Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 243

all crosses using the AABB tetraploids onto wheat or AABBDD hexaploids with
AABB tetraploids that do not require embryo culture and laboratory assistance to
handle self-fertility or chromosome doubling should fall under the breeder's do-
main of conventional breeding and not categorized as “pre-breeding”.
Wide hybridization returning to Triticale production, the classic wheat/barley
hybrids then additional Triticeae species that require embryo rescue and media
preparation skills, special plantlet regeneration strategies, handling care of the hy-
brid that is self-sterile with its advance via amphiploidy or backcrossing to affect
alien transfers is the true categorization of the term “pre-breeding” and has been in
operation since the start of such efforts highlighted in reports of Rimpau ( 1891 ),
Farrar ( 1904 ), Kruse ( 1967 , 1969 ), Islam et al. ( 1978 ), Sharma and Gill ( 1983 ),
Mujeeb-Kazi and Kimber ( 1985 ), Mujeeb-Kazi et al. ( 1987 , 1989 , 1996a , 2013 ),
Sharma ( 1995 ) and Mujeeb-Kazi and Hettel ( 1995 ). Outputs from such diverse
crossings require infrastructure and skills that are hard to find within the conven-
tional breeding professionals and requires expertise and controlled environment
facilities that permits adopting special tools for F1 crossing to be done, compat-
ibility to be harnessed, tissue culture to rescue the embryos, cytology to validate the
hybrid, growing conditions that promote vigorous growth, amphiploid induction
and/ or backcrossing before the BC or the amphiploid could be manipulated to
deliver advanced lines that have the potential to become varietal materials. These
facets are true “pre-breeding” steps that has been not been mentioned and the term
loosely used to chart program structures in developing world programs fostered by
international funding.
Breeding
An efficient protocol being followed in CIMMYT has been limited backcrossing
coupled with selected modified bulk for breeding program efficiency. Where novel
diversity becomes a donor of alleles the same protocol has been utilized effectively.
With the focus on bread wheat improvement these steps are as follows:
a. Bread wheat parent as the female crossed by the novel tetraploids to produce
pentaploid F 1 hybrids that upon limited backcrossing are advanced mediated by
selected bulk and ultimate cytology to generate hexaploid euploids (2n = 6x = 42,
AABBDD)
b. Similar crossing as in (a) of bread wheat by synthetic hexaploids to result in
F 1 's that are ABD AAB, ABD ABB(S) or ABD ABD to yield BC1's and upon
advance deliver euploids with the 42 complement.
Often with the D genome hexaploids the F1 hybrids exhibit necrosis this knowledge
of the necrotic genes is important to overcome this limitation. Also the tough rachis
requires a sizeable F2 population to allow for selecting free threshing derivatives.
Even though major efforts are on limited backcrossing use of F1 top-crossing in
wide crossing has been exploited significantly (Mujeeb-Kazi and Asiedu 1990 ).