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Plant Gene and Trait 2012, Vol.3, No.6, 28
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Table 2 QTLs and their effects for easy curing potential in flue-cured tobacco
QTL Name Linkage group Position (cM) Interval
LOD value Additive effect Dominant effect Variance (%)
qECP
-
1
-
1
c1
40.01
Tp179
-
Thdp023 2.43
0.327 6
-
2.537 6
9.26
qECP
-
8
-
1
c8
11.91
Sca87
-
Tp164b
3.17
0.883 1
-
2.236 8
8.87
qECP
-
8
-
2
c8
14.81
Tp069
-
Sca602
3.22
0.963 4
-
2.008 1
7.57
qECP
-
9
-
1
c9
0.100 0
Sca901
-
Tep1089 2.04
0.091 24
-
2.358 3
7.83
and the genetic distance up to the Sca901 is 0.1 cM.
Four QTLs have positive additive effects that indi-
cated the synergistic additive effect for improving
easy curing potential comes from Yunyan85, the
additive effect of
qECP
-
8
-
2
should be the maximum
value of 0.963 4; whereas the dominance effects of
the four QTLs were negative, which implied that the
four QTL were negative roles to the dominant gene
expression. The explained phenotypic variation of the
four QTLs were 9.26%, 8.87%, 7.57% and 7.83%,
respectively, of which
qECP
-
1
-
1
is the largest and
qECP
-
8
-
2
is the smallest.
2 Discussions
Constructing molecular linkage map is the basis for
mapping quantitative trait and molecular marker-
assisted selection breeding. Tobacco is not plentiful of
the polymorphism of molecular markers resulting
from the narrow genetic background. Therefore, there
is few number of molecular genetic map having been
constructed, and most of the genetic maps were
constructed by using the markers with poor stability
and low polymorphism, such as RAPD, RFLP and
other markers. In this study, SSR marker was
employed that have many advantages of co-dominant
inheritance, plentiful polymorphisms, easy to handle,
good repeatability, abundance and even distribution in
the genome. Although the constructed map of the
flue-cured tobacco only contains 17 linkage groups
and limited markers, this map can be further used as
frame map for saturating this map by development of
new SSR markers.
There is little study on QTL mapping for the important
traits in flue-cured tobacco. Easy curing potential is
one of the important traits in tobacco; it is the only
way to breed the good varieties with fine traits of easy
curing potential can produce high-quality tobacco with
plentiful flavor and yellow color to meet the needs of
the cigarette producers and consumers. QTL mapping
for the easy curing potential in flue-cured tobacco
would facilitate the genetic improvement of these
traits, but it hasn’t yet been reported on gene mapping
related to easy cuing potential. In the present study,
we adopted the yellowing index as the indicator of
easy curing potential in flue-cured tobacco to map the
gene linked to the trait and detect four QTLs related
to easy cuing potential by using composite interval
mapping analysis, sharing the contribution rate of
9.26%, 8.87%, 7.57% and 7.83%, respectively. This
would be the important clues to in-depth understan-
ding the genetic basis of the flue-cured tobacco, which
will be helpful to fine QTL mapping of easy curing
potential.
In general, the gene effect controlling quantitative trait
is considered to be similar, and same directional, as
well as the number of genes are considered to be
multiple, this assumption would be necessary for
traditional genetic analysis but it is an idealized model.
In fact, the expression of the quantitative trait is the
result of interaction of many QTL with the different
effects and different directions (Zhang et al., 2004). In
this study four QTLs with positive additive effects
would be conducive to pyramid the favorable genes to
obtain new hybrid combination with atavistic and
transgressive inheritance.
At present, the vast majority of studies on quantitative
trait still remain on some basic aspects such as the
marker identification, positioning, and mapping. The
expectation is still not yet to be realized to improve
breeding efficiency and develop lines or varieties at
large-scale by using marker-assisted selection. The
reason might be lack of the integration between
marker identification and assisted breeding in previous
studies on these two important aspects (Fang et al.,
2002). In this study, there were two QTLs tightly
linked with molecular markers, the QTL
qECP
-
8
-
2
located on linkage group 8, which closely linked to