapped (Bales-
dent et al. 2001). Rlm1 , Rlm3 , Rlm4 , Rlm7, and

Mapping Blackleg-Resistance Genes
Introduced from Related Species

All the previously described resistance genes on
N7 and N10 have been identified in B. napus .
Actually, resistance genes from other Brassica
species such as B. rapa , B. nigra, and B. juncea
have been introduced into B. napus through
interspecifc hybridization, and these resistance
sources are commonly used to develop com-
mercial canola cultivars. For instance, Rlm5 and
Rlm6 were introduced from B. juncea to B.
npaus and Rlm8 from B. rapa, although these
gene loci have not been characterized (Delourme
et al. 2006). Most domesticated strains of B.
rapa are susceptible to L. maculans , but a wild-
type accession, B. rapa subsp. sylvestris, is resis-
tant to L. maculans and this accession has been
used to introgress several blackleg-resistance