Biology Reference
In-Depth Information
source of resistance (Hayes et al. 2011c). Map-
ping of QTLs involved in the resistance to lettuce
drop has begun (Hayes et al. 2008b; Michelmore
2010).
et al. 2002). Post-transcriptional gene silenc-
ing (PTGS, RNAi) showed that within the same
gene cluster different domains of the resistance
gene silence different subsets of resistance speci-
ficities. For example, a fragment of the Dm3
gene LRR domain silenced Dm16 but not the
Dm6 gene. Conversely, a fragment of the NBS
domain of the Dm3 gene silenced Dm6 ,but
not the Dm16 gene (Wroblewski et al. 2007;
Michelmore 2010). Analysis of the lettuce EST
database identified more than 700 candidate
resistance genes that have sequences similar to
the known disease resistance genes (McHale
2008). These candidate resistance genes can be
classified into four broad groups: resistance gene
candidates ( RGCs ), signaling pathway genes,
defense response associated genes, and suscep-
tibility factors (McHale et al. 2009). Signifi-
cantly more RGCs co-localized on the molec-
ular linkage map with the mapped resistance
genes than did other categories of candidate
resistance genes. The candidate resistance genes
that co-localize with resistance phenotypes pro-
vide molecular markers that can potentially be
used in MAS. In addition to candidate resis-
tance genes, the genetic determinants of reac-
tions to effector molecules from bacterial plant
pathogens were mapped on LG 1, LG 8, and
LG 9 (McHale et al. 2009). The genomic loca-
tions of determinants provide potential targets of
pathogen effector proteins.
The reference molecular linkage map of let-
tuce was developed using RILs (recombinant
inbred lines) from the interspecies mapping pop-
ulation of L. sativa cv. Salinas
Bacterial Leaf Spot
Bacterial leaf spot usually has a minor effect on
lettuce crops, but damage might be significant
under conducive environmental conditions. The
disease, caused by the bacterium Xanthomonas
campestris pv vitians, was reported in the U.S.,
Europe, Australia, and Asia (Koike and Gilbert-
son 1997). All types of lettuces are suscepti-
ble to this disease. Infected plants show small
water-soaked leaf spots on outer leaves that later
become necrotic. If infected plants are harvested,
secondary decay organisms may colonize spot
lesions, leading to significant post-harvest losses
(Koike and Gilbertson 1997). Based on disease
incidence, two leaf-type cultivars (Grand Rapids
and Waldmann's Green) were identified as being
the least susceptible (Carisse et al. 2000). Further
testing revealed even lower disease incidence
and/or disease severity in the Latin-type cv. Lit-
tle Gem and the Batavia cv. Batavia Reine des
Glaces (Bull et al. 2007). A high level of resis-
tance was reported also in cvs. Salad Crisp (ice-
berg), Iceberg (Batavia), La Brillante (Batavia),
and seven iceberg-type breeding lines (Hayes
et al. 2008a). The mapping population devel-
oped from a cross between cvs. La Brillante
and Salinas 88 is being used to map a single
large-effect QTL (Hayes and Trent, unpublished
results).
L. serriola
accession UC96US23 (Truco et al. 2007). This
map was originally constructed from more than
700 AFLP and SSR markers and was later sat-
urated with approximately 14,000 loci to an
ultrahigh density genetic map (Truco et al.
2013). The 14,000 loci are based on transcribed
sequences from the microarray chip (Michel-
more 2010; Michelmore et al. 2010; Stoffel et al.
2012). The chip contains sequences for detect-
ing polymorphism in approximately 35,000 let-
tuce unigenes and facilitates high-throughput,
×
Studyofthe Lettuce Genome
In plant genomes, resistance genes are frequently
clustered (Michelmore and Meyers 1998). A
typical example of the resistance gene cluster
in lettuce was detected on LG 2, which har-
bors at least seven known Dm genes, including
Dm3 (Meyers et al. 1998a), which belongs to
the NBS-LRR group of resistance genes (Shen
Search WWH ::




Custom Search