For quality rice varieties, the endosperm should be free of chalkiness since the chalky grains have the lower density of starch granules compared to translucent ones and are more prone to breakage during milling. Also, both longitudinal and transverse cracks occur easily in chalky kernel when steamed, reducing palatability of cooked rice. Percentage of grains

with chalkiness (PGWC) in rice is quantitatively inherited. Identification of QTLs affecting PGWC and elucidation of its genetic mechanism would be useful for marker-assisted selection (MAS) to increase breeding efficiency. Up to now, there have been several studies on primary mapping of QTLs for PGWC (He et al. 1999; Tan et al. 2000). But no reports were found concerning stability analysis and fine mapping of QTLs affecting PGWC in rice.

In this study, we conducted a molecular marker-based analysis for the genetic control of PGWC using 66 Asominori/IR24 BC₃F₂ chromosome segment substitution lines (CSSLs) (Kubo

et al. 1999) in two years and four sites, and detected a stable expression QTL qPGWC8 with average R² of 23.7% underlying PGWC in the G1149-R727 interval on chromosome 8 (Table 1 and Fig. 1a). Then, a F₂ secondary population (BC₄F₂) was developed through backcrossing CSSL₅₀, one of the target CSSLs harboring qPWGC8 allele, with the genetic background parent, Asominori.

The F₂ population, composed of 1382 lines, was phenotyped for PGWC according the method of He et al. (1999). The result showed that the ratio of high PGWC lines: low PGWC lines fitted with 1:3 Mendelian inheritance segregation, indicating that high PGWC was controlled by a single recessive gene, pgwc8, in this population (Fig. 2). A genetic linkage map consisting of 6 SSR markers was constructed between RFLP markers G1149 and R727 for mapping precisely the pgwc8 gene. When the MAPMAKER/EXP 3.0 program was applied to combine the results obtained by SSR analysis and progeny tests, the pgwc8 gene was mapped between SSR markers RM477 and RM447 (Fig. 1b).

Acknowledgements

We thank professor A. Yoshimura, Kyushu University, Japan, for their kindly providing us the CSSL population and genotype data.

References

He P., S.G. Li, Q. Qian, Y.Q. Ma, J.Z. Li, W.M. Wang, Y. Chen and L.H. Zhu, 1999. Genetic analysis of rice grain quality. Theor Appl Genet 98: 502-508.

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.

Tan Y.F., Y.Z. Xing, J.X. Li, S.B. Yu, C.G. Xu and Q.F. Zhang, 2000. Genetic bases of appearance quality of rice grains in Shanyou 63, an elite rice hybrid. Theor Appl Genet 101: 823-829.