Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 242

Germplasm Choice for Wheat Improvement
The range of available genetic resources for improving wheat is enormous but if
the time span for the delivery of final products that translate into varieties is the
measure then the priority would follow intraspecific, interspecific and intergeneric.
At the intraspecific level T. dicoccum/T. dicoccoides followed by T. carthlicum
take lead for both durum and bread wheat improvement with T. spelta being a good
candidate for bread wheat.
At the interspecific level based upon homology the D genome heads the list
along the strategy of bridge crossing and direct crossing. For additive variation A
genome accessions come next and lastly usage of the B(S) from the Sitposis section
that is rarely exploited for applied goals of wheat production.
At the intergeneric level most of the tertiary gene pool species are genomically
far removed from the wheat A, B and D genomes. Hence parental choice of the alien
resource is paramount. If trait is present across various ploidy levels then the prefer-
ence of the species to be exploited would be the lower ploidy. Using such a strategy
the salt tolerance gene transfers from the diploids Thinopyrum bessarabicum or Th.
elongatum are preferred over the tetraploid source T. junceum or the decaploid Th.
ponticum . It is fortuitous that the diploid Th. bessarabicum is also resistant to the
UG99 pathotypes and thus for this paper it shall be discussed in detail.
Hybrid Production
Hybridization varies from the conventional to the radical combinations and thus
can be rather easy to highly complex. In general where polyploidy levels differ,
the higher polyploidy parent serves as the female and after crossing seed is set that
may mature and be shriveled or would have to have its embryo excised from after
10 to 15 days and give plantlets that are self-sterile requiring induced doubling or
backcrossing for further use in pre-breeding or breeding.
Pre-Breeding
Of late this term has been used to justify program organization in some internation-
al output projections. Unfortunately assigning the pre-breeding category to those
crosses where F1 embryo rescue is not needed needs to be separated and fitted
into the conventional hybridization category well within the work arsenal of any
wheat breeder. Intraspecifc crosses have across many decades being handled by
breeders where the classic examples are of the bread wheat/ durum wheat combina-
tions forming the pentaploid and their further exploitation. This remains a 100 %
field operation and calling it pre-breeding in our perception is inappropriate. Thus