21. Two major genes controlling the large grain size of IR8

IR8 and IR42 are high yielding rice cultivars with medium maturity and erect leaves. However, they differ in grain shape. IR8 has a large grain, being larger than others in both length and width. In contrast, IR 42 has a short and slender grain. From their hybrid derivatives, it was learned that the large grain size of IR8 was controlled by two major genes, one controlling grain length and the other controlling grain width.

To analyze the mode of inheritance, many F\4\ line groups from IR 42 x IR 8 were observed. Lines segregating only for grain size were used for gene analysis. One of the F\4\ lines (F-2-6) segregated into 19 plants with short to medium grain and 5 plants with long grain, giving a good fit to a 3:1 ratio (X²=0.06, P>0.75; Fig. 1). The difference in grain length between the short- grain and long-grain groups was comparable to that between IR8 and IR42, and those plants had almost the same grain width.

Another F\4\ line (F-28-3) segregated into 21 plants with slender to medium grain and 4 plants with wide grain, giving a good fit to a 3:1 ratio (X²=0.50, P<0.25). The difference in grain width between the slender-grain and wide-grain groups corresponded to that between IR42 and IR8, and those plants had almost the same grain length. Similar patterns were also observed in some other lines.

Grain size has been considered to be controlled by many minor genes. However, some examples of major genes controlling grain size were reported (Takeda and Saito 1980; Kuo 1982). In addition, Takita (1985) found that grain length and grain width were controlled independently and that some major genes were involved. A difficulty in gene analysis for such quantitative characters comes from the interaction of genes with major and minor effects. Hence, for detecting major genes, differences in minor genes should be eliminated. Although the breeding of isogenic lines is useful, this was done in the present study by the selection of F\4\ lines segregating only for grain length or grain width.

Fig. 1. Distribution of individuals in grain length and grain width. (F-2-6 and F-28-3 are F4 lines derived from IR42/IR8).

Kuo, Y.K., 1982. Genetic studies on grain length by partitioning method in rice. J. Agric. Res. China 31: 265-274.

Takeda, K. and K. Saito, 1980. Major genes controlling rice grain size. Jpn. J. Breed. 30: 280-282 (in Japanese).

Takita, T., 1985. Inheritance of grain size and the relationship between grain size and other characters in rice. Bull. Natl. Agr. Res. Cent. 3: 55-71 (in Japanese with English summary).