Rice Genetics Newsletter, Vol. 18

Rgn18

24.	Identification of a new gene for spotted leaf found in Japonica-Indica hybrid population of rice
	T. KUBO, T. KUCHIISHI and A. YOSHIMURA Plant Breeding Laboratory, Faculty of Agriculture, Graduate School, Kyushu University, Fukuoka, 812-8581 Japan

Disharmonious genic interaction in hybrids derived from the crosses of distantly related rice varieties causes various phenotypic abnormality, such as sterility, weakness, chlorosis and albino. In this study, we report a new phenomenon of genic interaction found in backcross populations between Asominori (Japonica variety) and IR24 (Indica variety).

Kubo et al. (1999) produced IR24 chromosome segment substitution lines (CSSLs) with Asominori genetic background by repeated backcrossing and marker-assisted selections. The CSSLs carrying an IR24 homozygous segment at the middle region of chromosome 2 showed spotted, drooping and somewhat yellowish green leaves at seedling stage under natural condition. The leaf spot was apparent on the second leaf of the seedling (Fig. 1). This feature was specific to the CSSLs of the region of chromosome 2. Therefore, we inferred that the donor parent IR24 has a gene located on the substituted region of chromosome 2 for spotted leaf, although the gene cannot be activated in IR24 genetic background. One of the CSSLs carrying an IR24 homozygous segment of chromosome 2 in BC₃F₄ was backcrossed with Asominori, and the BC₄F₂ population was used for analysis. BC₄F₂ population segregated into 65 normal and 18 spotted leaf plants at seedling stage. This result fitted to the expected 3:1 ratio (chi²=0.49), suggesting that spotted leaf was controlled by a single recessive gene. The gene was tentatively designated as spl12(t). Each individual of the population was genotyped using RFLP and SSR markers on chromosome 2. All IR24 homozygous plants at the two SSR markers RM106 and RM263 showed spotted leaf; the heterozygous and Asominori homozygous plants showed normal leaf. Thus, spl12(t) co-segregated with RM106 and RM263 (Fig. 2). Link-age relations of spl12(t) with the two RFLP markers on chromosome 2 were also detected. In

addition, the SSR marker RM263 was confirmed to be tightly linked with an RFLP marker C920 on chromosome 2 using recombinant inbred lines derived from the cross between Asominori and IR24 (Tsunematsu et al. 1996).

spl12(t) was apparently one of the complementary genes derived from IR24 genome. The other gene non-allelic to spl12(t) was supposed to exist in Asominori genome, and the identification of the gene is now under way using the reciprocal sets of substitution lines (Aida et al. 1997; Kubo et al. 1999).

This study was supported by Bio-oriented Technology Research Advancement Institution (BRAIN), Japan.

References

Aida, Y., H. Tsunematsu, K. Doi and A. Yoshimura, 1997. Development of a series of introgression lines of Japonica in the background of Indica rice. RGN 14: 41-43.

Kubo, T., K. Nakamura and A. Yoshimura, 1999. Development of a series of Indica chromosome segment substitution lines in Japonica background of rice. RGN 16: 104-106.

Tsunematsu, H., A. Yoshimura, Y. Harushima, Y. Nagamura, N. Kurata, M. Yano, T. Sasaki and N. Iwata, 1996. RFLP framework map using recombinant inbred lines in rice. Breed. Sci. 46: 279-284.