Plasmid-like mitochondrial (mt) DNAs have been reported in two maize cytoplasms: the cytoplasmic male-sterile cms-S (Pring et al., Proc. Natl. Acad. Sci. USA 74:2904-2908, 1977) and the fertile RU cytoplasms of 18 Latin American maize races (Weissinger et al., Proc. Natl. Acad. Sci. USA 79:1-5, 1982). In each case, the plasmid-like DNAs are observed in pairs of linear molecules with one molecule being appreciably larger than the other (Fig. 1).
Restriction endonuclease fragment patterns of five RU cytoplasmic maize races differ from the standard RU pattern by the presence or absence of up to three bands, and/or the degree of fluorescence in other bands (Weissinger et al., 1982). The spontaneous reversion of cms-S maize to fertility, correlated with the disappearance of the S-1 and S-2 molecules and changes in restriction fragment patterns, has been associated with a transpositional event (Levings et al., Science 209:1021-1023, 1980).
Mitochondrial DNAs from 31 accessions representing all the described taxa of teosinte were examined by agarose gel electrophoresis. Five of six Zea diploperennis accessions examined possessed two molecules designated D-1 and D-2 that migrated, respectively, to approximately the same positions, although slightly higher, as the R-1 and R-2 plasmid-like DNAs found in normal maize (Fig. 1). Differences in migration were greater between D-1 and R-1 than between D-2 vs. R-2 and S-2. The S-1 and R-1 molecules are distinguished by length, a BamHI site in R-1, and unique sequences. S-2 and R-2 have not been differentiated by length, molecular hybridization, heteroduplex analyses, or restriction sites.
BamHI restriction patterns of Z. diploperennis reveal three pattern groups. Those accessions containing D-1 and D-2 differ by nine or more bands from accessions lacking D-1 and D-2. Similar results were obtained with EcoRI. When D-1 and D-2 are present, two BamHI fragment pattern groups occur which differ only by the presence or absence of one band. The brilliantly fluorescent bands associated with total mtDNA digestion patterns of cms-S and RU cytoplasms are not seen with restriction endonuclease digestion of D-1 and D-2 bearing cytoplasms.
D. H. Timothy, C. S. Levings III, W. W. L. Hu, and M. M. Goodman
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