A genetical study on the nucleotide sequences of the nuclear DNAs which share homology with rice mitochondrial plasmid-like DNAs, Bl, B2, B3 and B4 was carried out. Restriction fragments of the nuclear DNAs hybridized with these plasmid-like DNAs showed polymorphisms in their length between Indica and Japonica rice cultivars. The hybridized signals found specifically in Indica or Japonica cultivars segregated in the F2 population derived from a cross between these two subspecies. The observed ratio of the nuclear homologues in the F2 population demonstrated that they were transmitted according to the Mendelian inheritance. The co-segregation of homologues was examined and the linkage was detected between the Bl-nuclear homologue of Japonica and the B4-nuclear homologue of Indica, and also between the nuclear homologues of B2 and B3 of Indica. The linkage between the Bl-nuclear homologue of Japonica and the B4-nuclear homologue of Indica was conserved in the different rice cultivars.
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