Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 242

phenotypic loci on oil content, which were presumably targeted by selection
in the transition from large-seeded, low-oil to small-seeded, high oil cultivars,
and allowed Leon et al. (1995b) to establish combined marker and
phenotypic (based on the hyp locus underlying a QTL for percentage of
kernel) assisted selection for high oil content in the backcross process, in
addition to marker-assisted background selection.
Codominant markers are the most useful for marker-assisted
backcrossing because selection among backcross progeny involves
identification of heterozygous individuals. Conversely, if a dominant marker
is used, it will only be informative if the dominant allele (conferring the
presence of a band) is linked to the donor parent allele. For selection of the
Pl6 gene conferring sunflower resistance to downy mildew race 730, Pankovi
et al. (2007) proposed the use of Pl6 tightly linked codominant cleaved
amplified polymorphic sequence (CAPS) markers in combination with
dominant markers developed from resistance candidate genes for increasing
the MAS efficiency in backcross programs.
Wild relatives of sunflower possess a high level of resistance to many
biotic and abiotic stresses (Jan and Seiler 2007). Marker-assisted backcross
breeding is also a very effective way of transferring genes or QTLs determining
these traits from wild donor genotypes into elite breeding lines by reducing
both the time needed to produce a commercial cultivar and the risk of
undesirable linkage drag. However, current breeding efforts in sunflower are
directed more towards the introgression of specific genes from wild species
through phenotypic evaluations, for example to diseases (Jan and Seiler 2007),
rather than through the use of molecular markers directly associated with the
desired trait. To facilitate and accelerate the introgression process and to
distinguish new pathogen resistance gene specificities, Slabaugh et al. (2003)
have proposed the identification of allelic variation in wild species in specific
candidate genes known to be associated with resistance to sunflower
pathogens such as RGCs. In addition, QTL and candidate gene analyses in
wild sunflower species are contributing towards identifying genes and QTL
for adaptation to salt or drought tolerance (Lexer et al. 2003a, b; Kane and
Rieseberg 2007) that could be exploited as a source of new genes to be
introgressed into cultivated sunflower. DNA-based markers have also been
useful for the characterization and verification of interspecific sunflower
hybrids (Natali et al. 1998; Binsfeld et al. 2001) and to identify introgressed
DNA fragments from wild species in interspecific progenies. Some of these
fragments have been related to increased levels of resistance to diseases such
as Sclerotinia sclerotiorum , Phomopsis, or downy mildew (Besnard et al. 1997;
Rönicke et al. 2004; Wieckhorst et al. 2008).
So-called “advanced backcross QTL analysis” (AB-QTL) has been
proposed for transferring QTLs for agronomically important traits from