The wild rice species Oryza meridionalis Ng is distributed in north Australia, and has long awns about 7.8-10.3 cm (Vaughan 1994). We developed O. meridionalis (Acc. W1625) introgression lines, which had the genetic background of a Japonica variety Taichung 65 (T65) and retained O. meridionalis chromosomal segments (Kurakazu et al. 2001b). We previously detected two genes for awn, An7 and An8 located on chromosomes 5 and 4, respectively (Kurakazu et al. 2001a). To further identify new genes for awn, we investigated awn using BC₄F₂ populations derived from the same cross as used for the construction of the introgression lines. With the aid of SSR markers (McCouch et al. 2002), we identified two new genes and one new allele in this study.

In the BC₄F₂-6 population, the awn co-segregated with SSR marker RM8078 on chromosome 1 with the ratio of 57 awned : 28 awnless. This result indicated that a single dominant gene controlled the awn character although the segregation of the gene was distorted. Further linkage analysis using SSR markers revealed that the gene for awn was located between RM8111 and RM8051 with map distances of 3.0 and 3.6 cM, respectively (Fig. 1). In the BC₄F₂-8 population that retained the O. meridionalis introgressed segment of a long arm region of chromosome 1, awn character was segregated into 53 awned and 32 awnless plants and was associated with SSR markers RM265 and RM237 on chromosome 1 (Fig. 1). This result indicated that a single dominant gene controlled the awn character. So far, no gene for awn has been identified on chromosome 1. Therefore, the genes for awn identified in BC₄F₂-6 and BC₄F₂-8 populations are newly designated as An9 and An10, respectively.

In the BC₄F₂-12 population, we found a dominant gene for awn on chromosome 8. It was linked to SSR marker RM3496 (Fig. 1). The map position of this gene was identical to that of An6, which was previously identified by using a cross between a Japonica variety Kinmaze and an Indica variety DV85 (Takano et al. 2001). Therefore, we named this allele An6-mer.

Morphology of homozygotes for An9, An10 and An6-mer in the respective mapping populations was compared (Fig. 2). Awns generated by An9 were approximately 5 mm to 3 cm in length, and observed mainly at the top of spikelets in panicle branches (Fig. 2B). An10 produced shorter and sparser awns than An9 (Fig. 2C). An6-mer conferred awns 2 to 4 cm in length (Fig. 2D).

So far, we have identified a total of 5 genes for awn from O. meridionalis, including An7 and An8 (Kurakazu et al. 2001a). An7 and An8 could work additively and expressed longer awns than each gene but shorter awns than O.meridionalis (data not shown). The difference of awn length of a pyramid line with An7 and An8 from O.meridionalis could be explained by the existence of three genes, An9, An10 and An6-mer identified in this study.

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.

Kurakazu, T., Sobrizal and A. Yoshimura, 2001a. RFLP mapping of genes for awn on chromosomes 4 and 5 in rice using Oryza meridionalis introgression lines. RGN 18: 28-30.

Kurakazu, T., Sobrizal, K. Ikeda, P.L. Sanchez, K. Doi, E.R. Angeles, G.S. Khush and A. Yoshimura, 2001b. Oryza meridionalis chromosomal segment introgression lines in cultivated rice, O. sativa L. RGN 18: 81-82.

McCouch, S.R., L. Teytelman, Y. Xu, K.B. Lobos, K. Clare, M. Walton, B. Fu, R. Maghirang, Z. Li, Y. Xing, Q. Zhang, I. Kono, M. Yano, R. Fjellstrom, G. DeClerck, D. Schneider, S. Cartinhour, D. Ware and L. Stein, 2002. Development and mapping of 2240 new SSR markers for rice (Oryza sativa L.). DNA Res. 9: 199-207.

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Vaughan, D.A., 1994. The wild relatives of rice. A genetic resources handbook. The International Rice Research Institute, Manila, Philippines. pp. 48-49.