Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 236

and QTL x environment (QTL x E) interactions can all contribute to biasing
the estimation of QTL effects and position, thus reducing the likelihood of a
successful outcome in a MAS program. QTL validation in independent
samples and in different genetic backgrounds and environments is, therefore,
necessary before using marker-QTL association in MAS programs. In
sunflower, QTL for Sclerotinia resistance have been validated across
environments (Bert et al. 2002), generations (Micic et al. 2005a; Vear et al.
2008), and genetic backgrounds (Rönicke et al. 2005; Micic et al. 2005b).
Micic et al. (2004, 2005a) and Vear et al. (2008) consistently identified QTLs
for Sclerotinia resistance on LG 8 and LG 16 (stem lesion), and on LG 1 and
LG 10 (capitulum lesion), respectively, across early F 3 and later F 6 (RIL
population) generations obtained from a cross involving the resistant lines
NDBLOS sel (Micic et al. 2005a) or PSC8 (Vear et al. 2008). The QTL on LG 8
was also detected in another mapping population developed from a different
resistant source (TUB-5-3234) (Micic et al. 2005b). In addition, Micic et al.
(2005b) cross-referenced previous studies of Mestries et al. (1998), Bert et al.
(2002), and Rönicke et al. (2005), and found that the same six linkage groups
(LG 1, 4, 8, 9, 10, and 13) carried QTLs for resistance against Sclerotinia
sclerotiorum in more than one of the five mapping populations considered.
However, the most important resistance QTLs appear to be on LG 1, LG 9,
and LG 10, which had a significant effect in at least three of the five
populations. For oil content, QTLs have also been validated across
generations, environments, and mapping populations. Oil content in
sunflower depends on both the percentage of kernel weight in relation to
whole achene weight and on the concentration of oil in the kernel. Mestries
et al. (1998) identified at least two QTLs for seed (achene) oil content
consistently in F 2 though F 4 generations from the cross GH x PAC2, whereas
Bert et al. (2003) detected four QTLs consistently in both F 2 and F 3 generations
from the cross between the lines PSC8 and XRQ. Both studies also reported
that one of the QTLs associated with oil content co-located with the B gene
determining apical branching. Leon et al. (2003) validated five QTLs for oil
content in the population ZENB8 x HA89, detected previously by Leon et al.
(1995a), through the evaluation of replicated progenies in four locations
within a target environment. Four of the QTLs were detected in at least two
locations. The QTL with the largest effect, which co-located with the hyp
gene that determines achene hypodermis color, was detected in all four
locations. QTLs for seed oil content centered on the B and hyp loci were also
identified by Tang et al. (2006a) in LG 10 and 16, respectively, in a different
mapping population developed from the cross RHA280 x RHA801.
QTL effects may also be environmentally sensitive. Variation in
expression due to QTL x E interaction remains a major constraint to the
discovery of QTL that will confer a consistent advantage across a wide
range of environments; however, if a QTL shows QTL x E interaction then