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The secondary gene pool of wheat contains polyploid species that share at least
one homologous genome with the cultivated types (Feuillet et al. 2008 ). Transferring
genetic material from secondary gene pool is comparatively more complex, there
are usually problems of hybrid seed death, female sterility of F 1 hybrids, and reduced
recombination (Ogobnnaya et al. 2013 ). To generate F 1 hybrids, embryo rescue is
required for crosses of the secondary gene pool with primary gene pool. This gene
pool for wheat includes polyploidy Triticum and Aegilops species, such as T.
timopheevii (AAGG) and the diploid S-genome (B genome) species from Aegilops
(Qi et al. 2007 ; Curtis 2002 ). In barley, the tetraploid species, H. bulbosum comes
under this category the closest relative of cultivated barley, after the subsp. sponta-
neum, and shares the ‘I genome’ with H. vulgare. and H. bulbosum. The secondary
gene pool of chickpea contains cicer echinospermum (Ladizinsky et al. 1988 ).
The tertiary gene pool is composed of more distantly related diploids and poly-
ploids with non-homologous genomes. They have no genome constitutions of the
cultivated species. Transfer of genetic material from this gene pool is very complex
(Feuillet et al. 2008 ). Usually special techniques such as, irradiation, gametocidal
chromosomes are needed for gene transfer. Embryo rescue is necessary in crossing
with species with tertiary gene pool (Jiang et al. 1994 ). This group for wheat
includes mostly germplasm of Triticeae that are not within the primary or secondary
gene pools. Most of germplasm in this group are perennials (Mujeeb-Kazi 2003 ;
Feuillet et al. 2008 ). Although tertiary gene pool resources are highly complex to
utilize but are important means for developing crop’s germplasm. The barley and
chickpea tertiary gene pool are composed of all other spices of their respective
genus not present in their primary and secondary gene pool. For some crop species
like lentils (Lens culinaris Medikus) differentiation between the primary, secondary
and tertiary gene pools is not straight forward as all the species and sub species can
be exploited quite easily for breeding purpose ((Ladizinsky et al. 1988 .)
5 Assessment of Gene Flow for Crop Improvement
Gene flow has been one of the main driving forces for crop improvement. Wheat
prominent selfing nature has produced a number of genetically differentiated popu-
lations around the globe due, among other reasons, to founder effect in combination
with local farmer’s trait selection. The first human settlers in the Fertile Crescent
adopted wheat as one of their main sustaining crops due to its single seed productiv-
ity (as bread wheat is believed to have evolved from a natural probably a single
cross). However, it was wheat plasticity and adaptive capacity the reasons behind
wheat worldwide spread. Wheat, carried by migrating populations, conquered the
most diverse environments resulting in populations with specific adaptations to a
wide range of environmental conditions. Very few crops can be as useful as wheat
to trace migrations and trade relationships of human groups, mostly in Africa and
Eurasia but, after 1500s also in America and Oceania. Since the Neolithic, wheat
spread-out with the first migrations, reaching Northern Europe by 3000 B.C. and
Q. Sohail et al.