Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 237

the selection of genotypes adapted to specific environments may be possible.
In sunflower, Leon et al. (2001) identified QTLs for days from seedling
emergence to flowering (DTF) associated with photoperiod (PP) response in
a population evaluated in six environments (locations, years, and sowing
dates) and they explicitly included a QTL x E interaction in the interval-
mapping model. These authors demonstrated that the two QTLs with the
strongest effect on DTF, identified across environments, also showed a highly
significant QTL x E interaction and were responsive to PP. Identifying these
QTLs could potentially allow the conversion of photoperiod-sensitive
germplasm to photoperiod insensitive.
Epistasis is another factor that must be considered during validation
studies. It causes the allelic effects of one QTL to be dependant on the
genotype at a different locus (Holland 2001). Tang et al. (2006a) provided an
example of complex interactions among QTLs associated with seed oil
concentration in sunflower, which could result from complex networks of
the underlying genes. However, these epistatically interacting QTLs for seed
oil content were not previously identified in other genetic analyses in
sunflower (Leon et al. 1995a, 2003; Mestries et al. 1998; Mokrani et al. 2002;
Bert et al. 2003). Studies in Arabidopsis suggest that epistatic QTLs are more
important than additive QTLs for fitness traits (Malmberg et al. 2005). In
contrast, studies designed to explicitly model epistatic interactions in maize
revealed that epistasis was of little or only moderate importance for
quantitative traits (Mihaljevic et al. 2005; Blanc et al. 2006). Although the
relative importance of epistasis is still under debate, it has sometimes reduced
the predicted gains from MAS (Holland 2001) and so its effect should clearly
be evaluated for MAS programs.
Where possible, mapping of QTL with a high level of resolution will
enable the identification of more tightly linked markers and the manipulation,
via MAS, of a smaller chromosomal segment carrying the QTL of interest.
Development of the high-density sunflower genetic map (0.8 cM/locus)
through the mapping of 2,495 high-throughput DNA marker loci (Knapp et
al. 2007) will contribute towards this objective. Availability of specific genetic
resources such as near-isogenic lines (NILs) differing in a genomic segment
containing a target QTL (QTL-NILs), advanced segregating populations
such as RILs, or RIL testcross populations (TC-RILs) will also help to estimate
with a greater accuracy QTL positions and their effects. In sunflower, Micic
et al. (2005a) re-estimated the position and effect of a number of QTLs for
Sclerotinia resistance in an RIL population developed from F 3 families in
which the QTLs were originally identified. However, not all the QTLs
identified in the F 3 were found in the RIL population. Pizarro et al. (2006)
developed QTL-NILs varying for target QTLs for seed oil concentration by
backcrossing a donor parent (a wild H. annuus ) to a recurrent parent (elite
high oil cultivar) combined with MAS. Comparing the phenotypes of the