Agriculture Reference
In-Depth Information
can be applied to early segregating generations, it
does not require elaborate sprouting-test facili-
ties, and is independent of the environment.
Wheat end-use quality consists of a group of
complex traits which are largely quantitatively
inherited. Further, growing environments usually
have an important infl uence on the end-use
quality phenotype. McCartney et al. (2006)
mapped QTLs for 41 end-use quality traits in the
population RL4452 × 'AC Domain'. In this study,
99 QTLs were reported on 18 chromosomes,
although 44 QTLs mapped to 3 QTL clusters on
chromosomes 1B, 4D, and 7D. On chromosome
1B, 14 QTLs mapped close to the
Glu-B1
locus.
The important role that glutenin storage proteins
play in end-use quality has been long established
(see review by Bushuk 1998) and validated by
others (Kuchel et al., 2006; Groos et al. 2007).
Twenty QTLs mapped close to a plant-height
QTL, likely the
Rht-D1
gene, and 10 QTLs
mapped close to a QTL for maturity on 7D, thus
demonstrating the important interactions between
genes controlling agronomic traits and end-use
quality. Major QTLs for protein content, water
absorption, and dough strength parameters were
also mapped. Noodle quality traits were mapped
to 5B and 5D, and the largest QTL for b* value
(yellow pigment color) mapped to 7AL, close to
the location of the phytoene synthase (
Psy-1
) gene
as reported by He et al. (2008). Groos et al. (2007)
reported seven QTLs for breadmaking quality
scores on eight different chromosomes and that
breadmaking QTLS were generally coincident
QTLs for dough rheology, protein content, or
fl our viscosity, indicating the interrelationship
between quality traits.
Selection for end-use quality is normally con-
ducted in later generations in a wheat breeding
program because of limited grain supply and the
cost of rheological and baking tests. Thus, MAS
for end-use quality at earlier stages of the breed-
ing process would be highly desirable. Marker-
assisted selection of specifi c glutenin subunits has
been used for some time with the intent to select
for dough strength (Radovanovic and Cloutier
2003). However, there has been limited use of
markers for more complex quality traits, possibly
because the relationship between QTLs and
end-use quality has been unclear. Recent mapping
efforts (Kuchel et al., 2006; McCartney et al.,
2006; Groos et al., 2007) should provide valuable
candidate QTLs for use in molecular breeding of
end-use quality.
FUTURE DEVELOPMENTS AND USES OF
QTL ANALYSIS AND MAPPING
Many researchers would agree that SSR markers
will continue to provide useful tools and insight
into the organization of the wheat genome and be
useful for resolving QTLs. It is reasonable to
assume that SNP markers covering the whole
genome is a technology that will evolve and, when
coupled with an inexpensive, highly multiplexed
detection platform, could overtake SSRs as the
most applied marker system. The advantage of
SNPs is the relative abundance compared to
SSRs, but this is balanced against the level of
polymorphism, which is lower than SSRs (Somers
et al., 2003). Currently SSR genotyping costs
range from $0.25 to $0.50 per datapoint (labor
cost excluded), depending on the level of multi-
plexing, source of consumables, and detection
platform. Single nucleotide polymorphism geno-
typing via the Illumina bead array system can
reduce this cost to approximately $0.05 per data-
point and can dramatically increase the through-
put with multiplex levels of 1,536 datapoints
per run.
Association mapping
Many research groups in wheat and other crop
species are turning their attention toward associa-
tion mapping (Flint-Garcia et al., 2003; Rafalski
and Morgante 2004). This is a natural extension
of chromosome interval haplotype analysis, the
difference being association analysis may be per-
formed as a full genome analysis. To accomplish
association analysis in wheat, the degree of linkage
disequilibrium (LD) is fi rst estimated. A mea-
surement of LD will predict the marker density
required across the genome to accommodate asso-
ciation mapping. It is estimated that LD decay in
wheat occurs over 2-5 cM (Chao et al., 2007;
