Unusual inheritance of evolutionarily-related double-stranded RNAs in interspecific hybrid between rice plants Oryza sativa and Oryza rufipogon

Endogenous, 14 kb double-stranded RNAs (dsRNAs) have been found in two ecospecies of cultivated rice (temperate japonica rice and tropical japonica rice, Oryza sativa L.) and in wild rice (O. rufipogon, an ancestor of O. sativa). A comparison of the nucleotide and deduced amino acid sequences of the core regions of the RNA-dependent RNA polymerase domains found in these three dsRNAs suggested that these dsRNAs probably evolved independently within each host plant from a common ancestor. These dsRNAs were introduced into F-1 hybrids by crossing cultivated rice and wild rice. Unusual cytoplasmic inheritance of these dsRNAs was observed in some F-1 hybrids; the evolutionarily related dsRNAs were incompatible for each other, and the resident dsRNA of an egg cell from cultivated rice was excluded by the incoming dsRNA of a pollen cell from wild rice. Coexisting dsRNAs in the F-1 hybrids segregated away from each other in the F-2 plants. However, the total amount of these dsRNAs in the host cells remained constant (ca. 100 copies/cell). The stringent regulation of the dsRNA copy number may be responsible for their unusual inheritance.