108 I. M. Ahmed et al.
(Wang and Chee 2010 ). One of the breeding strategies to overcome the problem of
linkage drag associated with wild genotypes during breeding programs is AB-QTL
analysis, which combines QTL detection with the introduction of favorable alleles
into the targeted variety (Tanksley and Nelson 1996 ). In barley, AB-QTL analysis
was first reported by Pillen et al. ( 2003 ) using a BC 2 F 2 population developed be-
tween the cultivar Apex and the wild accession ISR101-23 for various agronomic
and malting quality traits. Some of the other studies for improving drought toler-
ance in barley include Baum et al. ( 2003 ), Ceccarelli et al. ( 2004 ), Forster et al.
( 1997 ), Grando et al. ( 2001 ), and Ivandic et al. ( 2003 ).
Wild barley H. spontaneum has been recognized as an important source for
drought tolerance. A QTL identified on chromosome 4H from H. spontaneum con-
sistently increased grain yield across six test environments with an average yield
increase of 7.7 % (Pillen et al. 2003 ). Talame et al. ( 2004 ) identified two QTLs
on chromosomes 2H and 5H with relative yield increase ranging from 12 to 22 %
under dry conditions. These QTLs could be used as target chromosome regions for
the integration of wild barley genes for yield improvement under drought. Lu et al.
( 1999 ) suggested that drought tolerance in wild barley is related to their differ-
ing genetic abilities of osmotic adjustment under drought conditions. Thus, further
genetic mapping and marker-assisted transfer of the osmotic-adjustment genes har-
bored in the wild progenitor could improve resistance of cultivated barley grown in
water-limited environments.
Traditional QTL mapping or biparental QTL mapping based on a single segre-
gating population derived from two homozygous parental genotypes has been the
commonly used approach for genetic dissection of salt tolerance in barley and to
identify candidate genes (Mano and Takeda 1997 ; Xue et al. 2009 ; Ellis et al. 2002 ;
Witzel et al. 2009 ). This approach provides valuable information on genomic re-
gions that control quantitative traits but it also has limitations due to poor sampling
of the allelic variation present in the barley gene pool for each of the loci affect-
ing salt tolerance, lack of segregation, and poor resolution of this type of mapping
QTLs. Mano and Takeda ( 1997 ) identified QTLs controlling salt tolerance at ger-
mination and the seedling stage in barley by interval mapping analysis using marker
information from two doubled haploid (DH) populations derived from the crosses,
Steptoe × Morex, and Harrington × TR306. The results revealed that the QTLs
for salt tolerance at germination in the DH lines of Steptoe x Morex were located
on chromosomes 4H, 6H, and 5H, and in the DH lines of Harrington/TR306 on
chromosomes 1H and 5H. In both DH populations, the most effective QTLs were
found at different loci on chromosome 5H. Genetic linkage between salt tolerance
at germination and ABA response was found from QTL mapping. The QTLs for
the most effective ABA response at germination were located very close to those
for salt tolerance on chromosome 5H in both crosses. The QTLs for salt tolerance
at the seedling stage were located on chromosomes 2H, 1H, 6H, and 5H in the DH
lines of Steptoe x Morex, and on chromosome 5H in the DH lines of Harrington x
TR306. Their positions were different from those of QTLs controlling salt tolerance
at germination, indicating that salt tolerance at germination and at the seedling stage
was controlled by different loci.