Days to heading is an important agronomic trait of rice because it is highly associated with the regional and seasonal adaptability of rice cultivars. To identify genes controlling this trait, we developed a backcrossed population (BC₄F₂) from a cross between the Japonica rice variety Taichung 65, used as a recurrent parent, and the wild rice Oryza glumaepatula Steud., (IRGC Acc. No. 105668) used as a donor parent (Sobrizal et al. 1999). Two BC₄F₂ populations (BC₄F₂ 254 and BC₄F₂ 222) exhibiting a wide variation in days to heading were selected and planted under natural daylength conditions in the paddy field of Kyushu University Farm, Fukuoka, Japan. Seeds were sown on May 28, 1999 and transplanted on June 28, 1999. Each plant was monitored for the appearance of the first panicle. Days to heading was expressed as the number of days from sowing to

heading. Eight leaves (ca. 10g) were collected from each plant, after monitoring the heading date, for DNA extraction. One hundred six RFLP markers showing polymorphism between O. glumaepatula and Taichung 65 were used for the whole genome survey in the BC₃F₁ generation. The retained heterozygous regions were further evaluated in the BC₄F₁ generation (Fig. 1A).

The frequency distribution for days to heading of BC₄F₂ populations exhibited a bimodal distribution (Fig. 1B). The days to heading of Taichung 65 varied from 95 to 100 days, while for BC₄F₂ 254 and BC₄F₂ 222 ranged from 95 to 114 and 94 to 111 days, respectively. O. glumaepatula did not flower during the entire growing season under natural daylength condition, hence, no heading data is presented. BC₄F₂ 254 was classified into 20 early and 52 late heading individuals. This segregation agreed with 1 (early) : 3 (late) ratio (chi²=0.30), indicating that the late heading was controlled by a single dominant gene from O. glumaepatula. On the other hand, the segregation of 23 early and 55 late heading plants in BC₄F₂ 222 which agreed with 1:3 ratio (chi²=0.84) was observed, indicating that the late heading was governed by a single partially dominant gene from O. glumaepatula. F₃ tests supported the conclusion drawn from the F₂ populations. The two late heading genes identified in BC₄F₂ 254 and BC₄F₂ 222 were tentatively designated as Lhd1(t) and Lhd2(t), respectively. Linkage analysis of Lhd1(t) and Lhd2(t) with RFLP markers in BC₄F₂ and BC₄F₃ revealed that Lhd1(t) was tightly linked to RFLP marker XNpb27 on chromosome 6, whereas Lhd2(t) was linked to RFLP markers C383 and C145 on chromosome 7 (Fig. 1C).

Lhd1(t) appears to be located on a position corresponding to Hd3a (Yano et al. 1999), whereas the position of Lhd2(t) on chromosome 7 may be same as the region of the previously reported genes, E1 (Ichitani et al. 1998) and Hd4 (Yano et al. 1997).

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

References

Ichitani, K., Y. Okumoto and T. Tanisaka, 1998. Genetic analysis of the rice cultivar Kasalath with special reference to photoperiod sesnsitivity loci, E1 and Se1. Breed. Sci. 48: 51-57.

Sobrizal, K. Ikeda, P.L. Sanchez, K. Doi, E.R. Angeles, G.S. Khush and A. Yoshimura, 1999. Development of Oryza glumaepatula introgression lines in rice, O. sativa L. RGN 16: 107-108.

Yano, M., Y. Harushima, Y. Nagamura, N. Kurata, Y. Minobe and T. Sasaki, 1997. Identification of quantitative trait loci controlling heading date in rice using a high-density linkage map. Theor. Appl. Genet. 95: 1025-1032.

Yano, M., L. Monna, S. Kojima, H.X. Lin and T. Sasaki, 1999. Genetic dissection of genomic region for Hd3 into two loci, Hd3a and Hd3b, controlling heading date in rice. Breeding Research 1 (Suppl. 2): 156. (in Japanese)