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translocation, also novel rust resistance genes were incorporated into the wheat
genepool (McIntosh et al. 1995 ). Perennial ryegrass variety currently being devel-
oped by pastoral genomics for improved drought tolerance by inserting H+ – pyro-
phosphatase gene isolated from ryegrass (Holme et al. 2013 ).
Several suitable alleles have been identified in SHW and wild relatives and have
been or are being introgressed in modern germplasm. The most successful intro-
gressions of wild relative alleles into released varieties are related to pest and dis-
ease resistance (Hajjar and Hodgkin 2007 ). New leaf and stem rust resistance alleles
have been identified in synthetic wheats (Villareal et al. 1992 ; Aguilar-Rincón et al.
2000 ) and some of them have already been introgressed in modern released varieties
(Hoisington et al. 1999 ). New tolerance alleles for septoria, tan spot, karnal bunt,
fusarium, powdery mildew, Hessian fly, and other pests and diseases have also been
identified in SHW (Trethowan and Van Ginkel 2009 ).
Major genes controlling qualitative traits – for instance most of the disease resis-
tance genes – are more easily introgressed into modern varieties due to their easier
genetic control and expression. However, quantitative traits with polygenic control
can also be introgressed in modern backgrounds from wild relatives. For instance,
grain protein content QTL was successfully introduced in a modern durum wheat
background from its diccocoides relative resulting in an average increase of 15 g
kg−^1 in protein content (Chee et al. 2001 ).
Finally, primary synthetics tend to be better adapted than modern cultivars to
extreme abiotic stress conditions (Trethowan and Van Ginkel 2009 ). The potential
of the wild relatives to provide new sources of adaptation, yield potential and dis-
ease resistance to modern wheat varieties is confirmed by the high proportion of
genes from wild relatives in modern wheat backgrounds. About 20 % of the new
CIMMYT and up to 24 % of ICARDA material include synthetic background
(Ogbonnaya et al. 2013 ).
6 Gene Flow Constraints in Plant Breeding
Wheat and rice are the main staple crop in the world while legumes are among the
main sources of proteins in the most of developing countries and contributes 33 %
human dietary nitrogen requirements. However, to continue feeding the increasing
population wheat production has to be increased at least 1.6 % per year (Calderini
et al. 1999 ). Crop landraces and wild relatives have the potential of providing supe-
rior alleles that can help in increasing yields (Reif et al. 2005 ). However, some chal-
lenges have to be overcome by the breeders and pre-breeders to be able to fully and
efficiently identify useful alleles and make them available for breeding programs.
The main constraint for gene flow in breeding nowadays is the loss of genetic
diversity among the modern wheat varieties. Modern lines already adapted to
today’s cultural practices and ensuring the quality requirements of the wide range of
end-uses define the fastest and easiest way to introduce new alleles of interest in a
breeding germplasm without important undesirable linked alleles. However, the
Q. Sohail et al.