We searched for SNPs in 417 regions distributed throughout the genome of three Oryza sativa ssp. japonica cultivars, two indica cultivars, and a wild rice (O. rufipogon). We found 2800 SNPs in approximately 250,000 aligned bases for an average of one SNP every 89 bp, or one SNP every 232 bp between two randomly selected strains. Graphic representation of the frequency of SNPs along each chromosome showed uneven distribution of polymorphism-rich and -poor regions, but little obvious association with the centromere or telomere. The 94 SNPs that we found between the closely related cultivars 'Nipponbare' and 'Koshihikari' can be converted into molecular markers. Our establishment of 213 co-dominant SNP markers distributed throughout the genome illustrates the immense potential of SNPs as molecular markers not only for genome research, but also for molecular breeding of rice.

These two authors contributed equally to this work. SummaryWe hypothesized that cytoplasmic male sterility (CMS) in sugar beet may be the consequence of mitochondrial dysfunctions affecting normal anther development. To test the hypothesis, we attempted to mimic the sugar beet CMS phenotype by inhibiting the expression of mitochondrial pyruvate dehydrogenase (PDH), which is essential for the operation of the tricarboxylic acid (TCA) cycle. Screening with a cDNA library of sugar beet¯ower buds allowed the identi®cation of two PDH E1a subunit genes (bvPDH_E1a-1 and bvPDH_E1a-2). bvPDH_E1a-1 was found to be highly expressed in tap roots, whereas bvPDH_E1a-2 was expressed most abundantly in¯ower buds. Green¯uorescent protein (GFP) fusion of bvPDH_E1a revealed mitochondrial targeting properties. A 300-bp bvPDH_E1a-1 cDNA sequence (from 620 to 926) was connected to a tapetum-speci®c promoter in the antisense orientation and then introduced into tobacco. Antisense expression of bvPDH_E1a-1 resulted in conspicuously decreased endogenous bvPDH_E1a-1 transcripts and male sterility. The tapetum in the male-sterile anthers showed swelling or abnormal vacuolation. It is also worth noting that in the sterile anthers, cell organelles, such as elaioplasts, tapetosomes and orbicules were poorly formed and microspores exhibited aberrant exine development. These features are shared by sugar beet CMS. The results thus clearly indicate that inhibition of PDH activity in anther tapetum is suf®cient to cause male sterility, a phenocopy of the sugar beet CMS.