GIFU UNIVERSITY
Faculty of Agriculture, 1-1 Yanagido, Gifu 501-1193 Japan.
Nobuyoshi Watanabe.
Near-isogenic lines for GA3-sensitive semidwarfing genes (Rht12,
Rht14, Rht16, Rht18, and Rht19) in
durum wheat cultivar LD222 were developed for research purposes.
Near-isogenic lines for GA3-insensitive semidwarfing genes in
LD222 also were developed. The donors of the Rht genes were Cando
(Rht-B1b), a NIL of Maringa for Rht-B1c (Rht-B1c),
Saitama 27 (Rht-B1d), Krasnodari-1 (Rht-B1e), and
W6824D (Rht-B1f).
JAPAN INTERNATIONAL RESEARCH CENTER FOR AGRICULTURAL SCIENCES (JIRCAS)
Tsukuba, Ibaraki 305-8686, Japan.
Hiro Nakamura.
The quality of common wheat grain favored in bread and/or noodle-making quality is strongly affected by components of seed-storage protein, particularly the HMW-glutenin subunits. The HMW-glutenin 2.2 subunit controlled by the Glu-D1f allele is found frequently in Japanese common wheat cultivars and landraces. While investigating the factors affecting the distribution of this allele, the available data on HMW-glutenin alleles of common wheats from Japan were analyzed and compared with the data for intensity of winter habit and wheat flour hardness. The main factors affecting Glu-D1f allele frequency in Japanese wheats were the intensity of natural selection for winter habit and artificial selection for flour hardness. According to a study of the worldwide distribution of Glu-1 alleles in common wheats, the Glu-D1f is rare. However, the Glu-D1f allele was the most common Japanese wheat seed-storage protein allele. We know that Chinese wheat contributed to Japanese landraces, and Japanese landraces contributed to modern cultivars from Japan. However, Japanese and Chinese common wheats differ in their frequencies of the Glu-D1f allele. These results may be explained either by the founder effect or by a selective bottleneck in Japanese common wheat genetic resources.
HMW-glutenin subunits represent a group of common wheat seed endosperm proteins characterized by molecular weights of between 80,000 and 145,000 and a complex biochemical structure involving disulphide bonds. HMW glutenin, the protein constituent of flour of wheat that gives elasticity to a dough, is built up of at least 20 different subunits. Studies investigating HMW-glutenin subunit composition and/or its relation to bread-making quality have been madet in virtually all major wheat-producing countries. These studies involved both cultivars and landraces of common wheat. The results of these studies highlighted three important ideas: 1, the allelic variation for Glu-1 loci that exists in T. aestivum; 2, the relationship between HMW-glutenin subunit composition and wheat quality parameters; and 3, the association between allelic distribution and ecogeographical parameters. Because the HMW-glutenin composition of common wheat cultivars from many countries now has been published, an analysis of these data and that for Japanese wheats will contribute to our knowledge of the worldwide distribution of Glu-1 alleles. Our objective was to analyze this distribution in Japan, which is the most geographically remote region for common wheat production, concentrating mainly on HMW-glutenin allelic variation within common wheat and the factors which affect it in a worldwide context.
We know that common wheat cultivars from different countries differ in the frequency of Glu-1 alleles. The 2.2 HMW-glutenin subunit controlled by the Glu-D1f allele was frequently found among improved cultivars and in Japanese landraces in this study. Investigating the factors affecting the distribution of the allele, the available data on the HMW-glutenin alleles of common wheats from Japan were analyzed and compared to published data (1,380 cultivars from 21 common wheat-producing countries) on the worldwide distribution of Glu-1 alleles. The Glu-D1f has been reported to be a rare allele in the worldwide distribution of Glu-1 alleles. However, the Glu-D1f allele has been characterized as the most common Japanese wheat seed storage protein in this study. This study showed a specific difference in the frequency of the Glu-D1f allele for Japanese common wheat cultivars and landraces. The allelic frequency of this subunit was shown to be in excess of 35 % among improved Japanese cultivars, 25.3 % among Japanese landraces, but was found in only 1.8 % of 274 Chinese wheats in this study. Genealogical examinations revealed that the Glu-D1f allele was not only present in the Nisikaze-komugi prevalent in the Kyushu district (southern Japan) but also frequently appeared in its pedigree. The Glu-D1f allele was absent in Horoshiri-komugi found in the Hokkaido district (northern Japan), but carried only in a few of its remote ancestors. A noticeable geographical cline has been reported in the frequency of the Glu-D1f allele. To elucidate the factors involved in the establishment of this cline, we investigated the association of the occurrence of the glutenin Glu-D1f gene with both winter habit and with flour hardness. In Japan, the environment for cultivating common wheats is made diverse by the distance of the islands from north to south, by improved Japanese cultivars, and by locally grown landraces differentiating into distinct types of winter habit. The intensity of the winter habit in Japan is the most important factor for common wheat production in the field. Generally, wheats with a weaker winter habit (I~III) are grown in southern Japan; stronger winter habit (V~VII) wheats are grown in the north. A strong correlation was observed between intensity of winter habit and occurrence of the Glu-D1f allele. Improved cultivars with a weaker winter habit tended to carry the Glu-D1f allele more frequently, but this allele was absent in the cultivars with the stronger winter habit. The Glu-1 alleles have previously been reported not to be associated with ecogeographical parameters in a worldwide context. However, we have found that the Glu-D1f allele is associated with ecogeographical parameters in Japan, a finding of great interest to Japanese common wheat breeders and cereal chemists.
Flour hardness of common wheat grains is believed to be related closely to grain quality. Common wheat flour hardness is correlated with Japanese soft noodle-making quality and with hard common wheat cultivars having poor Japanese soft noodle-making quality. Thus, we investigated the relationship between flour hardness and the occurrence of the Glu-D1f allele in this study. We found that soft-flour cultivars tend to exhibit the Glu-D1f allele more frequently than hard-flour cultivars. Particular characters such as winter habit and wheat flour hardness also may be required as a breeding objective for common wheat in Japan. A current analysis of HMW-glutenin alleles shows a post factum status of these loci because, until recently, Japanese common wheat breeders did not manipulate the Glu-1 alleles intentionally. So, this analysis reflects the results of indirect changes in the genetic constitution of common wheat because of selection for related or linked traits in Japanese wheat-breeding programs. The average Glu-1 quality scores relating to good bread-making quality in Japan and China within the noodle-culture zones of the Far East have been shown to be less than those of known quality wheats from Europe, Australia, Canada, and the United States within the bread wheat-culture zones. Common parentage can influence the distribution of Glu-1 alleles. Frequent involvement of the same successful parents in crosses will result in a large similarity in genetic structure.
Chinese wheat is well known to have contributed to Japanese landraces, and Japanese landraces contributed to modern cultivars from Japan. However, the common wheats of Japanese and Chinese differ greatly in Glu-D1f allele frequency in this study. Our findings suggest that the geographical clines in frequency of the Glu-D1f allele observed among Japanese improved cultivars and landraces may be caused by natural selection for winter habit and/or by artificial selection for wheat flour hardness. Japanese common wheat is characterized by the high frequency of alleles such as Glu-B1g and Glu-D1f at the Glu-1 locus. Natural and artificial selection is thought to have narrowed the genetic base of Japanese common wheat. The frequent occurrence of the Glu-D1f allele would support this conclusion. This study has shown that the Glu-1 allele frequencies differ between the noodle-culture zones of the Far East and the bread-culture zones. Probably, the factor most influencing Glu-1 allele composition is a common wheat breeding strategy relating to bread-making quality in the bread-culture zone. The Glu-1 alleles have been reported to directly affect wheat-gluten quality. Japanese specific differences in Glu-1 patterns likely resulted from the intensity of selection pressure towards good Japanese soft-noodle making quality instead of selecting for good bread-making quality. As a consequence of its spread, adaptation, and phenotypic refinement, Japanese common wheat has developed a unique composition of Glu-1 glutenin alleles and a narrow common wheat genetic base; an illustration of the result of artificial breeding and/or adaptation to the different winter climatic conditions spanning the length of the Japan. Cultivars with superior bread-making quality have higher Glu-1 quality scores and tend to have limited genetic variation in their Glu-1 loci, such as the Glu-D1d gene demonstrating a narrowing of the genetic glutenin-protein variability when breeding common wheat for its bread-making quality.
The rich genetic diversity of common wheat landraces confers resistance to multiple diseases, environmental adaptation, and agronomic traits of economic significance. The Glu-1 alleles could serve as markers for genes involved in adaptation. The range of Glu-1 allelic variation in seed-storage proteins that is currently available to the wheat breeder is being extended by the introgression of glutenin alleles from primitive landraces and from alien common wheats. Linkage studies using SDS gel electrophoretic screening to determine the relationship between seed endosperm glutenin protein genes and genes for other agronomic characters might enable the incorporation of desired traits into new common wheat cultivars. Desirable traits may include improved cold tolerance, disease resistance, and improved quality for Japanese soft noodle-making. Investigating the contribution of the HMW-glutenin proteins that confer elasticity to dough by forming large aggregates to Japanese soft-noodle making will be of particular interest. Consequently, the Glu-D1f gene is being transferred into the many good soft-noodle making quality varieties at wheat-breeding programs in Japan.
The variability released by both environmental and genetic factors could easily lead to different subpopulations of the founders in turn, leading to the establishment of more than one reproductively isolated population from a single founder through population genetic mechanisms for common wheat. Each bottleneck was followed by a flush of rapid population growth, so, once again, there were optimal conditions for including genetic change. With this design from China, the common wheats were exposed to a selective bottleneck induced by the external environment, and a founder effect (because all populations went through a bottleneck of small size). Consequently, the selective bottleneck was extremely intense and, in fact, most ancestral populations may be become extinct in Japan. Although the selective bottlenecks discussed in this study were primarily induced by genetic environment, most of the predictions made in this study also would apply if the selective bottlenecks were induced by the external environment. The HMW-glutenin, subunit allele pattern of Japanese common wheat cannot be explained solely by the founder effect because, as already pointed out, the pattern also was affected by the artificial selection for bread-making or soft noodle-making quality in wheat-breeding programs. The ease with which genetic changes occur in Japan strongly implies a lack of genetic variability in natural populations of Japanese common wheats.
The theory presented here has increased the explanatory powers of wheat genetic revolution model of speciation and, more importantly, has generated testable predictions that can be examined in both the natural and artificial selection using current methodologies and systems.
Acknowledgments. The author thanks Dr. Hiroshi Fujimaki, professor at Tokyo University of Agriculture, for helpful discussion and comments. Thanks are due to the National Institute of Agrobiological Resources (NIAR) at Tsukuba, Japan, for providing common wheat samples in this study.