Culm length of rice is an important agricultural character related to the lodging resistance. To date, more than 60 dwarf mutants have been reported, but D53 is the only dominant mutation among them. Here we report a new dwarf mutant controlled by a dominant gene.

A mutant, KA-902 derived from cv. Taichung 65 mutagenized with methyl-nitrosourea (MNU), showed a semi-dwarfism and twisted leaves (Fig. 1 a and c). The progenies of selfpollinated KA-902 segregated three different phenotypes, wild, semi-dwarf and extreme dwarf types (Fig. 1 a, b and d). Semi-dwarf type was similar to the parent KA-902 and extreme dwarf type was sterile. We found no segregation from any wild type progeny, whereas semidwarf type progenies segregated the three phenotypes (Table 1). Thus semi-dwarf KA-902 would carry a heterozygous dominant dwarf gene.

To confirm the inheritance mode of KA-902, we crossed the semi-dwarf type plant with an indica cv. Kasalath. The F₁ plants showed two different phenotypes, wild and semi-dwarf

types in the ratio of 1: 1. In the F₂ progenies of wild type F₁, all plants were normal, but these of semi-dwarf type F₁ were segregated into the three different phenotypes (Table 2).

These results reveal that the semi-dwarfism of KA-902 is controlled by the single dominant dwarf gene, and its homozygote shows the extreme dwarf phenotype. The extreme dwarf type seedlings were frequently lethal. This may cause the low frequency of this dwarf type at

maturity (Table 1 and 2).

The phenotype of KA-902 differs from that of D53 mutant reported as tillered dwarf (Iwata et al. 1977, 1978, Iwata and Omura 1984). Then we designated tentatively this mutant gene Twisted dwarf1 (Td1).

References

Iwata, N., H. Satoh and T. Omura. 1977. Linkage studies in rice. Linkage groups for 6 genes newly described. Japan. J. Breed. 27 (Suppl. 1): 250-251. (in Japanese).

Iwata, N., T. Omura and H. Satoh. 1978. Linkage studies in rice. The sequence of genes at the eight and eleventh linkage groups. Japan. J. Breed. 28 (Suppl. 1): 170-171. (in Japanese).

Iwata, N. and T. Omura. 1984. Studies on the trisomics in rice plants (Oryza sativa L.). VI. An accomplishment of a trisomic series in japonica rice plants. Japan. J. Genet. 59: 199-204.