| We studied on SSR diversity in chinese major rice varieties, and mapping of QTLs for rice sheath blight resistance. The results were as follows:1. Forty SSR were used to compare the genetic diversity changes in 151 Chinese major rice varieties planted in 1950s and in recent 10 years. Of the 40 SSR loci, 39 were polymorphic while 1 locus (RM479) was found to be monomorphic. All the 39 polymorphic loci revealed a total of 213 alleles. The mean number of alleles per locus (Na) was 5.5 with a range from 2 to 11. Total genetic diversity index of Nei (He) varied greatly among loci from 0.309 at RM174 to 0.869 at RM418, with an average value of 0. 0.649. There exsited significant difference in SSR allelic diversity between indica and japonica subspecies, Indica had more variation than japonica both for Na and He. By comparison of the genetic changes in Na and He, it was found that the period 1950s had more alleles and higher He than that of period recent 10 years both for indica and japonica rice. The difference between two subspecies was significant in a tendency for Na over time (indica:z=2.677, P=0.007;japonica: z=3.441, P=0.001), but not for He (indica:z=1.471, P=0.141:japonica: z=1.932, P=0.053). AMOVA indicated that the genetic variation was significantly different (P＜0.05) between the periods 1950s and the recent 10 years, of which, more genetic variation was contributed by indica (Fst=0.050) and japonica (Fst=0.082) subsets. Using locus-by-locus AMOVA procedure, significant differentiations were observed in 13 loci for indica subset and 11 loci for japonica subset between periods. The study has found a part of the genetic alleles being lost in current major rice varieties as comparison with that of 1950s. Therefore, more alien elite genetic resources should be explored in the current program of Chinese rice breeding.2. It was conducted with a double haploid (DH) population consisting of 174 DH lines, derived from an elite combination, Maybelle×baiyeqiu. The DH population and their parents were grown in a randomized complete design with four replications in the years of 2005 and 2006. Sheath blight response ratings for the population and the parents were identified by an improved method of inoculation, which was carried out with short woody toothpicks incubated with a Rhizoctonia solani strain, RH-9, and inserted the third sheath in the late tillering/green ring stage of growth. A linkage map was constructed from the DH. The QTL mapping of sheath blight resistance was carried out by the method of interval QTL mapping. Two QTL for sheath blight resistance were detected in each year, and were located on chromosome 1 and 5, respectively. The QTL for sheath blight resistance on chromosome 1 was flanked by markers RM583 and RM575 on the basis of 2005 data, and by RM575 and RM580 using 2006 data. The phenotypic variation for rice sheath blight explained by two QTLs were 7.26％and 6.99％, respectively. The QTL for sheath blight resistance on chromosome 5 was located on the marker interval of RM274 and RM178 in term of 2005 and 2006 data, but they located on different locus. Based on the effect analysis of QTLs for resistance, the genotype of baiyeqiu and Maybelle both had negative additive effects or reduced sheath blight rating.
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