Agriculture Reference
In-Depth Information
has meant that with the required resistance has come many other linked genes, often
deleterious to yield and other traits, that were located on the same chromosome. This is
known as 'linkage drag'.
Two early methods used for promoting the transfer of smaller segments of chromo-
some from distantly related species have been spontaneous centric fusion and irradiation.
In the former, lines were generated that lacked a chromosome of bread wheat, but which
had an additional chromosome from the donor species. Natural, albeit rare, breakage
of chromosomes, usually at the centromere, at meiosis is often followed by subsequent
fusion of broken chromosome arms leading to rare translocations with a single chromo-
some carrying segments of both donor and recipient DNA. A very signifi cant example
of a centric fusion derived translocation was the transfer of the short arm of chromo-
some 1R from rye, with the major genes Sr 31, Lr 26 and Yr 9 for resistance to stem, leaf
and stripe (yellow) rust respectively, onto the 1B chromosome of wheat (Zeller, 1973).
This formed a translocation chromosome known as 1BL/1RS that is now carried in
many modern wheat varieties. This segment remains large, however, and also carries an
unfavourable quality gene that leads to 'sticky dough' during some industrial processes.
A second signifi cant example has been the transfer of the linked genes Lr 24 and Sr 24 for
leaf and stem rust resistance in wheat on a 3DL/3Ag e translocation from A. elongatum,
producing a line called Agent (Smith et al ., 1968) that has subsequently been used widely
in wheat breeding.
Ionizing radiation has been used to generate chromosome breakages that can then lead
to subsequent fusion of chromosome segments from donor and recipient species. Sears
(1956) used irradiated pollen to transfer the leaf rust resistance gene Lr 9 from Ae. umbel-
lulata, whilst Knott (1961) used irradiated seed to transfer the stem rust resistance gene
Sr 26 from Agropyrum elongatum into bread wheat. Ionizing radiation has also been used
to transfer resistance to powdery mildew into barley and oats from Hordeum bulbosum
(Pickering et al ., 1995) and Avena barbata (Thomas & Aung, 1978), respectively.
The reduction in size of donor chromosome segments to reduce linkage drag has been
aided by the discovery and use of a characteristic of some accessions of the wild grass
Triticum speltoides to suppress a gene Ph 1 which prevents pairing of non-homologous
chromosomes in wheat (Riley et al. , 1968). Using genetic stocks with the Ph suppressor
gene, it became possible to increase recombination between non-homologous (homoe-
ologous) chromosomes. Riley et al. (1968) fi rst used this technique to transfer the
yellow (stripe) rust resistance gene Yr 8 from Aegilops comosa to produce a line called
Compair.
Many examples of resistance transfer have been reported using chromotypes of wheat
where the chromosome 5B or 5BL arm carrying the Ph 1 gene is deleted (nulli-5B), or
the segment of chromosome 5BL with the Ph 1 gene is deleted (the ph1b mutant) (Sears,
1977). In both of these cases, homoeologous chromosome pairing occurs. A notably
successful case was the further development of the Sr 24 and Lr 24 genes for resistance
to stem and leaf rust resistance in wheat, already mentioned above. Sears (1973, 1978)
used this method to produce several 3DL/3Ag e translocations from A. elongatum , some of
which produced white seeded forms through breakage of the linkage to red grain colour
in the donor parent. These recombinant lines were used as the source for stem and leaf
rust in many past and present Australian wheat cultivars (Friebe et al ., 1996) where white
grained wheats are required.
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