III. Genetics of Morphological Traits

9. Dominant gene, Ssi1(t) located on the chromosome 1 shows the dm-type internode elongation pattern accompanied by a semidwarfness


1) Grad. Sch. Bioagr. Sci., Nagoya U., Nagoya. 464-01 Japan

2) Res. Inst. Biores., Okayama U.. Kurashiki, 710 Japan

Some of the rice dwarf mutants with a particular phenotype such as d1, d2 or d11 are known to have the characteristic internode elongation pattern named and categorized as "dm-type" by Takahashi and Takeda (1969). This peculiar elongation pattern is the result of inhibition of second internode from the panicle base. Although these dwarf mutants cause the whole plant dwarfism and somewhat abnormal morphologies, another dm-type mutant DMF-1 (original name Fu4M), induced from X-ray irradiated rice cultivar Fujimimori (Yamaguchi 1976), shows semidwarfism and it has fairly normal growth except inhibition of second internode elongation (Fig. 1).

To examine its inheritance mode and linkage relations, DMF-1 was crossed with Fujiminori and a linkage tester EM-20 showing shrunken endosperm (shr1-s) located on chromosome 1 (Yano et al. 1984)

F1s of these cross combinations showed dm-type stature, but in each F1 plant, both dm-type culms and normally elongated culms were contained. We decided to describe the dominant character of the mutant gene with the "expressivity" defined as the percentage of dm-type culm over total number of culms per plant. The values of the expressivity in F1 plants were clearly decreased compared with 100% of DMF-1 (Table 1).

In F2 generation of both cross combinations, three different phenotypes were observed, i.e. perfect dm-type (expressivity = 100%), imperfect dm-type (0% < expressivity < 100%) and normal type (expressivity = 0%) (Fig. 2). The ratio of these frequencies did not fit the 1:2:1 because the perfect dm-type were at the relatively high frequency. However, segregation ratio of sum of the perfect and imperfect dm-types on one hand and normal types on the other agreed with a 3:1 (Table 1 ). Then the mutant gene was designated tentatively as Ssi1 (short second internode 1) not using Sd- (semidwarf-) because such organ specific gene is unexampled in the category of semidwarf genes.

The results of F1 and F2 analysis suggest that Ssi1 as a heterozygote (Ssi1/ssi1) has an incomplete penetrance and shows a wide variation on the dm-type expressivity.

The results of linkage analysis between shr1-s and Ssi1 are shown in Table 2. The F2 populations were classified into four distinct groups based on two characteristics, shrunken endosperm and dm-type. The segregation ratio showed a significant difference in χ2L value for two-factor segregation and recombination value between Ssi1 and shr1-s was calculated as 10.12%. Thus, it is evident that Ssi1 controlling the dm-type internode elongation pattern is an incomplete dominant gene located on chromosome 1.



Takahashi, M. and K. Takeda. 1969. Type and grouping of internode pattern in rice culm. Memo. Agr. Hokkaido U. 7: 32-43. (In Japanese with English summary)

Yamaguchi, H., 1976. Morphological mutants. specially with reference to dm-type of internode elongation pattern, induced by X-ray irradiation to growing rice plants. Japan. J. Breed, 26(suppl.2): 31-32. (in Japanese)

Yano, M., Y. Isono, H. Satoh and T. Omura, 1984. Gene analysis of sugary and shrunken mutants of rice, Oryza sativa L. Japan. J. Breed. 34: 43-49.