Mitochondrial DNAs (mtDNAs) were isolated from several rice strains of Chinsurah boro II (fertile), Zhen Shan 97A (male sterile), Zhen Shan 97 (fertile), and Akihikari (normal japonica). Small plasmid-like DNAs, B1 and B2, which were found firstly in BT line, were cloned into plasmid vectors and mapped in terms of some of restriction enzymes. Plasmid-like DNAs were also observed in the mitochondria of Zhen Shan 97A and Zhen Shan 97 by agarose gel electrophoresis. To examine whether these plasmids were identical or not, hybridization analyses were carried out with B1 and B2 as probes. Consequently, it was suggested that two plasmids of Zhen Shan 97A were identical to B1 and B2, and one of the plasmids of Zhen Shan 97 was identical to B2. The smaller plasmid of Zhen Shan 97 had no homology to either B1 or B2. Co-existence of B1 and B2, while common, is not an obligatory feature. Because B2 was found in the fertile cytoplasm of Zhen Shan 97, an involvement of B2 with cytoplasmic male sterility (cms) might be denied.
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