Biology Reference
In-Depth Information
situations under both upland (Bernier et al. 2007)
and lowland conditions (Venuprasad et al. 2009;
Vikram et al. 2011). In rice, mapping and breed-
ing studies are increasingly using direct selec-
tion for grain yield as a criterion for identifying
major QTLs for grain yield under drought and to
improve grain yield under drought through con-
ventional breeding approaches (Verulkar et al.
2010; Mandal et al. 2010) or in marker-assisted
breeding (IRRI unpublished) targeted to improv-
ing the grain yield of popular high-yielding but
drought-susceptible varieties IR64, Swarna, and
MTU1010. In Brazil and Thailand also, direct
selection for yield has been successfully applied
in breeding for drought tolerance in upland and
rain-fed lowland conditions (Ouk et al. 2006).
grounds that enable the study to show high val-
ues of QTL effects under drought. For exam-
ple, some studies targeting the identification of
QTLs in upland drought-prone ecosystems have
used lowland varieties as recipients and screened
the population in upland drought situations. The
low adaptability of the lowland recipient vari-
ety resulted in large effects of the QTLs under
upland conditions. However, a desirable QTL
allele discovered in non-elite genetic material
and showing a large effect may not offer any
improvement in the target genetic background
because the allele may already be ubiquitous in
current varieties (Collins et al. 2008). The lack of
repeatability of QTL effects across different pop-
ulations (QTL
genetic background) and across
environments (QTL
×
environmental interac-
tion) has been another factor limiting the use of
QTLs in molecular breeding (Price et al. 2002;
Courtois et al. 2003, Lafitte et al. 2004; Bernier
et al. 2008), and this necessitates that donor
and recipient varieties be selected with appropri-
ate consideration. The recipient variety for QTL
studies should be an improved, high-yielding,
and popular variety in the drought-prone envi-
ronment. The selection of the drought-tolerant
donor should depend upon the duration of the
recipient variety. Most of the drought-tolerant
donors are of early duration and are likely to
reduce the duration of the recipient variety. Due
consideration for growth duration, in addition to
drought tolerance, as well as resistance to insects
and disease, should be given when selecting a
donor. Overall, complementation of the recipi-
ent and donor for different abiotic, biotic, and
grain quality traits will enable the improvement
of lines for more than one trait while improving
yield under drought stress.
A wide range of population structures can be
used for QTL mapping, the minimal require-
ment being the establishment of linkage dise-
quilibrium between defined genotypes (Peter-
son 2002). Plants that tolerate inbreeding can
accommodate especially great latitude for dif-
ferent population structures (Peterson 2002).
a feature that makes self-pollinated species
such as rice highly suitable for QTL mapping.
×
Identification of Large-Effect QTLs for
Grain Yield under Drought
The basic requirements of QTL mapping for
high grain yield under drought are the same as
in any other mapping study. However, the com-
plexity of genetic control for grain yield under
drought demands highly controlled field experi-
ments with uniform stress treatments. Similarly,
highly controlled non-stress control experiments
are needed for the assessment of yield potential,
plant type, grain type, flowering, and so forth.
This kind of experimentation helps in the identi-
fication of suitable lines from the mapping pop-
ulation combining high yield, drought tolerance,
and good plant type for marker-assisted back-
cross programs.
DevelopmentofMappingPopulations:
IdentificationofDonorsandRecipientVariety
The objective of any QTL identification program
should be the ultimate use of the identified QTLs
for improving grain yield under drought through
marker-assisted introgression of the identified
QTLs into an identified improved variety. How-
ever, most QTL identification studies on grain
yield as well as secondary traits have focused
more on developing mapping populations in very
low yielding, highly drought susceptible back-
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