22. An induced dwarfing gene, sd-7(t), obtained in Taichung 65

Kuo-Hai TSAI

Department of Agronomy, National Chung Hsing University, Taichung, Taiwan 40227, ROC

A semidwarf mutant, coded D65-31, was induced by X-ray (40 kR) irradiation of seed of Taichung 65 (abbr. T65). This mutant was about 21 cm shorter in height and 7 days earlier-heading than T65. It was backcrossed with T65 twice, and a fixed near-isogenic line D65-31, was obtained. This line was about 16 cm shorter and 3 days earlier in heading than T65 (Table 1). Probably, another induced gene or genes for early heading and dwarfism would have been eliminated during backcrossing. When this line was crossed with T65, the F1 was similar to T65, and the F2 segregated into 109 T65-like:40 dwarf types, giving a good fitness to the 3: 1 ratio. The recessive dwarfing gene of this mutant was assigned the gene symbol sd-7(t) in this note, and the near-isogenic dwarf line is coded T65sd-7(2), where (2) indicates the number of backcrosses.

As is well known, Taichung Native 1 carries the Dee-geo-woo-gen dwarfing gene, sd-1 (Suh and Heu 1978; Kikucht and Ikehashi 1984; linkage group III). Gene sd-1 was introduced into T65 by 8 backcrosses and an isogenic line T65sd-1(8) was established. Gene sd-1 is recessive in Indica crosses, but the F1 plants between T65 and T65sd-1(8) were about 15 cm shorter than T65, and the F2 segregated into 1 tall: 2 intermediate: 1 short type. The dominant allele of T65,+sd-1, seems to be incompletely dominant.

The agronomic characters of the three lines, T65, T65sd-1 (8) and T65sd-7(2), were compared (Table 2). Gene sd-1 had stronger reducing effects on upper internode length, panicle length and plant height than sd-7(t), and tended to increase panicle number per plant to some extent. Gene sd-7(t) did not affect panicle and upper internode elongation and tended to increase spikelet number per panicle. This gene may be useful in breeding, as is sd-6(t) reported by Hu (1987).

Table 1.Days to heading and plant height recorded in T65 and its
near-isogenic dwarf line (Winter season, 1986)
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                                         Difference  from   T65
                                      __________________________
Line        (Author's code)             Days to       Height
                                        heading         (cm)
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T65                                   0(= 128.7)    0(= 106.7)
T65sd-1(8)a       32-6/8                  -0.6        -27.2
T65sd-7(0)        D65-31                  -6.7        -21.2
T65sd-7(2)        D65-31/2                -2.9        -16.6
     LSD 5%                                0.4          0.8
         1%                                0.5          1.0
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a. Figures in parentheses show the number of backcrosses


Table 2.  Characters of near-isogenic lines with dwarfing gene 
          sd-1 and sd-7(t) as compared with those of T65
_____________________________________________________________________
                       Winter,1989          Summer,1988
Character         _________________________ _________________________    
                  T65 T65sd-1(8) T65sd-7(2) T65 T65sd-1(8) T65sd-7(2)
_____________________________________________________________________
Length(cm),
  Plant height     97       71        83     83     60          69
  Panicle          20       18        19     19     18          19
  Internode 1      40       29        40     33     25          33
        "   2      21       15        17     17     10          13
        "   3      11        6         5     10      5           4
        "   4       3        1.7       0.6    1     0.5         0.2
        "   5       0.5      0.5       0.4    0.4   0.3         0.2
Panicle no./plant  10       11.4       9.4   11    12.2         11
Spikelets/panicle  97       95       117     90     83          97
____________________________________________________________________

Table 3. Expected and observed segregation patterns in the F2 from 
         cross T65sd- 1(8) X T65sd-7(2)
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Genotype        Phenotype,   Planta   Frequency  No. of plants
                similar as   height   expected    Exp.  Obs.
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+/1+/1+/7+/7   T65             0      1/16       
+/1+/1+/7sd/7  F1:T65xT65sd-7  0      2/16
                                Tot   3             51.4   55
+/1sd/1+/7sd/7 Fl:T65sd-1XT65sd-7 -6  4/16
+/1+/1sd/7sd/7 T65sd-7(2)      -15    1/16
+/1sd/1+/7+/7  F1:T65XT65sd-1  -15    2/16   
+/1sd/1sd/7sd/7                -20    2/16
sd/1sd/1+/7+/7 T65sd-1(8)      -26    1/16
sd/1sd/1+/7sd/7                -26    2/16
                                Tot   12           205.5  207
sd/1sd/1sd/7sd/7 Double dwarf  -44    1/16          17.1   12
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a: Difference from the height of T65 (99 cm).
To know whether or not sd-1 and sd-7(t) are allelic, reciprocal crosses were made between T65sd-1 (8) and T65sd-7(2). The F1 plants were about 6 cm shorter than T65. The heterozygotes for sd-1 were about 15 cm shorter than T65 (incomplete dominance), while the heterozygotes for sd-7(t) showed the same stature as of T65. Therefore, the F1 plants heterozygous for both loci may be regarded as showing a tendency for overdominance. The F2 plants showed a range of plant height from T65-type to double-dwarf type (Table 3). When plants with intermediate statures were pooled to form a group, the F2 segregation pattern fitted a 3 tall : 12 interniediate: 1 short ratio, which was expected from the assumption of independence of sd-1 and sd-7(t).

There are about 60 dwarfing genes so far reported (RGN 1, p.32-34). The allelic relation of sd-7(t) with other dwarfing genes may be examined after testing its linkage relations with other markers to determine its linkage group.

References

H u, C. H., 1987. A newly induced semidwarfing gene with agronomic potentiality. RGN 4: 72- 74.

Kikuchi, F. and H. Ikehashi, 1984. Semidwarfing genes of high-yielding rice varieties in Japan. RGN 1: 93-94.

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