281more vulnerable than varieties having genetically different backgrounds. Breeders
should have a stock of resistance; may it even be in the gene banks only. To increase
the genetic diversity of any crop species rather broaden the crop gene pool is to take
the advantage of available genetic resources present in nature, the easiest way to
exploit are the landraces comparatively and the wild relatives.
Variation is the basis for improvement, so we need genetic variation for progress
in crop breeding. Currently there is not enough variation to break the yield barrier
or the traits/genes have not been discovered yet. Genetic variation in crops can be
created and enriched either by gene transformation, or by exploiting variation in the
wild wheat relatives through inter specific and inter-generic crosses. Wild relatives
of species often the progenitors of that species (Hajjar and Hodgkin 2007 ). Plant
breeders have been continuously exploiting the wild wheat relatives dominantly for
biotic stresses (Hajjar and Hodgkin 2007 ).
Many useful sources of disease and pest resistance are available in the wild rela-
tives of wheat (Zaharieva et al. 2001 ). Mujeeb-Kazi and Kimber ( 1985 ) have
reported the usefulness of wild relatives of wheat for wheat improvement. Among
the wild species of wheat considerable genetic diversity has been found in Ae. taus-
chii and T. turgidum (Xie and Nevo 2008 , Sohail et al. 2012 ). The wild relatives of
wheat have adapted to various environmentally harsh conditions for thousands of
years, there has been natural selection and only the fittest have survived. There are
a total of the 325 wheat wild relatives out of which around 250 are perennials and
75 are annual grasses of the tribe Triticeae (Dewey 1984 ). Very few have been uti-
lized for hybridization with wheat (Mujeeb-Kazi et al. 1996 ). Although many of the
wild relatives of wheat have not been hybridized with wheat, but have been used to
transfer useful genes to wheat.
Gene transfer from wild species to cultivated crops have been reported by many
researchers; such as resistance to stem rust and loose smut from Triticum tauschii,
Hessian fly resistance from T. timopheevi, T. monoccun, and T. turgidum (McFadden
1930 ). Goodman et al. ( 1987 ) summarized the transfer of important traits and genes
for different crops including bread wheat. He reported that high kernel protein from
Aegilops ovata, stem rust resistance from Ae. speltiods, T. timopheevi, T. monoccun,
and T. turgidum, leaf rust resistance from Ae. squarrosa, Ae. umbellulata and
Agrophyron elongatum, yellow rust resistance, powdery mildew resistance, winter
hardiness, leaf rust resistance, stem rust resistance from Secale cereale have been
transferred to bread wheat. Resistance to stem rust, powdery mildew and green bug
have been transferred from Secale cereale to bread wheat (Friebe et al. 1996 ;
Graybosch, 2001 ; Yediay et al. 2010 ).
Nevo et al. ( 2002 ) reported a high natural variation of traits in the population of
emmer wheat (Triticum dicoccoides). The Many of these traits could be useful
sources for wheat improvement to stresses such as salinity (Nevo et al. 1992 ),
drought (Peleg et al. 2005 ), powdery mildew (Moseman et al. 1984 ), leaf rust, stem
rust, stripe rust and tan spot (Pyrenophora tritici-repentis) (Chu et al. 2008 ).
The genetic diversity within the D genome of Ae. tauschii (Naghavi and Mardi
2010 ), which is much higher than within that of wheat (Reif et al. 2005 ). The D
Breeding and Genetic Enhancement of Dryland Crops