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different genetic background (Niew ohner et al.
1995; Moreno-Vazquez et al. 2003). To ensure
universal applicability of a marker in MAS, the
marker-gene association has to be tested in dif-
ferent genetic backgrounds, that is, in different
horticultural types of lettuce.
Although the potential for use of MAS in
plant breeding programs is extensive, applica-
tion varies widely among crop species. In crops
such as maize, rice, wheat, or barley, use of
MAS is well established, while in others, MAS
is used less frequently. Historically, a number of
different marker-systems have been applied for
lettuce genotyping, such as isozymes (Kesseli
and Michelmore 1986); restriction fragment
length polymorphism - RFLP (Kesseli et al.
1991); random amplified polymorphic DNA -
RAPD (Kesseli et al. 1994); sequence character-
ized amplified regions - SCAR (Maisonneuve
et al. 1994); retrotransposon-based sequence
specific amplification polymorphism - SSAP
(Syed et al. 2006); amplified fragment length
polymorphism - AFLP (Hill et al. 1996); simple
sequence repeats - SSR (Van de Wiel et al. 1999;
Rauscher and Simko 2013); target region ampli-
fication polymorphism - TRAP (Hu et al. 2005);
expressed sequence tag based SSR - EST-SSR
(Simko 2009); single-stranded conformational
polymorphism - SSCP (McHale et al. 2009);
single nucleotide polymorphism - SNPs (Kwon
et al. 2012); and single position polymorphism -
SPP (Stoffel et al. 2012). The majority of mark-
ers currently used in plant genetics are molecular
markers that allow high-throughput genotyping.
These markers have been used to construct sev-
eral molecular linkage maps of lettuce (Truco
et al. 2007) to map resistance genes (Paran
and Michelmore 1993; Meyers et al. 1998a;
McHale et al. 2009), and to develop assays for
MAS (Moreno-Vazquez et al. 2003; Simko et al.
2009). The following chapter describes progress
in mapping disease resistance genes and use of
MAS in lettuce breeding. Although most of the
information comes from public breeding pro-
grams, a brief description of MAS in the private
sector will also be provided.
Marker-Assisted Selection for
Disease Resistance
Precise assessment of a phenotype is critical for
development of assays for MAS. Therefore, it
is generally easier to develop marker assays for
monogenic traits that can be scored with high
confidence than for QTLs of complex traits. At
the present time MAS for diseases resistance
in lettuce is limited to simply inherited traits.
Assays were developed to select desired genes
and alleles for resistance to downy mildew, corky
root, lettuce mosaic virus, and lettuce dieback
(Figure 14.2).
Downy Mildew
Downy mildew, caused by the oomycete
Bremia
lactucae
Regel, is probably the most frequent and
destructive disease of cultivated lettuce world-
wide. The pathogen can infect lettuce at any
developmental stage, from young seedlings to
mature plants. Infected plants show yellow to
pale green areas on the adaxial side of leaves
that eventually become necrotic. When condi-
tions are favorable for growth of the pathogen,
sporulation appears predominantly on the abax-
ial part of the leaves (Figure 14.2A).
Two types of resistance to downy mildew are
known in cultivated lettuce: qualitative - based
on single dominant genes or resistance factors
(termed
Dm
and
R,
respectively), and quanti-
tative - based on multiple genes with minor
effect. More than thirty race-specific
Dm
genes
and
R
factors have been introgressed into lettuce
cultivars from wild species, and several of the
genes were mapped (McHale 2008; Michelmore
et al. 2010). Most of the mapped
Dm
genes are
clustered at three linkage groups (LGs);
Dm5/8
,
Dm10
,
Dm17
,
Dm43
, and
Dm45
at LG 1;
Dm1
,
Dm2
,
Dm3
,
Dm6
,
Dm14
,
Dm15
,
Dm16
, and
Dm18
at LG 2; and
Dm4
,
Dm7
,
Dm11
,
Dm44
,
Dm48
, and
Dm49
at LG 4 (Table 14.1). A single
Dm
gene,
Dm13
, is located at LG 3 (Paran et al.
1991; Maisonneuve et al. 1994; McHale 2008;
McHale et al. 2009; Michelmore et al. 2010;
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