Recombinant inbred lines (RILs) derived from the cross between Japonica variety Kinmaze and Indica variety DV85 were developed by single seed decent method. Subsequently, we have developed chromosome segment substitution lines by backcrossing RILs with recurrent parent Kinmaze. During the process of backcrossing, segregation of short panicle and awn were observed in the BC₂F₂ progeny from a selected BC₂F₁ plant in which DV85 chromosome segment retained in chromosomes 6, 7 and 8. For mapping, 71 BC₂F₂ plants were genotyped and their 71 BC₂F₃ lines (24 plants/line) were used for trait investigation.

In the BC₂F₃ lines, short panicle plants with about 15 cm length were clearly differentiated from the normal (or long) panicle plants with the length more than 20 cm (Fig. 1A). On the other hand, awn plants were clearly differentiated from the awnless plants (Fig. 1B). For short panicle, out of 71 BC₂F₃ lines there were 24 normal, 35 segregating and 12 short panicle lines

(chi² for 1:2:1 = 4.1). Normal plants were predominant to short panicle plants within the segregating lines. These results indicated that this trait was controlled by a single recessive gene. In this manner, awn also segregated with 1:2:1 ratio (chi² for 1:2:1 = 1.4) and awn plants were predominant within the segregating lines, indicating that awn was dominant and was under monogenic control.

Linkage mapping of two traits was performed using RFLP markers on chromosomes 6, 7, and 8. The gene for short panicle was located between RFLP markers R1440 and R1357 of chromosome 7, with genetic distance of 11.6 cM and 10.6 cM (Fig. 2 A). While, the gene for awn linked to G187 of chromosome 8 with 8.2 cM (Fig. 2 B). Since a short panicle gene sp is identified on chromosome 11 (Yoshimura et al. 1997), the short panicle gene newly identified on chromosome 7 in this study was designated as sp2. So far, an awn gene An4 is reported to be located on chromosome 8, but the map position of An4 on molecular map is not available (Sato and Tsukasaki 1984). Although there is a possibility that the gene for awn identified in this study is identical to An4, it was tentatively designated as An6(t) to avoid the inevitable confusion.

References

Harushima, Y., M. Yano, A. Shomura, M. Sato, T. Shimano, Y. Kuboki, T. Yamamoto, S.Y. Lin, B.A. Antonio, A. Parco, H. Kajiya, N. Huang, K. Yamamoto, Y. Nagamura, N. Kurata, G.S. Khush and T. Sasaki, 1998. A high-density rice genetic linkage map with 2275 markers using a single F₂ population. Genetics 148: 479-494.

Sato, S. and S. Tsukasaki, 1984. Genetical studies on awnedness gene An4 locating on the twelfth chromosome of rice plant. Japan. J. Breed. 34 (Suppl. 1): 288-289 (in Japanese).

Yoshimura, A., O. Ideta and N. Iwata, 1997. Linkage map of phenotype and RFLP markers in rice. Plant Mol. Biol. 35: 49-60.