Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 245

in use has been the D genome synthetics and their elite sub-sets 1 plus 2. Lesser
targeted usage also happens and special requests are met from the euploid reserve
holdings. One significant provision was of a synthetic hexaploid wheat stock to Cor-
nell University wheat program where after being crossed with the cultivar “Opata”
the famous ITMI population of 150 RIL's was developed . This population was the
conduit for developing the wheat microsatellite map (Roder et al. 1998 ). D genome
synthetic production has been of worldwide interest but large numbers have been
associated with the CIMMYT wide cross program where close to 1,200 spring and
winter habits synthetics have been produced (Mujeeb-Kazi et al. 2008b ) and these
numbers are increasing (Bonnett, Personal Communication with A Mujeeb-Kazi).
In depth details of the global inputs of various laboratories towards the D genome
stocks have been recently reviewed by Ogbonnaya et al. ( 2013 ) where the current
status of their practical utilization has also been elucidated.
The close relative genetic diversity beyond the D genome has exploited acces-
sions of the A diploid resources T. boeoticum , T. monococcum and T. urartu and to
a very limited extent the B(S) genome diploid Ae. speltoides of the Sitopsis section.
The A and the B(S) genome stocks have yet to be widely utilized.
The use of the SH route is categorized as bridge crossing as upon crossing se-
lected SH s with bread wheat for its improvement all three genomes are contributors
and allelic richness is harnessed from all three of the SH genomes, i.e., A, B and D
allowing for intraspecific and interspecific coverage to occur where the intraspe-
cific portion simulates the bread wheat/ durum wheat pentaploid breeding protocol.
DirectCrosses
The most efficient technique for exploiting Ae. tauschii variability for bread wheat
improvement is to achieve direct transfers from resistant/tolerant Ae. tauschii ac-
cessions to bread wheat. The methodology rapidly produces improved BC1 deriv-
atives with the six genomes (AABBDD), five of which (AABBD) resemble the
elite bread wheat cultivar used in the cross (Fig. 7.1 ). Aneuploidy in the BC does
surface and thus recovery of euploids (2n = 6x = 42) requires cytology. Advantages
of direct crossing have been elucidated by Cox et al. ( 1990 ) and has tremendous
potential to go beyond the D genome into the A genome diploids. For a targeted ap-
proach screening of the genomic resource is important for practical wheat produc-
tivity goals. Alonso and Kimber ( 1984 ), Cox et al. ( 1990 , 1991 ) and Gill and Raupp
( 1987 ) unequivocally placed priority on direct Ae. tauschii crossing with bread
wheat cultivars. Based on the transfer of stem rust resistance from Ae. tauschii to
the cultivar Chinese Spring Alonso and Kimber ( 1984 ) determined that one back-
cross onto the F1 hybrids reinstated 92 % of the genotype of the recurrent parent.
Where there are constraints to screening of the Ae. tauschii accessions, screen-
ing the synthetic hexaploids derived from T. turgidum/Ae. tauschii is an alternative
particularly where the durum parent is susceptible and thus the SH resistance is