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(Singh et al. 1989 ; Pukhalskiy and Bilinskaya 1998 ; Pukhalskiy et al. 2000 ).
Recently, studies have been conducted for trying to fine-map these loci (Chu et al.
2006 ) in order to develop diagnostic markers to select the most suitable modern
cultivar to cross with SHW avoiding the risks of hybrid necrosis.
In spite of the successful introgression of new alleles from wild relative back-
grounds through SHW or SDW, increasing breeder’s ability to more effectively
identify the best combinations remains a main objective to increase breeding effi-
ciency. For instance, studies have been carried out trying to identify biotic and abi-
otic resistance traits in Aegilops tauschii to increase the chances of expressing them
into SHW. The results of these studies showed there is no apparent relationship
between Ae. tauschii performance and physiological traits and their expression in
the synthetic lines obtained (Sohail et al. 2011 ). In fact, several major rust resistance
genes have been identified in Ae. tauschii (Villareal et al. 1992 ; Aguilar-Rincón
et al. 2000 ) but in some cases, the SHW developed with the Ae. tauschii lines
showed reduced or no resistance (Assefa and Fehrmann 2000 ). This indicates the
importance of assessing the mechanisms of interaction between the three genomes
to more effectively introgress genes of interest from wheat wild relatives.
Finally, one of the most important traits for wheat market value, end-use quality,
may be negatively affected when introgressing genomic regions from wild relatives.
In spite of the number of potential new quality alleles of interest already described
in SHW lines (Pflüger et al. 2001 ; Davies et al. 2006 ;), suitable wheat quality traits
for modern bread-making practices are not often linked to agronomic performance.
Since most of the suitable quality alleles identified and fixed in the modern varieties
are a result of artificial selection and do not provide actual fitness improvements,
natural selection may have negative or no effect on them. Therefore, a number of the
SHW selected on the base of other traits of interest will not carry suitable quality
alleles needing several backcrossing and selection events to select suitable lines
with acceptable quality. For instance, a higher frequency of soft grains has been
reported (Lillemo et al. 2006 ) in SHW as compared to modern lines indicating the
need of having this trait in mind when developing SDW. An analogous case would
be the 1BL/1RS wheat rye translocation. This translocation, as it was stated before,
introduced in the wheat genome several alleles of superior agronomic performance
and lines carrying it were widely used as parents in worldwide crossing blocks.
However, as a side effect, it also had a negative effect in wheat end-use quality,
mostly regarding the modern bread-making techniques (Pena et al. 1990 ).
7 Breeding Strategies
Plant breeding can be broadly defined as alterations caused in plants as a result of
their use by humans, ranging from unintentional changes resulting from the advent
of agriculture to the application of molecular tools for precision breeding. The core
of plant breeding is the selection of better types among variants, in terms of yield
and quality of edible parts; ease of cultivation, harvest, and processing; tolerance to
environmental stresses; and resistance to diseases and pests. Each of these aspects
of agronomic or food value can be dissected in many specific traits, each presenting
Q. Sohail et al.