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have helped to develop products through the
pyramiding of large-effect QTLs for grain yield
under drought. Efforts are also being made to
pyramid these QTLs along with QTLs affecting
other biotic and abiotic stresses, for the develop-
ment of NILs with multiple stress tolerance.
tunities for keeping grain quality traits of the
varieties intact, which is essential for retaining
favor with farmers. NILs developed with such
high background recovery can be disseminated
and adopted quickly because of their similarity
to the popular recipient parent. Introgression of
the large-effect QTL
qDTY
12.1
into recipient par-
ent Vandana was undertaken at the IRRI. Large
BC
2
F
2
and BC
3
F
2
populations segregating for
the QTL were screened with foreground and
background markers, and NILs with high back-
ground recovery were identified. The highest
yielding NILs showed a more than 4-fold aver-
age advantage over the recipient Vandana under
six upland drought experiments with varying
degrees of stress. Under severe drought stress,
qDTY
12.1
-introgressed Vandana lines showed a
yield advantage of 0.5 t ha
−
1
.
Marker-AssistedBackcrossing
The identification and transfer of large-effect
QTLs through marker-assisted backcrossing
(MAB) is suggested as a rapid approach for
developing tolerant versions of popular rice vari-
eties (Bernier et al. 2007). The use of molec-
ular markers permits the genetic dissection of
the progeny in each generation and increases the
speed of the selection process, thus increasing
genetic gain per unit time (Tanksley et al. 1989;
Hospital 2003). It also provides an efficient fore-
ground selection for the target locus, background
recovery for the recurrent parent genome, min-
imization of linkage drag surrounding the locus
being introgressed, and rapid breeding of new
genotypes with favorable traits (Neeraja et al.
2007). It has also been reported that screening
the populations with a few well-placed mark-
ers (two to four markers on a chromosome of
100 cM) provides adequate genome coverage as
observed in previous backcross programs (Viss-
cher et al. 1996: Servin and Hospital 2002). Such
targeted introgression leads to the development
of uniform lines that are similar to the recipi-
ent parent, with improved drought tolerance. In
the case of drought, it is important that the lines
developed at each stage of the backcross program
be tested under drought. This not only provides
additional data bearing on the effect of QTLs
in the backcross generation but also allows the
identification of the best NILs for further back-
crossing. With the few high-yielding varieties
such as IR64, IR36, MTU1010, Swarna, BR11,
and TDK 1 being highly popular among farmers
in drought-prone rain-fed areas, MAB introgres-
sion of the major drought-yield QTLs appears to
be a preferred strategy for improving such pop-
ular varieties. MAB also provides better oppor-
PyramidingofDroughtQTLs
Each of the individual drought-yield QTLs
shows a yield advantage of 300-500 kg ha
−
1
under moderate to severe drought conditions.
This yield advantage may look significant from
an academic point of view, but it is not high
enough to attract the attention of farmers and
motivate them to replace their currently culti-
vated varieties with newly developed varieties
with a single drought yield QTL. In order to pro-
vide farmers with economic yield advantages of
1.0 t ha
−
1
or more, it is necessary for two to
three or more drought yield QTLs to be pyra-
mided together. It is suggested that pyramid-
ing of more than one QTL from the same or
different donors working in the same genetic
background could be a good strategy for increas-
ing yield under drought to the maximum pos-
sible extent. Recently experiments have proved
that a combination of two to three drought-
yield QTLs has given a higher advantage than
a single QTL in IR64 (data not presented).
IR64 lines with two and three pyramided QTLs
showed a yield advantage of 1.2-1.5 t ha
−
1
over
IR64, under moderate to severe drought condi-
tions, while maintaining similar yield potential
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