Unique plasmid-like mitochondrial DNAs from indigenous maize races of Latin America

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Abstract
Mitochondrial DNA from 81 races of Latin American maize were examined by agarose gel electrophoresis. Twelve South American races each contained two plasmid-like mtDNA molecules similar to those of the cytoplasmic male-sterile S type ( cms -S). The plasmid-like elements from all 12 races, designated RU, appear to be identical. Both molecules appear in vitro as double-stranded linear DNAs terminated by repeated sequences arranged in reverse polarity (terminal inverted repeats). The larger molecule of the pair, R-1, contains about 7460 nucleotides. It shares considerable homology with the larger plasmid-like molecule of cms -S, S-1, but is about 1000 nucleotides longer than S-1, has a unique sequence of about 2576 nucleotides, and also contains a Bam HI recognition site not present in S-1, R-2, the smaller plasmid-like element, consists of about 5450 nucleotides and appears to share complete homology with S-2, the smaller plasmid-like molecule of cms -S. Neither pollen sterility nor any other trait has been associated with the R-1 and R-2 plasmid-like mtDNAs. The Bam HI restriction fragments of total mtDNA from the 12 RU cytoplasms display similar patterns, which differ only slightly but vividly from that of a normal maize standard, B73 × Mo17. Bam HI restriction analysis of 22 additional races produced arrays similar to those of the RU cytoplasms, but which lacked plasmid-like mtDNAs. The taxonomic significance of this digestion pattern and of the RU cytoplasms is discussed. One Mexican race, Conico Norteño, has been shown to contain the cms -S cytoplasm.