263rate, and intercellular carbon dioxide are associated with drought tolerance (Sohail
et al. 2011 ). On the other hand, wild relatives, synthetic hexaploid wheat (SHW),
synthetically derived wheat (SDW) or modern lines with alien translocations have
shown higher fitness than their modern counterparts under heat and drought stresses.
CIMMYT identified several SHW and SDW with superior adaptation to drought
conditions (Trethowan and Van Ginkel 2009 ). Wheat landraces have also been
exploited for tolerance against many abiotic and biotic stresses and are the source of
many important genes, such as those conferring drought tolerance, disease resis-
tance and quality traits (Reynolds et al. 2009 ).
4 Crop Gene Pool, Evolutionary Relationships
and Systematics
The concept of the gene pools was proposed by Harlan and de Wet ( 1971 ) and later
on described by Jiang et al. ( 1994 ), on the base of evolutionary distance from each
other, their genomic constitution and the idea of the three gene pools i.e. primary,
secondary and the tertiary gene pools, this classification is in relation to the culti-
vated species. The knowledge of the ancestry of cultivated crop species and variet-
ies is important for understanding variation and genetic diversity in their primary
and secondary gene pools and the potential for exploiting the valuable genes respon-
sible for disease resistance or stress tolerance, for developing new varieties with
resistance and tolerance to various stresses (Smale 1996 ).
The primary gene pool of a crop is composed of landraces, early domesticates
and wild species that hybridize directly with the cultivated species. Bread wheat
arose recently (6,000 to 8,000 years ago) from the hybridization of tetraploid
(Triticum turgidum) and diploid, Aegilops tauschii Coss.) so these two species con-
stitute the primary gene pool (Qi et al. 2007 ) for wheat and the diploid donors of the
A and D genome to bread wheat and durum wheat. The primary gene pool is often
preferred due the easiness of crossing and gene transfer (Mujeeb-Kazi 2003 ). The
chromosomes of this gene pool are homologous to the cultivated types and can be
utilized easily by breeding methods (Feuillet et al. 2008 ). For barely the progenitor
of cultivated barley (Hordeum vulagare subsp. spontaneum) belongs to this gene
pool. Similarly, the primary gene pool of chickpea includes Cicer arietinum ssp.
reticulatum and the variants of the domesticated chickpea.
During last few decades, many useful genes may have been lost in improving
varieties for specific environments, some of these lost genes can somehow be recov-
ered from the primary gene pool. The primary gene pool of wheat carries a highly
diverse and geographically widespread and sexually compatible germplasm (Feuillet
et al. 2008 ). However, only a small proportion of the existing genetic diversity of the
primary gene pool for most crop species has been utilized for crop improvement
(Tanksley and McCouch 1997 ).
Breeding and Genetic Enhancement of Dryland Crops