Molecular characterization of two endogenous double-stranded RNAs in rice and their inheritance by interspecific hybrids

We completely sequenced 13,936 nucleotides (nt) of a double-stranded RNA (d sRNA) of wild rice (W-dsRNA). A single long open reading frame (13,719 nt) containing the conserved motifs of RNA-dependent RNA polymerase and RNA hel icase was located in the coding strand. The identity between entire nucleot ide sequence of W-dsRNA and that of the dsRNA of temperate japonica rice (J -dsRNA, 13,952 nt) was 75.5%. A site-specific discontinuity (nick) was iden tified at nt 1,197 from the 5' end of the coding strand of W-dsRNA. This ni ck is also located at nt 1,211 from the 5' end in the coding strand of J-ds RNA. The dsRNA copy number was increased more than 10-fold in pollen grains of both rice plants. This remarkable increase may be responsible for the h ighly efficient transmission of J-dsRNA via pollen that we already reported . J-dsRNA and W-dsRNA were also efficiently transmitted to interspecific F1 hybrids. Seed-mediated dsRNA transmission to F2 plants was also highly eff icient when the maternal parent was wild rice. The efficiency of dsRNA tran smission to F2 plants was reduced when the maternal parent was temperate ja ponica rice; however, the reduced rates in F2 plants were returned to high levels in F3 plants.