Alien Introgression in Wheat Cytogenetics, Molecular Biology, and Genomics

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9.4 Chromosome Translocation s


Wheat– Aegilops chromosome addition lines have no practical appli cation in
agriculture. The Aegilops chromosome segment carrying the gene of interest must
be transferred to a wheat chromosome. This can be achieved by spontaneous or
radiation translocations, or homoeologous recombination induced either by removal
of the Ph1 gene on wheat 5B chromosome or use of the Ph1 suppressor in some
Aegilops accessions.
The most common type of sponta n eous translocation is the centric fusion or
Robertsonian translocation. Sears ( 1972 ) demonstrated that in wheat plants that are
monosomic for two chromosomes, both monosomes can misdivide at their centro-
meres at meiosis and rejoin to give chromosomes carrying arms from each original
chromosome. Shepherd and Islam ( 1988 ) listed spontaneous transloc ations in
common wheat involving chromosomes from Ae. markgrafi i (Greuter) Hammer
( Ae. caudata L.), Ae. comosa Sm. in Sibth. & Sm., and Ae. longissima (Schweinf. &
Muschl. in Muschl.) Eig. The fi rst report of using irradiation to transfer a disease
resistance gene was that of Sears ( 1956 ) who translocated a segment of an
Ae. umbellulata Zhuk. chromosome carrying the le a f r ust resistance gene Lr9 to
common wheat.


9.5 Homoeologous Recombination


Chromosome pairing behavior is under genetic control. However, it can be manipu-
lated to induce homoeologous recombination in order to achieve gene transfer
between chromosomes that normally do not pair. Meiotic pairing between homoeol-
ogous chromosomes of the three wheat genomes (A, B, and D) is normally pre-
vented by the action of several genes, the principal one being Ph1 on chromosome
arm 5BL (Okamoto 1957 ; Riley and Chapman 1958 ). Later, it was r ecognized that,
in addition to the gene on 5BL, several other genes on chromosomes 3D, 3A, and
4D acted to reduce homoeologous pairing. But genes also existed on other wheat
chromosomes (5BS, 5D, 5A, 3AL, 3BL, 3DL, and 2AS) that promoted homoeolo-
gous pairing (Sears 1976 ). Two additional pairing suppressors and nine promoters
on other chromosomes of wheat were later reported (Gale and Miller 1987 ).
Wheat lines lacking chro mosome 5B or poss essing the mutant gene ph1b (Sears
1977 ) (an interstitial chromosome deletion of the Ph1 gene) showed homoeologous
pairing between wheat chromosomes of different genomes and also with other spe-
cies in the tribe Triticeae. Transfers of disease resistance into wheat using the ph1b
mutant were reported from Ae. longissima (Ceoloni 1984 ; Ceoloni et al. 1988 ,
1992 ), Ae. triuncialis (Fan et al. 1995 ), and Ae. cylindrica Host (Bai et al. 1995 ).
In addition, the ph1b mutant was employ ed to reduce the size of Ae. speltoides
chromosome segments (see many examples li sted below). Wheat nullisomic for the
entire 5B chromosome ( carrying the Ph1 gene) was used to transfer disease resis-
tance from Ae. markgrafi i into wheat (Dyck et al. 1990 ).


P. Zhang et al.
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