Distribution and genetic variabilities of mitochondrial plasmid-like DNAs in Paramecium

Plasmid-like DNAs were isolated from mitochondria in many stocks of Para- mecium caudatum. Fifty-two among 68 stocks examined contained unique double-stranded DNAs (designated type I) ranging from 6.0 to 6.8 kb in length, and 12 stocks contained a set of 4 kinds of DNAs (8.2, 4.1, 2.8 and 1.4 kb; type II). Five of the stocks containing the type-I DNA had an additional 8.9-kb DNA (type III). No plasmid-like DNAs were found in the remaining 4 stocks. All these DNAs are linear and closely associated with proteins which can be removed only after proteinase-K digestion, suggesting that they exist as unique DNA protein complexes in mitochondria. Restriction and Southern blot analyses revealed a large sequence divergence among both type-I and -II DNAs. Type-I DNAs were found in every lineages of mitochondrial genomic DNA (mtDNA) which were inferred from restriction analyses. Type-II DNAs were, in contrast, scattered throughout the evolutionary tree of the mtDNAs. No obvious relations were found between the distribution of the DNAs and that of syngens (mating-type groups). Type-I DNAs were also found in other species of Paramecium, P. jenningsi and P. multimicronucleatum, and type-II DNAs in P. polycaryum.