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4) Breeding for resistance
to yellow mosaic virus and powdery mildew: These two severe diseases
are often epidemic in the warm regions west of the Kanto district.
Some cultivars highly resistant to these diseases have been
developed, such as Horoshirikomugi (Norin 114), Takunekomugi (Norin
115), which are resistant to wheat yellow mosaic virus, and
Ushiokomugi which is resistant to powdery mildew caused by
Erysiphe graminis de Candolle f. sp. tritici Em.
Marchal.
5) Breeding for resistance to preharvest sprouting: In Japan, the
prevention of preharvest sprouting damage is an important problem in
wheat production because harvesting time, early June to late July,
coincides with the rainy season. Japanese wheat cultivars, Igachikugo
Oregon and Zenkojikomugi (Norin 109), are mainly used as gene sources
to prevent preharvest sprouting.
6) Breeding for improved grain quality: Japanese wheat is mainly used
for making noodles. Superior cultivars suitable for noodle-making are
characterized by an excellent milling score, creamy-white flour, low
amylose content, etc. as well as low deterioration caused by rain.
The amylose content of flour shows a highly negative correlation with
the eating quality. Among the Japanese wheat cultivars, the breeding
line Kanto 107 and its parent Kanto 79 were found to show a lower
apparent amylose content (21.5% and 22.0%, respectively) than typical
wheat cultivars (28~29%) (Kuroda et al. 1989), and the wheat mutants
with a much lower apparent amylose content (14.0~16.7%) were induced
by ethylmethanesulphonate (EMS) treatment (Oda et al. 1992).
7) Breeding high-yielding wheat varieties using F1
hybrids: Initially, the male sterility from the Triticum
timopheevi cytoplasm and the fertility-restorer gene Rf3
from Triticum aestivum ssp. spelta var.
duhamelianum were used for F1 hybrids (Zeven 1967;
Wilson 1968; Tahir 1970; Tahir and Tsunewaki 1971; Araki 1990).
However, the F1 hybrids could not be used commercially
because they were susceptible to scab due to the T. timopheevi
cytoplasm. In addition, the Rf3 gene had incomplete
restoring ability and thus low production of F1 seeds.
Recently, extensive efforts to breed F1 hybrids have been
made using the Sv type cytoplasm from Aegilops kotschyi
and a wheat-rye translocation line with the 1 BL-1 RS chromosome
constitution (Toriyama et al. 1993; Nonaka et al. 1993). The
fertility-restorer gene Rfv1 for the Sv type
cytoplasm is carried on the short arm of chromosome 1B of wheat. The
translocation line with a 1BL-1RS chromosome constitution and normal
type cytoplasm (e.g., 911-B-8-11 in
Fig.
3) is used as a
female parent, and an 'W' variety is backcrossed "n" times to the
translocation line to develop the maintainer "A". In this process of
backcrossing, plants having the 1 BL-1 RS chromosome constitution can
be easily selected by using the leaf rust resistance gene Lr26
as a marker because it is carried on the 1 RS chromosome from rye.
Male sterile "A" line can be developed by crossing between the
maintainer "A" and "A" line having the S' type cytoplasm also
developed by backcrossing (e.g., (Sv)-Norin 26 x "A"
variety in Fig. 3). Almost all the wheat cultivars have a Rfv1
genotype because the Rfv1 gene is carried on the I BS
chromosome of wheat. Therefore, this breeding system does not require
the development of any specific restorer (Fig.
3).
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