Seed shattering is a primitive trait common in wild rice species. This trait has been eliminated in the process of rice domestication. Sobrizal et al. (1999) and Nagai et al. (2002) identified Sh3, which is commonly found in the backcross progeny of wild species. This is the first report of an inhibitor gene for Sh3.

In the process of developing O. glaberrima (Acc. IRGC104038) chromosomal segment introgression lines by continuous backcrossing with O. sativa cultivar Taichung 65 (T65) as a recurrent parent (Doi et al. 1997), we observed segregation of shattering plants in some of the BC₃F₃ populations. The characteristics of shattering plants were supposed to be caused by Sh3. Three BC₃F₄ populations obtained by self-pollinating seed shattering BC₃F₃ plants were used for the analysis of O. glaberrima allele of Sh3.

In the population analyzed, shattering and non-shattering plants totally segregated into 62:69 (Table 1). The degree of seed shattering was so remarkable that we could easily differentiate shattering from non-shattering plants. Since Sh3 was located on chromosome 4, RFLP markers tightly linked to Sh3 were used for linkage mapping. All 62 seed shattering plants in BC₃F₄ population were heterozygous at an RFLP marker R1427. Among 69 non-shattering plants, there were 29 homozygous plants for T65 and 40 homozygous plants for O. glaberrima. These results indicate that O. glaberrima allele of Sh3 tightly linked to R1427 caused shattering

and a recessive gene from O. glaberrima also tightly linked to R1427 inhibited the expression of the shattering gene. Linkage map of the genes for seed shattering and inhibitor is shown in Fig. 1. The allele of shattering gene from O. glaberrima identified in this study was designated as Sh3-gla because Sh3 from wild species was previously identified in the same map position (Sobrizal et al. 1999, Nagai et al. 2002). A recessive gene inhibiting the expression of Sh3-gla was tentatively designated as iSh3(t) (inhibitor for Sh3-gla). Our results indicate that O. glaberrima still has functional Sh3 and a mutation in iSh3 locus occurred during the process of domestication of O. glaberrima.

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

References

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Sobrizal, K. Ikeda, P. L. Sanchez and A. Yoshimura, 1999. RFLP mapping of a seed shattering gene on chromosome 4 in rice. RGN 16: 74-75.

Nagai, Y. S., Sobrizal, P. L. Sanchez, T. Kurakazu, K. Doi and A. Yoshimura, 2002. Sh3, a gene for seed shattering commonly found in wild rices. RGN 19 (in press).

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