Grain traits such as length and weight are not only related to production, but also have direct bearing on the marketability in rice. So far, quantitative trait loci (QTL) for grain traits in rice have been identified in many QTL studies by using all kinds of cross combinations in the sativa gene pool and AA genome species (Tan et al. 2000). However, genetic studies on the grain characters have not been reported in the interspecific progeny developed using O. minuta (BBCC) due to difficulty in developing progeny (Brar and Khush 1997). The aim of this study is to identify and characterize QTLs underlying grain characters in a backcross progeny derived from a cross between the O. sativa subsp. japonica cv. Hwaseongbyeo and O. minuta (2n = 48, BBCC, Acc. No. 101141).

An introgression line, WH79006 was produced from a single plant from BC₅F₃ families from a cross between Hwaseongbyeo used as a recurrent parent and O. minuta as a donor parent, which was subsequently self-pollinated for three generations (Kang et al. 2002). WH79006 resembled the O. sativa parent, Hwaseongbyeo, however it differed from Hwaseongbyeo in several traits including 1,000 seed weight and days to heading (Kang et al. 2002). These differences might be due to O. minuta introgressions. To detect the polymorphism between Hwaseongbyeo and O. minuta, 511 microsatellite markers of known chromosomal position were used for the parental survey. Of the 511 markers, 231 (45.2%) showed polymorphism. Of these 231 polymorphic markers, 45 SSR markers detected 28 independent O. minuta introgressions in WH79006 located on all chromosomes except for chromosome 2 (Jin et al. 2004). The high number of introgressions in WH79006 might be due to selection for O. minuta phenotype in the generation advancement (Kang et al. 2002). To evaluate the effects of these segments on the grain traits, 75 F_2:3 lines derived from the cross Hwaseongbyeo/WH79006 were developed and evaluated for grain traits. Bulked 300 brown rice of fully filled grains of each of the F_2:3 families were evaluated for seed length (sl), seed width (sw), seed thickness (st), 1,000 seed weight (tsw) and seed length/width ratio (lwr). The frequency distribution of phenotypes for four traits in the 75 F_2:3 lines are shown in Figure 1. All traits showed approximately normal distribution suggesting quantitative inheritance.

Forty-five SSR markers that detected O. minuta-specific introgressions in WH79006, were used for evaluation of the genotype of the F₂ population (Fig. 2). One-way ANOVA was carried out to detect association of introgression with the grain characters. QTL was declared if the phenotype was associated with a marker locus at P < 0.001 or with two adjacent marker loci at P < 0.05. A total of four QTLs for 4 traits were identified (Fig. 2, Table 1). The number of QTLs identified in this study is probably an underestimation because some of the chromosomal regions harboring introgressions might have been undetected with the SSR markers employed and the population size (75 lines) was small to detect QTLs of minor effects. The seed length QTL, sl11, was detected on chromosome 11 and the WH79006 allele increased seed length. This QTL, sl11 affecting seed length appears to share a similar region as the grain length QTLs (Tan

et al. 2000, Yoshida et al. 2002), and their relationships remain to be clarified. The other three QTLs associated with sw, tsw, and lwr, were detected on chromosomes 7, 7, and 10, respectively. The two QTLs, sw7 and tsw7 shared the similar region, and at these loci the WH79006 alleles increased weight and width. Because seed width and 1,000 seed weight are

related, it is quite possible that a single gene has a pleiotropic effect on two traits. Fine mapping of this cluster would be needed to differentiate between tight linkage and pleiotropy of the genes involved. These two QTLs have not been detected in the previous QTL studies between Oryza cultivars, indicating potentially novel alleles from O. minuta.

Acknowledgements

This study was supported by a grant from the Bio Green 21 project of the Rural Development Administration, Republic of Korea.

References

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