Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 244

7.5.3.2 Gene Pyramiding
Gene pyramiding is a genotype building strategy for simultaneous
manipulation of several genes or favorable alleles at QTL when they are
originally present in multiple parents (Hospital 2003). The final objective
would be to construct an ideal genotype by accumulating all the favorable
alleles at these loci in a single line. This strategy involves several initial
crosses between the parents. For example, Hospital (2003) proposed a two-
step procedure for the combination of four genes ( G1 to G4 ) into a single line.
In the first step, two lines that are homozygous for two target genes ( G1/G2
versus G3/G4 ) are developed by crossing pairs of lines (L1 x L2 versus L3 x
L4), followed by selection on the basis of linked markers of homozygotes
among F 2 or RIL progeny. In the second step, such individuals are crossed to
produce individuals that are homozygous at all target genes.
Gene pyramiding has been proposed as a useful approach to increase
the durability of resistance to pest and diseases, or to increase the level of
abiotic stress tolerance. Phenotyping assays have been used to pyramid
downy mildew and Sclerotinia stem rot resistance genes in sunflower
(Tourvieille de Labrouhe et al. 2004; Feng et al. 2007). For downy mildew
control, the preliminary results reported by Tourvieille de Labrouhe et al.
(2004) comparing different methods for obtaining durable resistance
indicated that gene pyramids were less effective in reducing the appearance
of new races compared to other control methods such as the use of
combinations of resistance Pl genes by alternation or in “multi-hybrids”.
Vear (2004) suggested that in order to increase the durability of these major
genes, they needed to be backed up by quantitative, non-race specific
resistance QTL. Markers will be essential for this strategy because it would
be impossible to select for these QTLs phenotypically when they are
combined with major resistance genes. Additionally, molecular markers
will be very useful for pyramiding tightly-linked resistance genes within
the same RGC cluster (Slabaugh et al. 2003). For partial resistances such as
Sclerotinia and Phomopsis, a very important step towards the improvement
of the levels of resistance is the use of MAS to combine different resistance
QTL. Studies reporting the identification, validation, and fine-mapping of
resistance QTL, as described in previous sections, will contribute to the use
of MAS for resistance QTL pyramiding.
7.6 Molecular Breeding in Private Sector Breeding Programs
Until relatively recently, sunflower had lagged behind other economically
important crops both in terms of the available molecular tools and its
genomic resources. The first linkage maps were based on RFLP markers
and these were generated by several private companies and/or consortia,