2. Semidwarfing genes of high-yielding rice varieties in Japan

1) Institute of Agriculture and Forestry, University of Tsukuba, Sakura-mura, lbaraki, 305, and

2) Okinawa Branch, Tropical Agriculture Research Center, Ishigaki, Okinawa, 907-01 Japan

Two distinct sources of semidwarfism have contributed to a break-through in the yield level of rice in 1960's in Japan. They are Reimei, a mutant induced from Fujiminori in northern japan (Futsuhara 1968), and Shiranui and its sister lines derived from a native dwarf, Jikkoku in southern Japan (Okada et al. 1967). These varieties were widely used as parents in cross-breeding programs and many short-statured high-yielding varieties have been developed.

Semidwarfism has also been introduced into high-yielding rice varieties in the tropics and other areas since 1960's. The Green Revolution is a direct achievement of an intensive use of the tropical semidwarfs. On the other hand, cumulative evidence has indicated that most of the short-statured varieties possess the same gene for semidwarfism, as a Taiwan-native variety Dee-geo-woo-gen (DGWG) was the common gene source (Hargrove 1979; Mackill and Rutger 1979; Chang and Li 1980). Short-statured mutant lines induced from tall native varieties, selected for high- yielding potential, also had the dwarfing gene at the same locus (Hu 1973). In conceiving breeding strategies for high-yielding varieties in Japan, it is of primary importance to investigate whether or not the Japanese semidwarf varieties have the same gene as DGWG.

The allelism test of semidwarfing genes in the Indica and Japonica groups is not always easy because of hybrid sterility and transgressive segregation for delayed maturity. We have overcome this difficulty by use of isogenic lines. By transferring the semidwarfism from Taichung native 1 having the DGWG gene and another semidwarfism from Shiranui with the gene of Jikkoku into a Japanese tall variety, Norin 29 through 4 times of backcrosses, two series of near-isogenic lines were obtained, which were desingated as SC 2 and SC 3 (Taichung Native 1/5* Norin 29) and SC 4 and SC 5 (Shiranui/5* Norin 29).

To investigate the genetic behavior of the semidwarfism, the near-isogenic lines were crossed with Norin 29 and the F\1\ and F\2\ plants were observed for culm length at National Institute of Agricultural Sciences in 1981. The long culm of Norin 29 was found to be partly dominant since the F\1\ plants had shorter culms than Norin 29. The F\2\ clearly segregated into 3 tall : 1 short types indicating that the semidwarfism of DGWG and that of jikkoku were each controlled by a recessive gene. Then, SC 4 and 5 were crossed with SC 2 and 3. The F\1\ plants had as short culms as of the parents and the F\2\ showed a narrow range of variation in culm length around the F\1\ and parental mean. This indicates that DGWG and Jikkoku have the same semidwarfing gene.

Furthermore, the semidwarfism of Reimei is known to be controlled by a single gene with incomplete dominance (Futsuhara 1968). When Reimei was crossed with SC2, the F\1\ plants had slightly shorter culms than of Reimei as SC 2 was shorter than Reimei, and the range of F\2\ segregation was between the parental values. This suggests that Reimei also has the same semidwarfing gene as DGWG although it has modifiers which increase the culm length.

It is known that Calrose 76, an induced semidwarf mutant grown in California, also has the same gene (foregoing note by J. N. Rutger). Suh and Heu (1978) have shown that the semidwarfism of a Korean variety Tongil (IR 8//Yukara/Talchung Native 1) is controlled by a single recessive gene, d-t, which is linked with the marker genes such as A (anthocyanin activator), Pp(brown pericarp), Pn(purple node), and Pau(purple auricle) of linkage group III with recombination values of 24.8%, 35.1%, 40.9% and 42.9%, respectively. It is of particular interest to find that all the genes carried by semidwarfs of economic importance are at the same locus as that of DGWG despite the differences in genetic background. There may be a potential danger of reducing genetic diversity by the frequent use of the same gene. Semidwarfing genes of economic use at non-allelic loci are now being searched for.

Chang T. T. and C. C. Li, 1980. Genetics and Breeding. In Rice: Production and Utilization. B.S.Luh(ed.) AVI Publishing Company, Inc., Westport. Connecticut. pp.87-146.

Futsuhara, Y., 1968. Breeding a new rice variety Reimei by gamma-ray irradiation. Gamma Field Symp. 7: 87-109.

Hargrove, T. R., 1979. Diffusion and adoption of semidwarf rice cultivars as parents in Asian Rice Breeding Programs. Crop Sci. 19: 571-574.

Hu, C. H., 1973. Evaluation of breeding semidwarf rice by induced mutation and hybridization. Euphytica 22: 562-574.

Mackill, D.J. and J. N. Rutger, 1979. The inheritance of induced-mutant semidwarfing genes in rice. J. Hered. 70: 335-341.

Okada, M., Y. Yamakawa, K. Fujii, H. Nishiyama, H. Motomura, S. Kai, and T. Imai, 1967. On the new varieties of paddy rice, "Hoyoku, Kokumasari and Shiranui", and notes on the parent varieties and their origins. Bull. Kyushu Agr. Expt. Sta. 12: 187-224.

Suh, H. S. and M. H. Heu, 1978. The segregation mode of plant height in the cross of rice varieties, VI. Linkage analysis of the semi-dwarfness of the rice variety "Tongil". Korean J.Breed. 10: 1-6.