Basic HTML Version
Triticeae Genomics and Genetics 2012, Vol.3, No.2, 9
-
24
http://tgg.sophiapublisher.com
14
together included QTLs for both PHST and dormancy.
In contrast, MQTL 5 at 96.41 cM was based on three
original QTLs for three original traits (PHST, dormancy
and GC). The results involving MQTLs mapped on
different individual chromosomes are described below
in greater details.
1.5.1 Seven MQTLs on chromosomes 3A, 3B and 3D
On chromosome 3A, 11 original QTLs were projected,
but only 8 of these 11 QTLs clustered as 2 MQTLs
(MQTL 1 and 2), positioned at 18.46 cM and 96.48 cM.
None of the remaining three original QTLs (58.1 cM,
81.03 cM and 119.49 cM) clustered with any other
QTL; these were, therefore, excluded from meta-QTL
analysis. Each of the two MQTL was based on four
original QTLs from 6 different studies, involving
PHST, GC and dormancy QTLs. In case of MQTL 1,
three of the four original QTLs belonged to seed
dormancy, suggesting that this locus may have an
important effect on seed dormancy in bread wheat.
The other MQTL (MQTL2) positioned at 96.48 cM
may control two associated traits (PHST and GC),
since the original QTLs for these two associated traits
clustered together to generate this MQTL.
On chromosome 3B, 8 of the 10 original QTLs were
found to represent 3 MQTLs, which were designated
as MQTL 3, 4 and 5 and positioned at 68.93 cM,
85.27 cM and 96.41 cM. MQTL 3 included two original
seed dormancy QTLs, while MQTL 4 included three
original QTLs for PHST. As mentioned above, MQTL
5 was based on three original QTLs for three traits
(PHST, dormancy and GC) from two separate studies.
On chromosome 3D, 9 of the 14 original QTLs repre-
sented only 2 MQTLs (MQTL 6 and 7) that were
mapped at 29.36 cM and 43.71 cM. MQTL 6 was
based on 3 original QTLs from two individual studies,
while MQTL 7 with a narrow window of 0.49 cM
involved clustering of 6 original QTLs (4 for PHST
and 2 for dormancy) from three different studies.
1.5.2 A solitary MQTL on chromosome 4A
A solitary MQTL designated as MQTL 8 was mapped
at 75.75 cM with a narrow CI of 0.40 cM on chromo-
some 4A, and involved 11 original QTLs (7 QTL for
PHST and 4 QTL for dormancy).
1.6 Confidence intervals (CIs) in meta-QTLs and
original QTLs
The confidence intervals for MQTLs were narrow
(mean: 2.67 cM; range 0.40 to 6.55 cM) than those for
the original QTLs (mean: 23.56 cM; range- 10.0 to
53.6 cM; for details see Table 2). MQTLs with the
most precise and narrow CI of 0.40, 0.49, 2.32, 2.36
and 2.61 were located on chromosomes 3B, 3D and
4A. With respect to the reduction in size from mean
initial CI to MQTL CI, the gain in accuracy in terms
of coefficient of reduction (Mean initial CI/MQTL CI)
ranged from 2.7 (3A; MQTL 1) to 55.3 (3D; MQTL 7).
An average decrease in CI ranged from 19.6 cM to 5.0
cM on chromosome 3A, from 32.5 cM to 2.43 cM on
chromosome 3B, from 18.5 to 2.0 cM for chromosome
3D and from 14.88 cM to 0.40 cM for chromosome
4A. On an average, meta-QTL analysis led to
reduction in the size of CI by a factor of 8.8 and thus
increased the precision of QTL mapping.
2 Discussion
2.1 Markers for MAS based on MQTLs
It may be recalled that during the present study 36
QTLs were condensed into 8 MQTLs. Each of these 8
MQTL had a relatively narrow confidence interval
(CI), thus providing markers that are more closely
associated with the corresponding MQTL. Since these
MQTLs are based on different original QTLs reported
in different environments, it is also suggested that
MQTLs may be more stable across different environ-
ments and are therefore more suitable not only for
MAS, but also for searching candidate genes or for
map-based cloning. It should be noted that at least one
of the flanking SSR markers for each of the eight
meta-QTLs was also reported as the closest marker to
QTLs reported in one or more of the initial studies.
For instance, MQTL 1 (chromosome 3A) was located
within the same marker interval (Table 2), to which
one of the 3A QTL reported earlier belonged (Liu et
al., 2008; 2010). Similarly, MQTL 3 (chromo- some
3B) was located in the same interval (Table 2), in
which one of the 3B QTL for dormancy was reported
earlier (Mares et al., 2009).
2.2 Homoeology between MQTL on 3A, 3B and 3D
Since three of the four chromosomes used for meta-
QTL analysis belonged to the same homoeologous
Triticeae Genomics and Geneti s Provisional Publishing